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TS7.2

EDI
Dynamics and structural evolution of fold-and-thrust belts and accretionary prisms: an interdisciplinary approach

Fold-and-thrust belts and accretionary prisms are some of the most recognizable large-scale geological features occurring all around the globe. They mostly develop along convergent plate boundaries. Fold-and-thrust belts may also form along passive margins or other super-critical slopes by a gravitationally driven stress field. Fold-and-thrust belts may involve the basement of continental lithospheres to build entire mountain ranges or just the uppermost sedimentary sequence detaching along stratigraphic décollements. Although these different types of fold-and-thrust belts vary in spatial extent, longevity of their formation, and rock types involved, their dynamics and structural evolution strongly depends on the same internal and external effects, such as rheological and rock mechanical properties, temperature and surface processes, allowing to compare them with each other and to develop common mechanical predictions.
Fold-and-thrust belts have been intensely investigated to decipher their short- and long-term evolution. However, there are important questions that yet have to be fully understood: i) What is the effect of inherited structures within the basement, the upper sequence and potential décollements, and how can those inheritances be detected? ii) How are transient and long-term rheological/mechanical characteristics comparable during the formation of a fold-and-thrust belt? iii) Do present day fold-and-thrust belts reflect local, transient conditions, and how are large-scale, long-term tectonic processes affecting their evolution?
The here proposed session tackles to answer these questions by an interdisciplinary approach. We look forward to receive abstracts focusing on the short- and long-term dynamics and structural evolution of fold-and-thrust and belts by means of structural fieldwork, seismics and seismology, analogue and numerical modelling, rock mechanics, geomorphology and thermochronology as well as quantification of uncertainties in order to improve our understanding of fold-and-thrust belts across spatial and temporal scales.

Convener: Sandra BorderieECSECS | Co-conveners: Jonas B. Ruh, Christoph von Hagke, Esther Izquierdo LlavallECSECS, Olivier Lacombe
Presentations
| Thu, 26 May, 13:20–18:29 (CEST)
 
Room D1

Thu, 26 May, 13:20–14:50

Chairpersons: Sandra Borderie, Olivier Lacombe

13:20–13:21
Introduction

13:21–13:28
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EGU22-7288
Olivier Lacombe et al.

Stylolite are rough structures developed by pressure solution, usually related either to burial stress or to tectonic contraction. The stylolite roughness, i.e., the difference in height between two points separated by a given distance along a track, yields quantitative information about the normal stress applied to the stylolite plane. The stress magnitude is accessed by applying signal analysis such as Average Wavelet Coefficient (AWC) or Fourier Transform (FT) onto a stylolite track, returning a characteristic length (cross-over length, Lc) at which two regimes of self-affine properties switch. In the case of a sedimentary, bedding-parallel stylolite, Lc scales to the magnitude of the vertical principal stress σ1, hence to the burial depth at the end of the life of the studied stylolite. When applied to bedding-parallel stylolite populations in foreland basins and fold-and-thrust belts, this Stylolite Roughness Inversion Technique / paleopiezometer (SRIT) allowed estimating the maximum burial experienced by a strata before σ1 became horizontal at the onset of tectonic contraction. We have collected hundreds of stylolites in the Meso-Cenozoic carbonate sequence along a wide SW-NE transect across the Umbria Marche Apennine Ridge (Apennines, Italy). On this dataset we conducted stylolite roughness inversion with both FT and AWC in order to quantify the maximum burial reached in the different parts across the belt before the Apenninic contraction begun. Doing so, we observed some discrepancies between Lc values obtained by either one or the other signal analysis method, implying a user dependent choice of the method based on best fit of the treatment and on consistency between all results. In order to (1) find the source of this difference, (2) correct this effect and (3) assess whether it impacts the derived vertical stress magnitude, we built virtual composite stylolites by assembling consistent stylolite tracks together to increase the range of roughness covered by the signal analysis. We present a statistical comparison of the results of the application of SRIT on single tracks and on composite ones. In both cases, the resulting depths are of the same order of values, and within the range of uncertainties, allowing a confident reconstruction of the pre-contractional burial depth across the Umbria Marche Apennine Ridge. However, the resulting Lc are closer for the two regression methods in the case of composite stylolites. The new approach therefore reduces the risk associated with a choice between signal analysis methods related to the user and expands its easiness of application.

Key words: paleopiezometry, stylolites, compressive deformation, folding, vertical stress, compaction and burial depth

How to cite: Lacombe, O., Labeur, A., Beaudoin, N. E., and Callot, J.-P.: Assessing burial history using stylolite roughness paleopiezometry with a twist in the Umbria-Marche Apennine Ridge , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7288, https://doi.org/10.5194/egusphere-egu22-7288, 2022.

13:28–13:29
questions

13:29–13:36
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EGU22-110
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ECS
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On-site presentation
Thomas Gusmeo et al.

The integration of low-temperature thermochronological and thermal maturity analyses constrains the maximum temperatures experienced during burial by the sedimentary fill of the central sector of the Greater Caucasus basin and the timing of its structural inversion. Raman spectroscopy, illite percentage and stacking order in illite-smectite mixed layers, illite crystallinity index, and Rock-Eval Pyrolysis analyses indicate that the maximum paleotemperatures experienced by the Greater Caucasus basin fill increase progressively from about 100 °C in the southern foothills of the central Greater Caucasus to close to 400 °C approaching the axial zone of the orogen. Apatite fission-track and apatite and zircon (U-Th)/He analyses along the same transect yielded ZHe ages between about 137 and 5 Ma, AFT central ages between about 37 and 4 Ma, and AHe ages between about 10 and 2 Ma, with progressively younger ages approaching the axial zone of the Greater Caucasus. Statistical inverse modelling of thermochronological data, integrating thermal maturity results and all other geological and geochronological constraints available, points to episodic exhumation during structural inversion of the central Greater Caucasus basin. Such basin was first partially inverted in Late Cretaceous/Paleocene times following Northern Neotethys closure along the Sevan-Akera suture zone; renewed basin inversion occurred since Middle-Late Miocene times as a consequence of far-field compressional stress transmission from the Arabia-Eurasia hard collision along the Bitlis-Zagros suture zone. It should be emphasised that this sequence of events applies only to the central portion of the Greater Caucasus and by no means should be extended to the other parts of such a large and complex orogenic system.

How to cite: Gusmeo, T., Cavazza, W., Zattin, M., Corrado, S., Schito, A., Alania, V., and Enukidze, O.: Burial and exhumation history of the Georgian sector of the central Greater Caucasus, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-110, https://doi.org/10.5194/egusphere-egu22-110, 2022.

13:36–13:37
questions

13:37–13:44
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EGU22-7262
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On-site presentation
Marion Roger et al.

The Carpathians fold-and-thrust belt resulted from the accretion of sediments during subduction of the European slab in the Cenozoic, and the collision of the ALCAPA and Tisza-Dacia blocks with the East European margin in the Early-Middle Miocene. In this study we unravel the history of nappe stacking and the thermal evolution of the Ukrainian Carpathians wedge by combining low-temperature thermochronology and tectono-stratigraphic analysis.

We collected 11 sandstone samples for (U-Th)/He and fission-track dating on detrital apatites and zircons. The resulting thermochronological data were modelled using QTQt to constrain the time-temperature paths independently for several nappes. We compare the results with the burial histories of the respective nappes as derived from their stratigraphy.

All zircon (U-Th)/He (ZHe) ages in our samples are older than the depositional age of the corresponding strata; they are thus non-reset and thought to mark sediment provenance. We identified two groups of ZHe ages; a younger group with ages around 130 Ma to 90 Ma in the inner nappes, and an older group with ZHe ages around 450 Ma to 200 Ma in the outer belt. Potential sources for these zircons are thought to be the Tisza-Dacia basement and its sedimentary cover for the younger ZHe age group, and the East European shield and/or its sedimentary cover for the zircons with older ZHe ages in the external nappes. Apatite (U-Th)/He (AHe) ages are mostly <20 Ma and show a trend of progressive younging toward the outer nappes. Apatite fission-track (AFT) ages display a similar pattern with overall younging of the central age from the inner to the outer nappes, except for the outermost Skyba nappe where AFT central ages are around 10 My older than in the adjacent Krosno nappe.

Modelling the sample time-temperature paths from AHe, ZHe and AFT data permits to unravel the chronology of nappe stacking. Most of the AFT samples are partially reset, allowing to better constrain the burial and exhumation pathways. The inner Magura and Marmarosh nappes started cooling from a peak burial temperature of 100 ± 5°C at 40 to 29 Ma. Our two samples from the following nappes, Burkut and Rakhiv, show younger cooling ages with an onset at 24 ± 2 Ma and at ~10 Ma, respectively, and significantly higher burial temperatures (≈150 °C) than the other nappes, provoking the total resetting of AFT ages. The next nappe, Krosno, started cooling between 21 and 17 Ma from peak burial temperatures of 100 ± 8°C. The Skyba nappe started cooling between 22 and 16 Ma. Whereas the onset of this cooling is similar to the Krosno nappe, the maximum burial temperature of Skyba is higher (130 ± 5°C) and at odds with the minor thickness (<1500 m) of the sedimentary overburden.

These results indicate potential out-of-sequence thrusting and/or significant erosional exhumation in the innermost nappes. For the middle nappes, burial by tectonic overthrusting and/or kilometre-scale syn-tectonic sedimentation is required. The higher temperatures experienced by the outermost nappes can resulted from overthrusting and complete erosion of the middle nappes.

How to cite: Roger, M., de Leeuw, A., van der Beek, P., and Husson, L.: Reconstruction of the Ukrainian Carpathians fold-and-thrust belt from low-temperature thermochronology and tectono-stratigraphic analysis. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7262, https://doi.org/10.5194/egusphere-egu22-7262, 2022.

13:44–13:45
questions

13:45–13:52
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EGU22-8497
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Virtual presentation
Paolo Ballato et al.

The Iranian Plateau represents a NW-SE striking, elongated, elevated (mean elevation of ~1.8 km), arid, mostly internally drained (~65 % of its area) and aseismic morphotectonic feature of the Arabia-Eurasia collision zone. With a crustal thickness up to ~65 km, the southern plateau margin includes the High Zagros Mountains and the plate suture zone. The northern plateau margin, instead, consists of the Urumieh-Dokhtar Magmatic Zone and the western Alborz-Talesh Mountains from SE and NW Iran, respectively, which exhibit a crustal thickness ranging from 40 to 50-55 km. The plateau interior is characterized by a low-topographic relief morphology with six major, mostly internally drained intermontane sedimentary basins. The backbones of these basins are mainly represented by the Sanandaj-Sirjan Zone. Plateau uplift commenced after ~17 Ma, as documented by the occurrence of Lower Miocene shallow-water marine sediments of the Qom Formation within the plateau interior. Although the Iranian Plateau represents the second largest collisional plateau after Tibet, the chronology of the events and the mechanisms that built it are poorly constrained.

In this study, we combine a new low-temperature thermochronologic dataset including apatite fission- track and apatite (U-Th-Sm)/He ages from the northern plateau sectors and its interior with structural and stratigraphic data from different intermontane basins and literature thermochronology data. Combined, this information shows that after a mild phase of post late Eocene contractional deformation, collisional deformation started in the early Miocene along the plate suture zone to the south and in the middle Miocene (~16 Ma) in the Talesh-Alborz Mountains to the north. Subsequently, around 12-10 Ma, deformation jumped in the plateau interior over a rather large area including the Urumieh-Dokhtar and Sanandaj-Sirjan zones, apparently without a specific pattern of propagation. Upper plate deformation occurred mostly through the reactivation of older NE-dipping structures that led to the topographic growth of several mountain ranges spanning a wavelength of ~50-60 km. This was associated with the compartmentalization of the upper plate and the development of different intermontane basins. There, basin filling processes inhibited intrabasinal deformation and faulting along the major range-bounding faults producing the smoothed, low-relief landscape typical of an orogenic plateau.

Combined, these results provide new information concerning the mechanisms and the timing of the lateral, orogen-perpendicular, growth of the Iranian Plateau

How to cite: Ballato, P., Balestrieri, M. L., Dunkl, I., Balling, P., Paknia, M., Heidarzadeh, G., Biralvand, M., Zeilinger, G., Ghassemi, M., Faccenna, C., and Strecker, M.: Deformation, uplift and exhumation across the northern sectors of the Iranian Plateau: insights from low-temperature thermochronology data and intermontane basins fill units, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8497, https://doi.org/10.5194/egusphere-egu22-8497, 2022.

13:52–13:53
questions

13:53–14:00
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EGU22-9211
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ECS
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On-site presentation
Chloé Bouscary et al.

In mountain accretionary wedges, it is generally considered that the preservation of a topography in mechanical equilibrium is modulated by the activation of faults, sometimes internal to the prism, sometimes frontal. The folds of the Himalayan foothills correspond to the most frontal structures of the Himalayan prism. Understanding the timing of the initiation and the activity of these frontal folds can provide valuable information on the deformation sequences within the range (reactivation of the MCT, prograde sequences and transfer to frontal folds, ...) in response to tectonic and climatic forcing. Late Cenozoic climatic changes, including glaciations, might have impacted the denudation of the Himalayan range. The study of recent deformation rates is thus key for understanding lateral variations in deformation along the entire Himalayan arc, which will bring new constraints on the interactions between tectonics and surface processes at different scales time, as well as deepen our understanding of the seismic behaviour of the range.

 

Here we quantify exhumation rates in the Himalayan foothills using luminescence thermochronometry, which is a recently developed very-low temperature thermochronometer applicable between tens of years and a few hundred kyr. In contrast to classical methods, it can resolve thermal histories from the upper few km of the Earth’s crust, allowing spatial variations in exhumation rates across the Himalayas to be deciphered on sub-Quaternary timescales. An extensive data set of more than 40 Siwalik rock samples, from Western Nepal to Eastern Bhutan, was measured to complement other thermochronometric data and understand the sub-Quaternary deformation on the Himalayan foreland.

 

The results show along-strike variations in exhumation rates in the Himalayan foothills during the late Quaternary, with exhumation rates across the sub-Himalaya varying locally independently of precipitation trends and changes in the modern convergence rates. These along-strike variations may suggest that over the last 300 kyr, Himalayan shortening has not only been accommodated by the most frontal faults along the Himalayan range.

How to cite: Bouscary, C., King, G., Grujic, D., Lavé, J., Hetényi, G., and Herman, F.: Late Quaternary deformation of the sub-Himalaya on 100 kyr timescales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9211, https://doi.org/10.5194/egusphere-egu22-9211, 2022.

14:00–14:01
questions

14:01–14:08
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EGU22-10064
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ECS
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Virtual presentation
Sarah Falkowski et al.

The spatio-temporal history and control of uplift and incision of the eastern flank of the Central Andean Plateau margin is a point of controversial discussion. For example, ca. 4 Ma incision has been suggested for canyons in Bolivia and South Peru (>1250 km apart) and could be interpreted as either tectonically or climatically driven.

To evaluate the sensitivity of cooling ages to climatic and/or tectonic driven erosional exhumation, we build upon previous work and contribute new low-temperature thermochronometer data from three, up to 190-km-long transects from the eastern Andean Plateau to the Subandean Zone in southeastern Peru. The transects extend from the plateau down the San Gabán, Marcapata, and Tres Cruces valleys and include both valley bottom and interfluve samples. This is different from previous work and allows for an evaluation of age-elevation relationships along and across strike.

We present new thermochronometer dates from 46 apatite (U-Th)/He (age range ~1–41 Ma), 23 zircon (U-Th)/He (~4–284 Ma), 21 apatite fission-track (~3–63 Ma), and 11 zircon fission-track (~14–37 Ma) bedrock samples, as well as thermal models. All samples are interpreted in the context of sample elevation and neighboring structures.

We discuss the Miocene–Pliocene exhumation history of the Eastern Cordillera, including differences in the exhumation magnitude between the transects that are ca. 50–100 km away from each other. Based on age-elevation and age-distance relationships of the different thermochronometers and thermal models, we find that causes of exhumation and canyon incision cannot be as clearly identified and separated in time as previously suggested. However, plateau incision in the latest Miocene or later is consistent with climate enhanced incision and needed to explain the relationship between apatite (U-Th)/He and higher-temperature thermochronometer ages. Future work will integrate thermo-kinematic and erosion models to help gain further insight into the deformation history of the area.

How to cite: Falkowski, S., Ehlers, T., McQuarrie, N., Buford Parks, V., Glover, C., and Cárdenas, J.: Along-strike variations in the timing and magnitude of exhumation in the eastern Peruvian Andes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10064, https://doi.org/10.5194/egusphere-egu22-10064, 2022.

14:08–14:09
questions

14:09–14:16
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EGU22-7548
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ECS
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Virtual presentation
Willemijn S.M.T. van Kooten et al.

The transition from the Eastern Cordillera to the Santa Barbara System in NW Argentina is characterized by the inversion of pre-existing Cretaceous and Paleozoic structures. Within this complex fold-and-thrust belt, the Miocene Cianzo basin with its rich sedimentary and structural record tells the tale of Andean reactivation of an extensional fault system and the incorporation of the former Salta rift basin into the orogenic wedge. In the Cretaceous, the intracontinental Salta rift was widely distributed in NW Argentina, with multiple sub-basins radiating from a central high. The Cianzo basin, at that time situated at the northern margin of the ENE-WSW striking Lomas de Olmedo sub-basin, impressively shows the transition from condensed post-rift strata on top of the rift shoulder to thick, proximal syn- and post-rift strata of the Salta Group in the adjacent half-graben. These sediments were buried by up to 3 km of clastics from the approaching orogenic wedge, causing apatite (U-Th-Sm)/He (AHe) and, in part, apatite fission track (AFT) ages to be reset. In the Miocene, deformation reached the Eastern Cordillera, gradually dissecting the foreland basin along pre-existing faults and forming local depocenters such as the Cianzo basin, which is surrounded by inverted normal faults and basement block uplifts. Its unique structural setting and complete sedimentary record provide an excellent natural laboratory to study fold-and-thrust belt formation through reactivation of pre-existing structures. Although the structural and sedimentary characteristics of the Cianzo basin have been studied in detail, low-temperature thermochronology data to quantify deformation processes is lacking. We provide AHe, AFT and zircon (U-Th-Sm)/He (ZHe) cooling ages from 39 samples from the Cianzo basin and adjacent areas. Jurassic ZHe ages from the Aparzo ranges may reflect pre-Salta Group exhumation of the rift shoulder, an event which is also recorded in thick, proximal agglomerates that were shed into restricted depocenters. AHe and AFT data document a Middle-Late Miocene onset of rapid exhumation of the Abra de Zenta, Hornocal and Aparzo ranges that border the Cianzo basin. Furthermore, AHe and AFT ages constrain the sequence of deformation for large folds such as the Cianzo syncline. The new dataset refines the timing of reactivation of pre-existing normal faults that now bound the Cianzo basin and sheds a new light on the propagation of the Eastern Cordillera fold-and-thrust belt in time and space.

How to cite: van Kooten, W. S. M. T., Sobel, E. R., del Papa, C., Payrola, P., Starck, D., and Bande, A.: Constraining the formation of the fault-bound Cianzo basin, NW Argentina using low-temperature thermochronology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7548, https://doi.org/10.5194/egusphere-egu22-7548, 2022.

14:16–14:17
questions

14:17–14:27
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EGU22-7974
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solicited
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On-site presentation
Susanne Buiter and Nicolas Molnar

A series of influential papers in the 1980’s showed how the long-term evolution of fold-and-thrust belts and accretionary wedges (here collectively termed orogenic wedges) can quantitatively be described as striving towards a mechanical equilibrium defined by their internal and basal material strengths (Dahlen 1984, Dahlen et al. 1984, Davis et al. 1983). Unstable orogenic wedges will deform to adjust their basal and surface slopes to a critical taper angle, defined by a wedge that is at the verge of failure everywhere. Critical taper theory has been confirmed by analogue and numerical experiments and found numerous successful applications in field studies.

The success of critical taper theory forms a framework that allows investigating non-critical behaviour of orogenic wedges. Previous numerical and analogue studies pointed out that: (1) Only portions of orogenic wedges may be at failure at any given time, separating critically stressed from non-critical segments (Lohrmann et al. 2003, Simpson 2011). As these wedges still observe a critical taper, this may indicate that it is the critically stressed segments that define the overall wedge shape. (2) Numerical experiments often attain a critical taper at lower shortening percentages than analogue experiments. We speculate that this may be related to larger amounts of strain softening generally used in numerical setups and/or the number of shear zones that forms at equivalent shortening (which is controlled by numerical resolution and analogue material properties). This non-criticality is thus likely only a transient state.

We here ask the question whether structural inheritance from earlier compressional or extensional deformation phases may lead to longer-term non-critical wedge behaviour by favouring out-of-sequence thrusting or shear zone propagation into the foreland. To address this question, we combine a review of previous dynamic wedge experiments with new analogue experiments that investigate the influence of inherited shear zones and variations in material properties on wedge evolution. We shorten quartz sand layers overlying a weak basal microbeads layer with a non-deformable backstop. The backstop has two independently moving parts, allowing to alternate thin- and thick-skinned deformation. We find that the reactivation of basement shear zones formed in earlier deformation phases is short-lived and does not affect thrusting to a degree that would distinguish these wedges from those without inheritance. We extend these experiments by including variations in internal material properties and weaker shear zones, remaining however in the domain of brittle orogenic wedges.

Non-critical wedge behaviour may only be a transient state, but could occur frequently owing to variations in material properties or structural inheritance, which are to be expected in regions of inter-plate shortening of former rift regions. Our contribution hopes to highlight the potential for future modelling studies of orogenic wedges to examine how non-critical wedge behaviour could play into the evolution of fold-and-thrust belts and accretionary wedges.

How to cite: Buiter, S. and Molnar, N.: Dynamic experiments investigating non-critical orogenic wedges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7974, https://doi.org/10.5194/egusphere-egu22-7974, 2022.

14:27–14:28
questions

14:28–14:35
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EGU22-7027
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ECS
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On-site presentation
Nicolas Molnar and Susanne Buiter

When orogeny reactivates extensional structures or uplifts pre-existing depocenters in the foreland (inversion), the overall nature, dimension, and geometry of these rheological heterogeneities represent one of the main controlling factors in the spatio-temporal evolution of foreland fold-and-thrust belts. Relationships between inversion structures in the foreland and far-field stresses caused by orogenic fronts have long been identified (e.g., Ziegler, 1989, Geol. Soc. Spec. Publ. 44). However, conditions that facilitate or hinder basin inversion in these settings remain unclear, mainly due to the intrinsic complexity of analysing multiple overprinted geological events.

We use novel laboratory experiments of basin inversion to investigate how compressional stresses are transferred across a heterogeneous crust. More specifically, we determine how the presence of multiple extensional basins in the foreland controls the location, occurrence, and sequencing of foreland thrusts. Quantitative analysis of our experiments allows us to define conceptual models for comparison and application to natural examples where geological interpretation remains partially conjectural due to their intrinsic complexity, such as the permo-carboniferous troughs beneath the Swiss Molasse basin or the inverted Broad Fourteens Basin in the North Sea.

Our experiments are built in a modelling apparatus with a mobile backstop, using quartz sand to model brittle crustal materials and glass microbeads to simulate a weaker basal detachment layer. Velocity discontinuities at the base are created by attaching multiple thin basal sheets to the mobile wall during extensional phases (pulling). The location of each extensional basin is defined by the lengths of the basal sheets. During extension, the resulting graben-like structures are progressively filled with microbeads to create a sedimentary infill that is less competent than the surrounding rock. The basal sheets are completely detached from the mobile wall before the initiation of the shortening phase (pushing). Topography, surface and lateral deformation is quantified employing a high-resolution particle imaging velocimetry (PIV) system.

We present results of shortening multiple extensional basins at fixed distances from the orogenic front. Detailed analysis shows that extensional basin faults are not reactivated during shortening, but instead inversion is characterised by an initial squeezing of the basin fill and subsequent formation of either frontal or back thrusts that localise along the microbead-sand interface, leading to the overall uplift of the basins. This mechanism occurs independent of the distance of the basin to the orogenic front. However, when several grabens are present, the extent of shortening that each extensional structure localises differs greatly between experiments, showing variability according to the number of basins and their distance to the orogenic front.

When compared to reference models with a homogeneous crust, our results show that the presence of multiple extensional basins in the foreland exerts a first-order control on the evolution of propagating fold-and-thrust belts. Thrust location and sequencing evolve differently, with frontal thrusts developing along pre-existing basins boundaries at early stages, and subsequent stages of back thrust formation characterising wedge thickening at the hinterland of the extensional basins.

How to cite: Molnar, N. and Buiter, S.: Analogue modelling of inversion tectonics: investigating the role of multiple extensional basins in foreland fold-and-thrust belts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7027, https://doi.org/10.5194/egusphere-egu22-7027, 2022.

14:35–14:36
questions

14:36–14:43
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EGU22-8489
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ECS
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On-site presentation
Anna-Katharina Sieberer et al.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. In this contribution, we focus on the internal deformation of the DI and its eastern continuation towards the Dinarides. Using a series of crustal scale analogue models, we investigate the effect of Jurassic E-W extension on the NW-SE directed shortening of the DI during Alpine orogeny.

The brittle and brittle-ductile analogue experiments can be grouped in two sub-series. In sub-series A, the platform-basin topography has been created by pre-scribing an initial strength contrast between platforms and basins followed by one stage of indentation. In sub-series B, graben structures were developed through an initial extensional phase, subsequently followed by compression. The evolving grabens were syn-kinematically filled up to different thicknesses depending on the material used; either with quartz sand up to a platform/basin thickness ratio of 0,75 or with feldspar sand or glass beads up to the initial, non-stretched crustal thickness. The brittle upper crust of platforms was simulated with quartz sand, the brittle to ductile middle crust by either glass beads or by a mixture of polydimethylsiloxane (PDMS) silicon putty and quartz sand. In both sub-series, variations in the orientation of rheological boundaries with respect to the convergence direction have been modelled. This (oblique) basin inversion allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust.

Modelling results of (oblique) basin inversion confirm the localisation of deformation in areas of lateral strength contrasts (Brun and Nalpas, 1996), as transitions from platforms to basins represent. Spacing of in-sequence thrusts is larger on platforms and smaller in basins, visible in both, models of sub-series A (inversion of strength difference only) and B (inversion of strength difference and actual normal faults). The vergence of in-sequence structures varies from mostly foreland directed using glass beads as basal detachment, to pop-up structures using putty, to a combination of both using quartz sand only. Based on our modelling observations, we propose that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform/basin configuration.

To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out (for details on structural and thermochronological data see the contribution of Klotz et al. in session GD8.4). According to field observations, the shortening direction along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system, external eastern Southern Alps, Italy), changes locally from top SSW to top SSE along strike. We infer that the variability of shortening directions along these thrust faults depends on inherited geometries and is not the result of different deformation phases. One possible conclusion from this observation is that the number of deformation phases in the Southern Alps may have been overestimated so far.

References:

Brun, J.-P., Nalpas, T. (1996). Graben inversion in nature and experiments. Tectonics. v. 15, no. 3, p. 677-687.

How to cite: Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Internal deformation of the Dolomites Indenter, eastern Southern Alps: Orthogonal to oblique basin inversion investigated in crustal scale analogue models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8489, https://doi.org/10.5194/egusphere-egu22-8489, 2022.

14:43–14:44
questions

Thu, 26 May, 15:10–16:40

Chairpersons: Christoph von Hagke, Sandra Borderie

15:10–15:17
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EGU22-11171
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On-site presentation
Prokop Závada et al.

The concept of detachment folding was developed between the '60s and '80s and generally describes displacement and buckling deformation of a competent layer above a weak, usually low viscosity horizon during tectonic shortening. From this definition, and based on the Biot-Ramberg theory, it is clear that geometrical parameters of such folds depend on contrasting rheology in both layers or on the rheological gradient in a complex multilayer. These systems were originally studied in association with the thin-skinned deformation and salt tectonics, and recently with regards to large-scale lithospheric deformation. In the latter, the rapidly heated lower crust is partially melted and a thin melt layer at the MOHO depth serves as the detachment horizon during collision and shortening. 

Our experimental work contributes to the understanding of the geometrical, kinematic and dynamic behaviour of such types of detachment folds as this deformation process strongly depends on a thermally dependent rheological gradient and nonlinear shortening velocity. The natural prototype for our models is for example the Chandmann or Bugat metamorphic domes in central Asia (CAOB). Our aim is to parametrize the style of such crustal-scale detachment folds depending on the rheological properties of the layered crust and the thermal gradient.

For this purpose, we developed an apparatus for thermal analogue models capable of producing thermal gradients and programmable shortening. Paraffin wax is used as the analogue for the partially molten lower crust. The advantage of this material is, that it reproduces the temperature-controlled rheological stratification of the crust in hot orogens with a melt layer at the bottom (at Moho) superposed by partially molten crust. The upper crust is represented by a granular mixture of low-density cenosphere particles and silica sand, respectively. To keep the models properly dynamically scaled, we take into account the relationship for progressively decelerated plate convergence in the orogens. 

With increasing of both, basal heating and shortening rate, the folds' finite geometry converges to a system of pseudo-symmetric folds, cored by various amounts of the melt with respect to their position in the fold sequence. The dynamics of the fold amplification also depends on the position in the array of folds and is described by four evolutionary steps; initial perturbation, amplification with the melt inflow into axial zones of the folds, locking and simple vertical extrusion. 

With decreasing intensity of basal heating, the total melt amount is lower, deformation is more localized and converges to brittle-ductile coupling. Typical products are thrust systems on a local scale or pop-up structures on a large scale. Melt is localized in form of small fingers underneath the pop-up structures. Relatively colder and slower models display homogeneous thickening.

A higher degree of heating results in melt redistributed along the axial planes of folds. Analysis of the layer interfaces curvature, paths and tortuosity of the material particles in these high-temperature experiments (based on resultant displacements calculated by the PIV method) also revealed asymmetrical evolution of the P-T-t paths for associated limbs of the pseudo-symmetrical folds.

 

How to cite: Závada, P., Krýza, O., Schulmann, K., Lexa, O., and Shu, T.: Modes and geometry of crustal-scale detachment folds in hot orogens – insights from physical modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11171, https://doi.org/10.5194/egusphere-egu22-11171, 2022.

15:17–15:18
questions

15:18–15:25
|
EGU22-12281
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ECS
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Virtual presentation
Sepideh Pajang et al.

Long-term tectonics numerical modelling at accretionary margin scale is a powerful tool to retrieve the influence of many parameters such as the spatial variations of the frictional properties along a simplified interface and its feedback on the deformation. Despite significant analogue and numerical studies on the evolution of accretionary prism, none of them account for heat conservation or temperature-dependent rheological transitions. Since Makran is one of the thickest accretion prisms in the world, the contribution of heat to the rheology of the prism cannot be ignored. Here, we solve for advection-diffusion of heat with imposed constant heat flow at the base of the model domain to allow the temperature to increase with burial. We start with a simple setup of one décollement layer to capture how the brittle-ductile transition affects the structures and geometry of the accretionary prism.

Our results show that a mature brittle-ductile wedge forms four different structural segments that can be distinguished based on topographic slope and deformation. An initial purely frictional segment is characterized by an imbricated zone and active in-sequence thrusts faults at the toe of the wedge. Its topographic slope is controlled by the basal friction of the décollement and is consistent with the critical taper theory prediction. The presence of the smectite-illite transition (dehydration reaction) leads to a flat topographic slope by the drop of friction. This flat segment produces little internal deformation and appears during the early stage of the accretionary prism formation. The third segment is marked by an increase of the topographic slope that begins with the onset of internal distributed viscous deformation in between brittle structures. Viscous deformation appears once the base of the model reaches 180°C while the décollement remains brittle. We refer to that segment as the brittle-ductile transition where both brittle and ductile deformation co-exists within the wedge together with high internal deformation. The last segment of deformation corresponds to the onset of the ductile deformation along the décollement by reaching a temperature of 450°C with an approximate flat zone without effective internal deformation. The topographic slope is again consistent with the critical taper theory, considering that a viscous décollement is equivalent to a brittle décollement of extremely low friction.

Knowing the impact of temperature transitions, we include more complexity in our simulations to increase the relevance of the models with the Makran accretionary prism. We calibrate the basal heat flow from BSR visible along seismic profiles. An intermediate décollement, essential for underthrusting to occur at the rear of the wedge, is added to the simulations. We show that the onset of underthrusting is controlled by the brittle-ductile transition. As tomographic models on land indicate packages with a higher velocity at depth, seamount subduction is another hypothesis tested. We conclude that the subduction of large seamount is accompanied by deep-rooted listric normal faults, whose location migrates through time. Seamount subduction also permits the formation of a large thrust slice zone and lateral variation of basal-erosion which can be followed in seismic profiles of Nankai and Makran subduction zones.

How to cite: Pajang, S., Le Pourhiet, L., Cubas, N., Khatib, M. M., and Heyhat, M.: The brittle-ductile transition signature in accretionary prism, insight from thermomechanical modeling. Application to Makran, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12281, https://doi.org/10.5194/egusphere-egu22-12281, 2022.

15:25–15:26
questions

15:26–15:36
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EGU22-3252
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ECS
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solicited
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Virtual presentation
Tasca Santimano and Russell Pysklywec

Fold and thrust belts and prominent orogens are found primarily along continental plate boundaries. Our knowledge of how these orogens are formed is based on the deformation of the upper crust. However, continental interiors also exhibit fold and thrust belts that may not be related to plate boundary collision. In these intraplate settings, structural heterogeneities in the deep lithosphere have been identified as an important factor in the formation of these belts. Particularly, inherited deep zones of weakness may initiate orogenesis in continent interiors. Aside from structural heterogeneity, the rheological strength of the lithosphere also has a primary role affecting the kinematics of deformation in the lithosphere. To investigate the interplay of rheology and pre-existing structures, we designed a set of physical scaled analogue experiments in a convergent setting that tests (a) the presence and absence of a pre-existing weak zone in the lithospheric mantle and (b) the effects of the rheological strength of the lithospheric mantle. The tectonic evolution of the model is recorded to acquire a time series data set of the velocity field, strain in the model, and the development of structures in the upper crust. Results show that a weak zone in the lithospheric mantle allows deformation to be accommodated by displacement along this zone and is transferred into the overlying lower and upper crust, regardless of lithosphere strength. In contrast, a model absent of a weak zone accommodates deformation by folding and thickening of the viscous layers. The viscous lithosphere in models with a strong lithospheric mantle tends to buckle creating a sequence of brittle faults in the upper crust. Specifically, the rheology of the lithosphere dictates the distribution of strain. Our results are further used to interpret the genetic formation of an intracontinental fold and thrust belt found on Ellesmere Island in the Canadian Arctic Archipelago.

How to cite: Santimano, T. and Pysklywec, R.: Fold and thrust belts in an intraplate setting- An interplay between rheology and inherited deep structures. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3252, https://doi.org/10.5194/egusphere-egu22-3252, 2022.

15:36–15:37
questions

15:37–15:44
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EGU22-11768
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ECS
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Virtual presentation
Extensional faulting in the crests of growing anticlines – examples from seismic data and discrete-element modelling
(withdrawn)
Torsten Hundebøl Hansen et al.

15:44–15:45
questions

15:45–15:52
|
EGU22-10312
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ECS
|
On-site presentation
Costantino Zuccari et al.

Understanding how folds and faults nucleate and grow is key to unravelling the tectonic and seismic evolution of both active and fossil fold-and-thrust belts (FATB). The progressive growth of folds and the transition from folding to faulting in FATBs are complex phenomena that reflect the combined effects of numerous deformation processes and boundary conditions. To better understand these complexities, we studied a folded sequence within the shortened Mesozoic carbonate multilayer succession of the seismically active Italian Eastern Southern Alps (ESA). The studied mesoscopic folds are parasitic to hanging- and footwall hectometric folds associated with regional-scale S-verging thrusts. We aimed to constrain (1) the folding style of the area, (2) the parameters governing the transition from folding to faulting through time, (3) the seismic vs. aseismic behaviour during the folding-faulting transition in carbonate-dominated fold-and-thrust belts, and (4) the overall tectonic framework of the ESA. Our approach relied on i) the structural analysis of symmetric and asymmetric folds and of the locally associated thrusts to assess the overall structural style and derive geometrical constraints upon the documented deformation features, ii) XRD analysis of the deformed carbonate multilayer to define its mineralogical composition and to establish the influence thereof upon deformation, and iii) mechanical modelling based on the Finite Element Method (FEM) to study the factors governing fold symmetry versus asymmetry.

Our field analysis shows that folds evolve from symmetric and open to south-verging asymmetric and close to tight before eventually being decapitated by discrete faults. This occurs once fold forelimbs exceed ~80°, which corresponds roughly with when the ratio between fore- and backlimbs dip angle exceeds ~3.3.  The mesoscopic thrusts that dissect asymmetric folds firstly localise along the gently dipping backlimbs, exploiting clay-rich beds therein, and then propagate toward the foreland by cutting across the steep forelimbs, producing cataclastic domains. Layer-parallel shearing and cataclasis are the dominant deformation modes during thrusting along the backlimb and forelimb, respectively. FEM modelling, used to constrain the transition from symmetric to asymmetric folding, shows that it is mainly controlled by (i) the thickness and vertical distribution of different rock types and (ii) the growth of first order folds at larger scales. In multilayer sequences we observe that small scale folds are initially symmetrical, forming under pure shear conditions. However, these structures may later become passively rotated in simple shear as they become parasitic to the growth of larger scale folds.

Finally, we propose a scenario of fold growth and transition from folding to faulting that has implications on the tectonic evolution of fold-and-thrust belts, including the coexistence of seismic and aseismic deformation during progressive shortening.

How to cite: Zuccari, C., Viola, G., Simpson, G., Curzi, M., Aldega, L., and Vignaroli, G.: Fold evolution and the transition from folding to faulting: New insights from the carbonate multilayer succession of the Italian Eastern Southern Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10312, https://doi.org/10.5194/egusphere-egu22-10312, 2022.

15:52–15:53
questions

15:53–16:00
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EGU22-8263
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ECS
|
Virtual presentation
Animesh Das et al.

The outer wedge of the Indo-Burma wedge (IBW) has resulted due to oblique subduction of the Indian Plate below the Burma. In this study, we will use the analysis of outcrop-scale structures from Tripura-Mizoram fold belt (TMFB) to evaluate the structural evolution of the outer wedge of IBW. TMFB belongs to the widest section of the outer wedge that stretches from east to west for around 270 km (along 23.5° N latitude). The first order structure of the outer wedge is characterized by a series of north-south trending anticlines-and-synclines of varying tightness. Analysis of our field observations provide a detailed understanding on the evolution of the first-order structure of the outer wedge of IBW. Weshow that the style of folding progressively becomes complex towards the hinterland direction of the wedge. The complexity of the fold structure is defined by the development of different geometries of folds, including refolding of earlier structures. Interestingly, different geometries of folds towards the hinterland share a uniform orientation of folds axes, implying pure shear deformation. Our field observations allow us to infer that the outer wedge sediments of IBW have deformed in a ductile manner over a shallow decollement, lying beneath the Neogene sediments of the outer wedge. We attribute the ductile behaviour of the outer wedge sediments to the dominance of weak shale horizons and high pore fluid pressure in the entire Neogene sequence of the outer wedge. To gain a complete understanding on the style of the strain distribution within TMFB, we performed scaled laboratory modelling under oblique convergence. We used Polydimethyl Siloxane (PDMS) to simulate the viscous rheology of the Neogene sediments. Model results show strong consistency not only with the existence of across-strike variations in the tightness of fold patterns from east to west but also provides a strong basis for explaining the occurrence of along-strike variations of deformation intensity in the outer wedge of IBW, which gradually increases southward with narrowing the width of the wedge.

How to cite: Das, A., Roy, S., Dasgupta, S., and Bose, S.:  Structural evolution of the outer Indo-Burma Wedge: Insights from field observations and laboratory experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8263, https://doi.org/10.5194/egusphere-egu22-8263, 2022.

16:00–16:01
questions

16:01–16:08
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EGU22-5905
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ECS
|
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On-site presentation
Francho Gracia Puzo et al.

The southern Pyrenean Zone shows a classical thin-skinned fold-and-thrust belt. Particularly interesting is the thrust sequence detached in the Upper Triassic low-strength level cropping out in the central-western sector of the chain (Leyre-Orba thrust system). Along its mapped trace, the Leyre thrust cleanly places the Cretaceous units on top of the Eocene (syn-tectonic) marls of the Jaca basin. However, in the footwall of this thrust there is a series of smaller-scale faults related to the main thrust and involving exclusively the marly units.

Understanding the deformation that marls (often lacking structural indicators) have acquired is a subject of interest to many geoscientists, given the role that these rocks play in geological storage systems. Knowing the state of the rock fabric may be essential to understand variations in its expected physical properties. In particular, Anisotropy of Magnetic Susceptibility (AMS) is a technique that can be successfully (although perhaps not easily) applied on these lithologies, provided that clay minerals present on marls are responsible of the magnetic signal of the fabric.

The marls of the Arro-Fiscal formation show fracturing and a pervasive cleavage along a width of hundreds of meters along strike the Leyre fault. The penetrativity of cleavage gradually increases with the proximity to the main fault, as demonstrated by Boiron et al. (2020). However, new data presented in Gracia-Puzo et al. (2021) and this communication show that the deformation gradient is not a single progression, but that there rather are variations in the intensity of deformation, as indicated by the magnetic fabric data of the marls, what can also be correlated with outcrop observations in the field.

A second look at the outcrops, after considering AMS data, has permitted to detect faults that a priori were not observed, since they involve the same monotonous lithology in both walls. Therefore, they are not presented in previous cartographies. In outcrop view, we can detect areas where the marls have undergone significant deformation, with a very penetrative, almost slaty cleavage. These deformation zones are metric in thickness, and the less intense pencil cleavage in the marls can extend several tens of meters in thickness across strike.

For realizing the presence of these faults, the study of the magnetic fabric has revealed as a useful tool, since it gives a very accurate picture of strain conditions (Parés, 1999; Gracia-Puzo, 2021). In this presentation, microscopic data are also added. Together with the AMS dataset, they are aimed to characterize the fabric of the deformed marl, and thus to understand how deformation has developed in the context of the foreland of the Leyre thrust and the Arro-Fiscal Formation, at different scales. Gathering AMS data, thin sections and field observations, we conclude that a symmetrical deformation shadow is observed in these faults, involving both the foot-wall and hanging-wall.

Finally, in this work, we aim to characterize these strained beds, which differ in scale and geometry from other known deformed marls, thus extending our knowledge of shale cleavage formation.

How to cite: Gracia Puzo, F., Aubourg, C., and Casas Sainz, A.: Reverse fault propagation in shales and associated decametric deformation gradients. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5905, https://doi.org/10.5194/egusphere-egu22-5905, 2022.

16:08–16:09
questions

16:09–16:16
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EGU22-5906
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On-site presentation
Antonio Casas et al.

Interpretation at depth of geological structures and cross-section reconstruction of fold and thrust belts requires either (1) constraints derived from geophysical exploration (seismic, gravimetric or, in some cases, magnetic) or borehole data, or, alternatively, (2) assumptions about the geometrical model that help to accept or discard, or, eventually, to evaluate the feasibility of possible solutions. In this sense, 3-D reconstructions can help to correct and modify the reconstruction at depth of the main structural traits of a structure or a set of structures. Factors to take into consideration include the consistency in shortening figures along strike for each thrust sheet and the whole set of thrust sheets, and the deformation associated with thrust fronts, together with the consistency in constraints referred to the relative chronology between the different thrust slices. In this work we present the results of a 3-D high-resolution modelling of the Leyre thrust (Southern Pyrenees), confronting different possible models of its structure at depth, and showing the usefulness of 3-D reconstruction. The interest for its study lies in the strong along-strike changes observed, that must be linked to the particular kinematics of this sector of the Pyrenean chain. The proposed geometrical reconstruction benefits from the outstanding outcrops along the Esca valley transect and the existence of geophysical low quality data that, nevertheless, allow to establish some limits to the maximum depth of particular horizons.

The Leyre thrust is a plurikilometric, E-W striking, shallow-dipping, South-verging thrust located within the Eocene Jaca-Pamplona basin and detached at depth in the Upper Triassic evaporites (the regional décollement for many thrust systems in this area). The overall geometry of the outcropping segment of the Leyre thrust is a low-angle ramp of the Cretaceous-Paleocene competent units (folded and cut with high-angle ramp geometry), onto the Eocene marls that show pervasive slaty cleavage related to the thrust front. A second thrust sheet can be inferred at depth also involving the Cretaceous-Paleocene sequence. Furthermore, a back-thrust linked to a box-fold anticline appears in the hangingwall of the main thrust. This box folds shows a strong eastwards plunge, and disappears laterally towards the East. Finally, a slightly oblique thrust (WNW-ESE) ramps up the box-fold, with increasing displacement from West to East. The connection between this latter thrust and the back-thrust at the rear front of the box-fold is probably related with the warping of the fault surface and (possibly) a clockwise rotation of the uppermost thrust sheet.

All in all, the 3-D reconstruction proposed allows to update and contrast some of the tectonic models classically proposed for the area (see e.g. Labaume et al., 1985), reducing the number of superimposed thrust sheets and relating their geometry with an overall break-back (or hanging-wall-sequence) kinematics triggered by the blocking of movement at particular thrusts and the upward steepening of thrust surfaces. Development of (hardly-to-detect) thrust surfaces in the marls located in the footwall of the frontal thrust would be the manifestation of the last movements of the thrust system.

How to cite: Casas, A., Gracia-Puzo, F., and Aubourg, C.: Along strike structural changes in thin-skinned thrusts: 3-D approach to the Leyre thrust (Southern Pyrenees). , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5906, https://doi.org/10.5194/egusphere-egu22-5906, 2022.

16:16–16:17
questions

16:17–16:24
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EGU22-8958
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Virtual presentation
William A. J. Rutter et al.

The Bristol Channel contains major Variscan thrusts juxtaposing distinct tectonostratigraphic terranes:  the Upper Carboniferous Rhenohercynian, Culm (south) and Sub-Variscan Foredeep, Coalfield (north). There is agreement the contrasts across the Channel are not restricted to this, since underlying marine Devonian differs from continental ORS and Lower Carboniferous radiolarian-chert differs from the Main Limestone. The famous basins were mapped intricately over 100+ years by the UK geological survey and by academics across Europe, and questions about their juxtaposition date back to 1895 in the Quarterly Journal of the Geological Society, London.

Our aim, is to use structural styles and shortening to determine an upper limit for displacements upon the major thrusts. We investigate the magnitudes of shortening from south to north through the Culm and north Devon basins, and from west to east across SW Dyfed, central South Wales, Bristol, Mendips, Oxfordshire, and Kent, using an immense legacy of sections drawn by various authors, including the recent basin dynamics group of Wales.

Estimates corrected for Mesozoic negative inversion show 45% shortening due to accommodation-chevron and box folding in the Culm, 40% due to folds, back-thrusts, and fore-thrusts in the north Devon basin, 30% beneath northern parts of the Channel, and 33% along the strike of the foredeep from Wales to Kent. There is also great contrast in deformation style, between the Culm continuous-folds and the foredeep with reactivated faults, rounded folds, and thrusts, related to preferential slip along seams within central parts of the Middle Coal Measures.

Shortening can be 70%, close to underthrusts in the southern Culm; adjacent to regional thrusts along the north Devon coast; and, proximal to disturbances within the foredeep. This intensity of composite deformation would not be out of place close to tectonic-scale thrusts, between these terranes. Additionally, thrusts of this scale are detectable on regional seismic profiles and were the topics of recent studies. Structural inspection reveals significant 1km-scale displacements along NW-SE strike-slip faults common to both terranes and upon WSW-ENE oblique-ramp thrusts local to the Vale of Glamorgan and Severn estuary. WNW-ESE frontal ramps with ~10km-scale displacements are considered candidate ‘stems’ to tectonic-scale thrusts and are found in Gower, Devon, and inner Channel.

Further investigations could elaborate the style of transmission of major thrust displacement from beneath the hinterland into the foredeep, whether by reactivation, decapitation, translation, and rotation of structural fabrics. There are complications of Mesozoic negative and Cenozoic positive inversions to consider in section restoration and adjustments are required to reveal how large displacements were dissipated exactly.

Reservations are that shortenings in hanging-walls can be poor indicators of displacement magnitude upon individual thrusts within sequences. Nevertheless, we conclude there is nothing contrary to the occurrence of a 100km-scale displacement, especially if accounting the tectonic-scale dimension and 300-500km geographic separations of modern terranes analogous to facies equivalent to the Culm and foredeep.

How to cite: Rutter, W. A. J., Miliorizos, M. N., Melis, N. S., and Reiss, N.: Structural styles and shortening estimates for the inverted external British Variscides to determine maximum thrust displacements., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8958, https://doi.org/10.5194/egusphere-egu22-8958, 2022.

16:24–16:25
questions

16:25–16:32
|
EGU22-4607
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ECS
|
On-site presentation
Adeline Marro et al.

The arcuate Jura Mountains Fold-and-Thrust Belt (FTB) is situated in the NW Alpine Foreland and its formation is related to the Alpine orogeny. The western part of the Jura FTB, investigated here, is situated in France to the north of the Geneva Basin (Switzerland). The geothermal project “GEothermie2020”  of the larger Geneva area allowed us to re-assessed the structural geology and the kinematic evolution of the internal part of Western Jura FTB from the Geneva Basin (Switzerland) to the Bienne Valley (France).

Stratigraphic harmonization, new geological and tectonic maps, new seismic interpretation, and a new near top Basement surface were used to construct a kinematic model. This model using forward modelling techniques has been developed in the software Movetm by Petroleum Experts. The forward model relies on fault-bend fold, trishear, and fault-parallel flow algorithms, and provides a valid and balanced cross-section. The model is constrained by surface, well and seismic data. Therefore, the depth of the near base Mesozoic horizon has been well constrained by seismic depth-converted lines. Thus, we can show, that the top basement under the Jura domain is dipping 1.7° to the SE, whereas under the Geneva Basin it is dipping between 2.7°-3.3° to the SE. The results of our modelling show a shortening of 23.6 km for the western Internal Jura FTB along a basal detachment and a forward stepping deformation accompanied by minor back-stepping thrust sequences. The first deformation is attributed to the thrusting of the Crêt de la Neige anticline followed by the Crêt Chalam thrust and its imbrications. Then, the Tacon thrust and finally the Bienne thrust nucleate. Imbricate fault-bend folding explains the high southern slopes of the anticlines found in this area. In addition to the primary décollement level situated at the base of the Keuper Group evaporites, three other detachment levels are found in marly layers. Using such a multiple thrust horizon approach avoids having to introduce thick unaccounted for evaporitic duplexes in the Keuper units, basement horst, or inverted Permo-Carboniferous grabens. The change in dip of the top basement located under the SE flank of the Crêt de la Neige anticline, at the transition of the Jura FTB to the Molasse Basin, is considered to be linked to a preexisting Paleozoic normal fault and could correspond to the northern edge of a suspected Permo-Carboniferous graben interpreted on seismic lines under the Geneva Basin. This step can be considered as an initiation point for structures developing in the detached cover.

How to cite: Marro, A., Sommaruga, A., Hauvette, L., Borderie, S., Schori, M., and Mosar, J.: Tectonics of the Western Jura Fold-and-Thrust Belt: from the Geneva Basin to the Bienne Valley (France). Mapping and forward modelling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4607, https://doi.org/10.5194/egusphere-egu22-4607, 2022.

16:32–16:33
questions

16:33–16:40
|
EGU22-4105
Jon Mosar et al.

The Jura Mountains in France and Switzerland are a classical thin-skinned fold-and-thrust belt (FTB), which developed as part of the Alpine orogenic foreland, together with the Western Alpine Molasse Basin. The Molasse Basin initiated as a flexural basin and evolved into a wedge-top Basin following the initiation of the main foreland décollement level. The Jura FTB thus forms the frontal portion of the Alpine foreland, which enjoyed a transport of some 30km towards the foreland along the main décollement in the mechanically weak Triassic salt-rich evaporites.

Overall the Jura FTB behaves as a mechanical wedge in hydrostatic conditions, that is propagating towards the Alpine foreland. Wedge-internal accommodations, due to changes in the surface topography and the basal décollement inclination, as well as in the basal friction, are operated by oscillating forward and backward stepping sequences of thrusting and related fold development. Basement topography associated with inherited faults leads to a kinematic preconditioning of the structures developing in the detached cover. Analogue modelling has helped show that oblique steps in the basement topography lead to the formation of normal and reverse faulting and oblique fold structures in the cover.

Herein we will discuss the link of different types of faults observed in the field, such as normal faults, inverted inherited faults, thrust faults and strike-slip faults, to major tectonic processes such as flexural bending, rifting, faulting due to steps in basement topography, and thrusting inside a mechanical wedge.

Works on relative chronology of faults, combined with new results from kinematic section modelling and data on published and new deformation ages from calcites (using U-Pb) make it possible to assess the timing of deformation. It is thus possible to show that thrust faults and strike-slip faults, as well as, normal and inverted faults were active at different times and witness superposed events. Deformation in the Jura FTB is partitioned and distributed along discrete faults that clearly operate in a forward and backward oscillating manner. We further can identify different structural domains that can be considered as distinct tectonic nappes. These domains are bound by major strike-slip faults (acting as inherited, rigid boundaries), progressive en-echelon relay zones and major thrusts. The present-day deformation involving both the detached cover and the mechanical basement will be discussed.

How to cite: Mosar, J., Schori, M., Borderie, S., Hauvette, L., Marro, A., Radaideh, O., Sommaruga, A., and Ursprung, A.: The Jura Fold-and-Thrust Belt: timing and kinematic of faulting and thrusting , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4105, https://doi.org/10.5194/egusphere-egu22-4105, 2022.

Thu, 26 May, 17:00–18:30

Chairpersons: Olivier Lacombe, Christoph von Hagke

17:00–17:01
questions

17:01–17:08
|
EGU22-7144
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ECS
|
On-site presentation
Etienne Large et al.

The pattern of active deformation of Himalayan frontal structures is complex with out-of-sequence reactivations in the chain and development of scarps associated to earthquake ruptures reaching the surface in the piedmont. We analyze passive seismic records using the Horizontal-to-Vertical Spectral Ratio method along three North-South trending profiles of the Darjeeling Himalayan piedmont, revealing subsurface structures down to 600 meters and imaging the Siwalik sedimentary rocks / recent deposits interface. We find evidence for a thrust fold system hidden beneath the plain correlated to geomorphologic scarps revealed by topographic profiles. These morphological surfaces are incised by large rivers of the piedmont by several tens of meters during phases of low sedimentation, thus slightly emerging in the plainThese scarps are supposedly induced by thrust deformation related to great earthquakes propagating south of the morphological front and inducing subsurface ruptures in the piedmont. This interpretation is comforted by the lateral correlation of the imaged thrusts and associated folds with previously evidenced fault scarps associated to active thrusts of the Darjeeling piedmont. In the piedmont of East/Central Nepal, oil company seismic profiles image similar thrusts and associated folds that can also be correlated to both local incision of small rivers draining the southern flank of the Siwalik hills, and uplift evidenced by a previously analyzed leveling profile.

The long-term kinematic evolution of this hidden thrust fold belt is slow, with a tectonic uplift of the hangingwall lower than the subsidence rate of the foreland basin, i.e., less than ~ half a millimeter per year. The evolution of the hidden structures corresponds to that of an embryonic thrust belt affected by a layer parallel shortening (LPS) acting in the long term with a shortening rate of the order of 5-10% of the shortening rate of the whole Himalayan thrust system. An aseismic deformation associated with the LPS structures that could absorb the entire deformation of the embryonic thrust belt in East/Central Nepal is suggested by the comparison of the long term structural evolution with geodetic and paleoseismological studies. The amplitude of this aseismic deformation is however too limited to significantly reduce the seismic hazard in the Himalayan piedmont.

How to cite: Large, E., Huyghe, P., Mugnier, J.-L., Jouanne, F., Guillier, B., and Chakraborty, T.: Development of a hidden fold and thrust belt in the Himalayan piedmont and distribution of active tectonics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7144, https://doi.org/10.5194/egusphere-egu22-7144, 2022.

17:08–17:09
questions

17:09–17:16
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EGU22-104
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ECS
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Virtual presentation
Arun Ojha et al.

Understanding how the geological architecture of the Himalaya has been constructed demands well-constrained tectonic models supported and validated by field observations. The channel flow model has been used to explain the structural architecture in many different sectors of the Himalaya. However, the model appeared to have failed in the classic example of the Himachal Himalaya, so in 2007 Webb proposed the Tectonic Wedge Model. But the key to this model is the existence of a regional-scale recumbent anticline, illustrated in many different papers and in textbooks. This structure is known variously as the Phojal fold, or the Sikhar nappe, or the Kalath anticline. Webb’s Tectonic Wedge Model requires the South Tibetan Detachment (STD) to also be recumbently folded, along with the Phojal fold. Our detailed field observations are contrary to the regional structures proposed in the existing models. First, it is evident that ‘photo geology’ has produced an optical illusion. Field mapping shows that the Phojal Fold is a north-east vergent reclined back fold. Thus, despite having been developed on the km-scale, the Phojal Fold has nothing to do with the formation of the earlier formed recumbent folds. There is no doubt that an early period of recumbent folding has produced regional-scale structures in the NW Himalaya. These folds post-date the first recognized period of Barrovian metamorphism. However, because the axial-plane cleavage of the recumbent folds is a pressure-solution cleavage, it can be inferred that these metamorphic rocks had cooled and been exhumed to shallow crustal levels prior to the start of the early recumbent folding event. The second period of Barrovian metamorphism was associated with the STD, which post-dates the earlier recumbent folds, but pre-dates back folding. The underpinning of the Tectonic Wedge Model has been removed. Hence the validity of the model is on trial.

How to cite: Ojha, A., Srivastava, D., and Lister, G.: Evidence of back folding in the Himachal Himalaya: A reassessment of the tectonic models in light of new evidence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-104, https://doi.org/10.5194/egusphere-egu22-104, 2022.

17:16–17:17
questions

17:17–17:24
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EGU22-5086
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ECS
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On-site presentation
Xinpeng Wang et al.

Western Sichuan Depression is located in the west of the Sichuan Basin and has shown great gas prospects for many years. Due to the characteristics of multiple stages of tectonic evolution and multiple directions of tectonic distribution, petroleum geological conditions are extremely complex in this area. In this paper, we use geological data, seismic data, well logging, and petroleum geological data to study the tectonic characteristics and controls on hydrocarbon accumulation in the middle section of the Western Sichuan Depression. The middle part of the Western Sichuan Depression is dominated by thrust structures, including thrust structures formed by the combination of thrust faults, fault triangular belt, and thrust imbricate structures, as well as fold deformation related to thrust faults, such as snake-head structure and slippage fold. The study area is characterized by east-west zonation and up-down stratification. In the process of formation and evolution, Western Sichuan Foreland Basin mainly experienced three tectonic periods, namely the Indosinian period, the Yanshan period, and the Himalayan period. Multi-stage tectonics, the change of force source, and principal stress direction also lead to the formation of tectonic series and tectonic belts with different trends. Most of the traps of the Leikoupo Formation in the region had their embryonic form in the late Indochinese period, which was further developed in the Yanshanian period, and basically formed in the early Himalayan period. Therefore, the tectonic conditions and accumulation conditions were well arranged, forming self-generation and self-accumulation or a combination of self-generation and self-accumulation and up-generation and sub-accumulation reservoir formation mode. The superior accumulation conditions and tectonic conditions make the Marine strata of the Leikoupo Formation of the Western Sichuan Depression show more favorable exploration potential. This study reveals structural style of the piedmont belt in the foreland basin and establishes reliable evidences for the further development of regional structural and accumulation models, which are crucial for further oil and gas explorations.

How to cite: Wang, X., Chen, S., Zhang, C., and Li, W.: Tectonic Characteristics and Controls on Hydrocarbon Accumulation in Middle Section of Western Sichuan Depression, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5086, https://doi.org/10.5194/egusphere-egu22-5086, 2022.

17:24–17:25
questions

17:25–17:32
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EGU22-9386
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ECS
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Virtual presentation
Francesca Stendardi et al.

The Epiligurian wedge-top basins of the Italian Northern Apennines fold and-thrust belt have been thoroughly investigated in the past from a sedimentological and paleogeographic perspective leading to the identification of several regional unconformities and the appreciation of their significance to track down the complex evolution of the accretionary wedge. Among these, a major Burdigalian unconformity has been recognised as a key regional element marking an abrupt shift from a deep marine (pre-Burdigalian) to platform (post-Burdigalian) environment during the progressive uplift of the accretionary wedge. We integrate these studies by providing a solid structural framework wherein to set this evolution. We investigated the pattern and the kinematics of the deformation structures deforming the Epiligurian Units both in the pre- and post-Burdigalian sequences exposed in the Emilia-Romagna Region of the Northern Apennines. Field investigations were integrated with the remote sensing of lineaments mapped at the regional scale to unravel the significance of the Burdigalian unconformity during the thickening and later dismantling of the Northern Apennines wedge. Fieldwork data document the existence of different structures and lineament trends affecting the pre- and post-Burdigalian sedimentary sequences. For example, the lowermost units of the pre-Burdigalian sequence are affected by top-to-the SE, NE-SW-striking reverse faults defined by planar slip surfaces associated with thin clastic damage zones. These reverse faults are cut across by scattered normal faults accommodating centimetric to decimetric throws and associated with clusters of disaggregation deformation bands. The post-Burdigalian succession, instead, is affected by more systematic trends of both reverse and normal faults. The reverse faults are oriented either NE-SW or WNW-ESE, with a general NW or NE tectonic transport, respectively. The crosscutting normal faults strike from NW-SE to NE-SW and are associated with extension-oriented NE-SW and NW-SE, respectively. Normal faults are locally decorated by calcite slickenfibres and syn-kinematic calcite veins, documenting structurally controlled circulation of mineralising paleofluids. All the structures affecting both the pre- and post-Burdigalian sequences are linked to a tectonic evolution encompassing syn-orogenic compression and post-orogenic extension, with the latter accompanied by local instabilities during overall thinning of the transiently supercritical wedge. To assess the significance of our results on a regional scale, a remote sensing analysis of tectonic and morphological lineaments was performed by systematically mapping lineaments within a study area of 200 km² at an observation scale varying from 1:50.000 to 1:5.000. Statistical analysis of open-access dataset focused on reverse and normal faults, confirming the significant lineament orientation variations indicated by field data. NE-SW striking normal and reverse faults define the pre-Burdigalian dataset, whereas NE-SW-striking normal faults and NW-SE-striking compressional structures define the post-Burdigalian dataset. Preliminary results from the combination of field and remote sensing made it possible to not only differentiate tectonic and morphological elements and to identify the preferential trend of deformation structures, but to also conclude that the polyphasic tectonic evolution of the Epiligurian Units during the NE-verging accretion of the Northern Apennines wedge accommodated significant changes in stress field orientation and faulting regime in the pre-and post-Burdigalian period.

 

 

How to cite: Stendardi, F., Ceccato, A., Vignaroli, G., and Viola, G.: Syn-to post-accretionary tectonic history of the wedge-top Epiligurian Units (Northern Apennines, Italy) as constrained by structural and remote sensing analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9386, https://doi.org/10.5194/egusphere-egu22-9386, 2022.

17:32–17:33
questions

17:33–17:40
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EGU22-11281
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On-site presentation
Hugo Ortner and Anna-Katharina Sieberer

Synorogenic sediments have often been used to constrain nappe movements and geodynamic processes. This contribution presents a case study from the Alps, in which synorogenic deposition (Lech-, Rossfeld-, Losenstein-, and Branderfleck Fms., Gosau Group) is affected by processes on different scales, that have led to a bewildering multitude of interpretations. We add another one.

Presently, the Northern Calcareous Alps (NCA) are a thin-skinned fold-and-thrust belt in the external part of the Austroalpine unit, which represents the upper plate during Cenozoic Alpine orogeny. However, orogeny started in the late Early Cretaceous, when large parts of the Austroalpine and the entire NCA were in a lower plate position. This major geodynamic change also controlled deposition of synorogenic sediments.

Prior to Cretaceous onset of subduction, the NCA were still in sedimentary contact with the underlying lithosphere. Most paleogeographic reconstructions show the NW edge of the Adriatic microplate at the transition from a passive margin in the SW to a transform-dominated margin in the N, as a consequence of Jurassic-Cretaceous opening of the Alpine Tethys. These transform faults apparently offset oceanic units dextrally to the east, however, they have sinistral kinematics, as a result of the northward propagating opening of the Atlantic Ocean.

At the turn from the Early to the Late Cretaceous, the NCA of the external Austroalpine had already been affected by major nappe movements in the foreland of an intracontinental subduction that had initiated along sinistral, roughly E-striking intracontinental transform faults within the Adriatic microplate (Stüwe and Schuster, 2010). Thrusting had propagated across the Adriatic plate to its northern transform boundary (Ortner and Kilian, 2021 in press).

As a consequence, oceanic crust in the N neighboured continental crust S of a transform zone. When shortening resumed in the early Late Cretaceous, continental lithosphere was subducted and replaced by oceanic lithosphere. Thus, the foreland thrust belt became an accretionary wedge. Its surface subsided to bathyal depth, as the surface of oceanic crust is isostatically in a depth of about 4.5 km below sea level, and the surface of continental crust is typically near sea level (e.g., Kearey et al., 2009).

Synorogenic sediments were deposited throughout shortening. They were affected by (i) ongoing contraction associated with tear faulting on the local scale, (ii) thickening of the orogenic wedge by emplacement of thrust sheets on the regional scale, and (iii) subsidence of the thin-skinned wedge controlled by replacement of continental by oceanic lithosphere. Such a multi-scale explanation may solve the long-disputed question of the tectonic setting of the Cretaceous synorogenic sediments of the NCA.

References

Kearey, P., Klepeis, K.A., Vine, F.J., 2009. Global tectonics (3rd ed.). Wiley-Blackwell, Oxford.

Ortner, H., Kilian, S., 2021 in press. Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of western Austria and southern Germany. Int. J. Earth. Sci. https://doi.org/10.1007/s00531-021-02128-3

Stüwe, K., Schuster, R., 2010. Initiation of subduction in the Alps: Continent or ocean? Geology, 38, 175-178. https://doi.org/10.1130/G30528.

How to cite: Ortner, H. and Sieberer, A.-K.: From foreland thrust belt to accretionary wedge: Synorogenic sediments monitor a changing geodynamic setting in the Northern Calcareous Alps of the European Eastern Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11281, https://doi.org/10.5194/egusphere-egu22-11281, 2022.

17:40–17:41
questions

17:41–17:48
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EGU22-3639
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ECS
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Virtual presentation
Edoardo Barbero et al.

The occurrence of topographic relief along the subducting plate is thought to play a significant role in controlling the architecture and the deformation processes of subduction complexes Seamounts and seamounts chain represent topographic reliefs of the seafloor whose ultimate fate is the interaction with subduction complexes at convergent plate boundaries. Geophysical data (e.g., von Huene & Lallemand, 1990) and numerical modeling (e.g., Ruh et al., 2016) demonstrate that subducting seamounts contribute to modify the frontal part of the subduction complexes controlling the morphology of the frontal wedge, the fore-arc subsidence, the deformation and stress pattern, the triggering of tectonic erosion, as well as the migration and localization of the basal décollement. Complementary data with respect to geophysics and numerical modeling dataset can be derived from structural investigations on seamounts accreted in ancient accretionary prism or within collisional belts.

Here, we present the results of a multiscale (from map- to micro-scale) structural study of the western Durkan Complex in the Makran Accretionary Prism (SE Iran) that has been recently interpreted as including fragments of Late Cretaceous seamounts with the aim to shed light on the mechanism of accretion of seamount materials and the factors controlling the localization and propagation of the basal décollement. The results from the Durkan Complex indicate a polyphase deformation history characterized by three main deformative phases (D1, D2, and D3), likely occurred from the Late Cretaceous to the Miocene-Pliocene (?). The D1 is characterized by sub-isoclinal to close folds associated to an axial plane foliation and shear zone, and likely represents the underplating of seamount fragments at shallow to intermediate levels of the Makran accretionary prism. The D1 shear zones are preferentially composed of volcaniclastic rocks derived from successions representing seamount slope and cap. The D2 deformation stage is characterized by open to close folds with sub-horizontal axial plane and likely developed during the progressive exhumation up to shallow structural levels of previously accreted seamount fragments. The D1 and D2 structures are unconformably sealed by a late Paleocene – Eocene siliciclastic succession that is, in turn, deformed by W-verging thrust faults typical of the D3 phase. This phase likely testifies for a Miocene (?) -Pliocene (?) tectonic rework of the accreted seamount fragments with the activation of out-of sequence thrusts.

In conclusion, our findings indicate that seamounts are deformed within subduction complexes during the underplating and subsequent exhumation at shallower structural levels. As a general rule, the stratigraphic architecture of the subducting seamount, in particular the occurrence of thick volcaniclastic successions, likely controls the position of the basal décollement of the prism during the underplating phase.

 

Ruh, J. B., Sallarès, V., Ranero, C. R., Gerya, T., 2016. Crustal deformation dynamics and stress evolution during seamount subduction: High-resolution 3-D numerical modeling. Journal of Geophysical Research: Solid Earth 121(9), 6880–6902. https://doi.org/10.1002/2016JB013250

von Huene, R., Lallemand, S., 1990. Tectonic erosion along the Japan and Peru convergent margins. Geological Society of America Bulletin 102 (6), 704-720.

How to cite: Barbero, E., Pandolfi, L., Delavari, M., Dolati, A., Di Rosa, M., Zaccarini, F., Saccani, E., and Marroni, M.: The deformation pattern of subducting seamount: insights from the structural evolution of the Late Cretaceous Durkan Complex in the North Makran domain (Makran Accretionary Prism, SE Iran), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3639, https://doi.org/10.5194/egusphere-egu22-3639, 2022.

17:48–17:49
questions

17:49–17:56
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EGU22-9529
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ECS
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Virtual presentation
Olivia Lozano Blanco et al.

The Taiwan thrust-and-fold belt results from the oblique arc-continent collision between the Eurasian Plate and the Luzon Volcanic Arc. In this context, inherited structures from the E-directed underthrusting Eurasian continental margin are being reactivated, causing uplift of crystalline basement rocks and the formation of transverse zones that influence the evolution of the structure, seismicity and topography of the Taiwan thrust-and-fold belt. The depth and geometry of the crystalline basement-cover interface of the continental margin are partly constrained by seismic velocities and the locations of earthquake hypocenters. However, further constraints are needed in order to obtain a better resolved location and geometry of this interface since this would improve the understanding of the deep structure of the thrust belt.

In this work, we investigate the geometry and position of first-order discontinuities of the underthrusting Eurasian continental margin to understand i) the role of the crystalline basement and the inherited structures in the deformation and, ii) the overall crustal geometry resulting from this collision. The approach we use is based on FFT and derivative based analyses, and 2D modelling of absolute gravity and magnetic anomalies. Techniques such as the calculation of the averaged power spectra have allowed us to infer the depth to the top of the most important discontinuities through the analysis of their gravity and magnetic wavelength signature. Results, obtained over different datasets show an upper interface at ~6 km depth and a lower at ~13 km depth. These are average values that we have better constrained through modelling, integration with other structural studies and comparison with tomography and seismic data. Results have helped us to improve the comprehension of the crustal structure of Taiwan and the Eurasian continental margin.

This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency of the Ministry of Science and Innovation of Spain. Olivia Lozano acknowledges funding from the same agency through contract PRE2019-091431.

How to cite: Lozano Blanco, O., Ayarza, P., Álvarez-Marrón, J., and Brown, D.: Depth to main crustal interfaces calculated through potential field data analysis and modelling: Implications for the study of inherited structures in Southern Taiwan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9529, https://doi.org/10.5194/egusphere-egu22-9529, 2022.

17:56–17:57
questions

17:57–18:04
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EGU22-4911
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ECS
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On-site presentation
Mateus Rodrigues de Vargas et al.

Taiwan orogen records a singular geological context where different stages of convergence are preserved. From south to north, the Taiwan region records the transition from oceanic and continental subduction of the SE China passive margin to its collision with the Luzon Magmatic Arc.

This study aims to define the thermal, compositional, and structural inheritance of the Chinese SE passive margin onto the processes of continental subduction and early collision. We combined geological and geophysical data (e.g., crustal thickness, seismicity, gravimetry, and magnetic anomaly maps) to propose a structural rift domain map of the SE China margin and its inversion. Open access seismic data is currently being interpreted to identify key tectonostratigraphic sequences, showing the crustal architecture and the tectonic-sedimentary evolution of the region. By building these offshore-onshore transects, we aim to capture the along-strike variations of the trench morphology. Initial results suggest a northward thickening of the accretionary prism in relation to the change from oceanic to continental subduction.

This work is part of an ongoing Ph.D. thesis in which the analysis of the results will not only focus on establishing new first-order tectonic models for the early collision but also on better constraining the control of former rifted margin on the locus of the deformation in this tectonically active zone.

How to cite: Rodrigues de Vargas, M., Mohn, G., and Tugend, J.: The role of the SE China passive margin for the formation of Taiwan orogen, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4911, https://doi.org/10.5194/egusphere-egu22-4911, 2022.

18:04–18:05
questions

18:05–18:12
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EGU22-6798
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Virtual presentation
Brian K. Horton and Andrés Folguera

Andean orogenesis is expressed in the diverse deformational records of crustal structures and sedimentary basins in western South America.  Here we summarize retroarc structural styles within the Andean orogenic belt and foreland basin system through consideration of regional contractional fault geometries, their kinematic interactions with other structures, and the comparative involvement of crystalline basement and sedimentary cover rocks.  In assessing the controls on structural style, we emphasize the importance of precursor conditions and employ the concept of tectonic inheritance to identify four factors that influence Andean deformation.  (1) Structural inheritance involves the reactivation of preexisting faults or basement fabrics and accompanying inversion of sedimentary basins.  (2) Stratigraphic inheritance is exemplified by the preferential localization of interconnected thin-skinned structures above regional décollements developed in wedge-shaped stratigraphic packages versus isolated basement-involved thick-skinned fault structures formed in provinces with limited cover strata.  (3) Rheological properties guide the activation of new structures by means of the integrated strength, rock and mineral composition, fluid content and pressure, and associated mechanical heterogeneities and anisotropies that define crustal and lithospheric architecture.  (4) Thermal structure in the form of initial thermal conditions and later thermal perturbations (such as cooling/heating episodes related to arc magmatism, subducting slab dynamics, or lithospheric removal) can promote inboard advance or outboard retreat of deformation.  Spatial and temporal variations in the relative importance of these four inherited attributes likely resulted in a complex evolution of structural styles during Andean shortening. 

The major styles include: (1) thin-skinned fold-thrust systems affecting principally cover strata with ramp-flat structures above gently dipping regional décollements that ultimately root in middle to upper crustal levels; (2) thick-skinned basement-involved block uplifts delineated by isolated high-angle reverse fault structures that penetrate deeply and may root in the lower crust; (3) pre-Andean (preorogenic) and (4) Andean (synorogenic) extensional basins that have been inverted by fault reactivation during later shortening; (5) upper-crustal backthrust belts linked to deeper foreland-directed structures; and (6) salt-involved contractional structures with weak décollement horizons that facilitate lateral flow of evaporite facies.  These structural styles are not mutually exclusive and may overlap in time and space.  We propose that evaluation of the contrasting roles of structural, stratigraphic, rheological, and thermal inheritance will help explain how numerous Andean structures do not bear simple relationships to the history of plate convergence, subduction, and magmatism along the western margin of South America.

How to cite: Horton, B. K. and Folguera, A.: Structural styles and tectonic inheritance in the Andean fold-thrust belt and foreland basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6798, https://doi.org/10.5194/egusphere-egu22-6798, 2022.

18:12–18:13
questions

18:13–18:20
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EGU22-3681
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ECS
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Virtual presentation
Gaia Siravo et al.

GPS data suggest that the NW South America corner forms a semi-rigid and distinct tectonic block (Northern Andean Block) drifting at 0.6 cm/yr NE-ward along regional dextral strike-slip faults that bound an oceanic terrane accreted in Late Cretaceous times to western Ecuador and Colombia. This is consistent with an average 0.76 cm/yr Quaternary slip rate obtained from field investigation along the main strike-slip faults. Nevertheless, pure thrust tectonics characterize the external (eastern) Northern Andes deformation front from Ecuador to Colombia. Thus, the relevance of strike-slip versus thrust tectonics during Cenozoic times and their relation with oceanic terrane accretion are unclear. 

The incertitude on the magnitude of a hypothetical Cenozoic strike-slip deformation is reflected by the variable interpretations of the tectonic regime that generated the Ecuadorian Interandean Valley. This tectonic depression, blanketing the eastern side of the Cordillera Occidental, has been variably considered as due to extensional, thrust, or strike-slip tectonics.

Paleomagnetism may represent an important tool to unravel the Cenozoic tectonic history of the Northern Andean Block, as peculiar patterns of vertical axis rotations arise from strike-slip and thrust tectonics.

Here we report on the paleomagnetism of 31 mid-upper Eocene to upper Miocene mainly volcanic sites from the Cordilleras Occidental and Real of southern Ecuador. Eleven sites show that the western Cordillera Occidental underwent a 24°±10° clockwise (CW) rotation with respect to South America after late Miocene, while no rotation occurred further east. We relate the regional CW rotation to the emplacement of the Cordillera Occidental nappe onto the continental sediments of the Interandean Valley. As rotation and continental sedimentation onset ages are similar, we interpret such tectonic depression as a narrow flexural basin formed ahead of the advancing nappe front.

Previous authors find a post-Cretaceous 28°±9° CW rotation of the Coastal forearc that is statistically indistinguishable from the 24°±10° Neogene CW rotation documented by us in the Cordillera Occidental and Interandean Valley, implying that the whole W Ecuador Andean chain was detached and rotated over a mid-crustal detachment during the last 10 Ma. Eocene-Miocene paleomagnetic inclination values are systematically consistent with those expected for South America, thus excluding latitudinal terrane drift. Our results show that thrust tectonics prevailed over strike-slip displacement in the southern Ecuadorian Andes during the late Cenozoic.

Finally, we note that the orogenic reentrant-salient sequence of the Nazca trench / Andean chain from northern Chile to Ecuador mimics closely the margin of the Archean–Paleoproterozoic Amazonian Craton and other minor cratons of South America. Considering our results on a continental scale and in combination with previous paleomagnetic data from the Andean belt we infer that the stiff crust of the Amazonian Craton behaved as a foreland indenter, hampered inland deformation propagation, and caused the formation of what we call the “Ecuadorian Orocline”, arisen by opposite-sign nappe rotations around the Craton apex.

How to cite: Siravo, G., Speranza, F., Mulas, M., and Costanzo Alvarez, V.: Significance of the northern Andean Block extrusion and genesis of the Interandean Valley: Paleomagnetic evidence from the “Ecuadorian Orocline”, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3681, https://doi.org/10.5194/egusphere-egu22-3681, 2022.

18:20–18:21
questions

18:21–18:28
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EGU22-13531
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Virtual presentation
Pierpaolo Guarnieri et al.

A new tectonic model is presented to explain the tectonostratigraphic evolution of the Paleoproterozoic Karrat Group in central West Greenland and the polyphase deformation, magmatism and metamorphism in the Rinkian orogen recorded in Paleoproterozoic rocks and Archaean complexes. The Karrat Group (from c. 71°00’ to 73°00’ N) formed in an intra-cratonic sag basin after c. 2000 Ma with basal quartzites of the Qaarsukassak and Mârmorilik formations unconformably overlaying Archaean gneisses of the Rae Craton. From 1950 to 1900 Ma a carbonate platform represented by the Mârmorilik Formation developed toward the south, while rift related alkaline volcanic rocks represented by the alkaline member of the Kangilleq Formation and syn-rift siliciclastic and volcaniclastic sediments of the Nûkavsak Formation were deposited to the north. The rifting was succeeded by a back-arc system, represented by the transitional member of the Kangilleq Formation. Concomitantly with development of the back-arc system, arc-related granitoids of the Prøven Intrusive Complex (PIC) intruded into and along the basal contact of the Karrat Group around 1900 Ma with major pulses at c. 1870 and c. 1850 Ma. The Karrat Group and the magmatic arc rocks underwent HT-metamorphism at c. 1830–1800 Ma during the collisional phase of the Rinkian orogen. The metamorphic grade increases from greenschist facies in the south, to granulite facies in the north, where the metamorphism is associated with migmatization and emplacement of the S-type Qinngua leucogranites. Extensive thrust emplacement and folding characterize the Rinkian orogen south of the PIC and the eastern boundary of the magmatic arc is reworked along a top to ESE shear zone post-dating the HT-metamorphism. The ESE-ward emplacement of allochthonous thrust sheets during an early stage of thin-skinned tectonics is followed by NE-ward emplacement of basement nappes and finally by a NW-SE compression stage resulting in tectonic inversion of basin normal faults.  The back-arc extension and Cordilleran-type magmatism were driven by eastward subduction of oceanic crust during the Trans-Hudson Orogeny resulting from the convergence of the Superior, Meta Incognita and Rae Archean cratons between 1870–1800 Ma. The Karrat Group north of the PIC together with the time-correlative Piling Group of Baffin Island (Canada) probably represented the passive margin succession of the Rae craton that evolved into a forearc setting during the Trans-Hudson Orogeny. The Rinkian orogen is an example of Cordilleran-type tectonics resulting from the deformation of the Rae continental margin intruded by magmatic arc granites during subduction, followed by HT-metamorphism in the upper plate and the structuring of a back-arc fold and thrust system antithetic to the subducting plate.

How to cite: Guarnieri, P., Rosa, D., Thrane, K., Kokfelt, T. F., Sørensen, E. V., and Baker, N.: Paleoproterozoic Cordilleran-Type Tectonics in central West Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13531, https://doi.org/10.5194/egusphere-egu22-13531, 2022.

18:28–18:29
questions