Fascinating feedback relationships between surface processes and tectonic deformation have long been highlighted for convergent settings. Mountains influence local climate, with precipitation increasing with mountain height and focusing at windward-facing slopes. The resulting erosion reduces the elevation and width of mountain belts, in turn leading to a focussing of tectonic deformation and exhumation at eroding regions. Thus, in convergent settings, erosion and tectonic deformation show positive feedback by enhancing each other. In comparison, the role of surface processes in extensional settings has received less attention, which does not mean that erosion or sedimentation might not equally affect tectonics deformation during extension. In this presentation, i will review theoretical expectations, discuss numerical experiments, and pose questions on how, when, and where surface processes interplay with tectonic deformation during extension.

How: The removal of material by erosion is expected to decrease vertical crustal stress and reduce brittle strength (which is the main process leading to focussing of deformation in shortening). Sedimentation conversely increases brittle strength. However, sediments of low thermal conductivity in extensional basins can trap heat, increasing crustal temperatures, and reducing viscous crustal strength. Will brittle strengthening or viscous weakening dominate during sedimentation? And during rifting, is erosion the controlling surface process, or sedimentation, or both?

When: Usually, subsidence needs to create accommodation space before sedimentation occurs and rocks should uplift before they can be eroded. This would imply that surface processes need time to start up and cannot play a decisive role in initial stages of deformation. This then begs the question: once an extensional system starts to deform in a certain style, can surface processes still change the style? For rift basins, we find from numerical experiments that sedimentation favours symmetric basins over asymmetric half-graben and single basins over distributed deformation. For rifted margins, i have found that sedimentation promotes hyperextension by forming wide areas of thinned continental crust, thus supressing early break-up. These experiments point out that surface processes seem to be able to exert a control on the style of rifting. But at which stage in rift evolution do surface processes start to play a role? And is there a crucial timing, after which erosion and sedimentation no longer influence the extensional style?

Where: Analogous to convergent tectonic settings, erosion of rift footwalls can enhance tectonic deformation and, on a large-scale, turn a ‘passive’ margin ‘active’ in a tectonic sense. Footwall uplift provides a sediment source region, linking erosion to offshore sedimentation. For rifted margins, where does deposition of sediments (whether they are brittle strengthening or viscous weakening) play the most influential role in the rifting process? Can strong near-footwall sedimentation suppress footwall uplift, thus providing a negative feedback in the system?

How to cite: Buiter, S.: A discussion on how, when and where surface processes interplay with extensional tectonic deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8665, https://doi.org/10.5194/egusphere-egu21-8665, 2021.

Models that couple tectonics and surface processes commonly predict that efficient erosion and sedimentation help focus crustal deformation onto fewer, longer-lived faults. However, because their geomorphic parameters are difficult to calibrate against real landscapes, the sensitivity of tectonic deformation to a realistic range of surface process efficiencies remains poorly known. Here we model the growth of structurally simple half-graben structures subjected to fluvial incision of specified efficiency and sedimentation. Numerical simulations predict that infinitely-efficient erosion and deposition (i.e., complete surface leveling) can more than double the maximum offset reached on a master normal fault before crustal strain localizes elsewhere. Further, leveling footwall relief tends to promote the migration of strain towards the hanging wall to form new grabens instead of horsts. 

         To test whether the efficiency of river incision can vary sufficiently across real rifts to exert a control on tectonic styles, we analyze the profiles of rivers draining half-graben footwalls and horst blocks in the Basin & Range, Taupo, Rio Grande, and East African Rift. We adapt the standard methodology of equilibrium river profile analysis to account for spatial variations in uplift expected from crustal flexure in a fault-bounded block. Erosional efficiency (EE) is defined as the inverse of the (dimensionless) slope of uplift- and drainage area-corrected river elevation plots.  Measured EEs range between ~0.1 and ~4, reflecting natural variability in lithology, climate, and uplift rates across sites. Incorporating EEs within this documented range in numerical simulations, we find that increasing EE can increase the maximum throw on half-graben master faults by ~50%. Changing EE also affects the geometry of subsequent faults, with lower EEs favoring the transition from half-graben to horsts. These models predict that rifting in a colder, stronger continental crust is less sensitive to surface processes and requires even lower EE to develop horst structures. Our simulations are consistent with a compilation of EE, crustal strength proxies, and fault characteristics across real rift zones. These results suggest that natural variability in climatic conditions and surface erodibility has a measurable impact on the tectonic makeup of Earth's plate boundaries.

How to cite: Olive, J.-A., Malatesta, L., Behn, M., and Buck, R.: Measurable impact of river incision on rift tectonics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5193, https://doi.org/10.5194/egusphere-egu21-5193, 2021.

Normal faulting acts as a forcing on the morphodynamics of alluvial rivers by changing the topographic gradient of the river valley and channel around the fault zone. Normal faulting affects river morphodynamics either instantaneously by surface rupturing earthquakes, or gradually by continuous vertical displacement. The morphodynamic responses to normal faulting range from longitudinal bed profile adjustments to channel pattern changes. However, the effect of faulting on river morphodynamics and morphology is complex, as they also depend on numerous local, non-tectonic characteristics of flow, river bed/bank composition and vegetation cover. Moreover, river response to faulting is often transient. Such time-dependent river response is important to consider when deriving relationships between faulting and river dynamics from a morphological and sedimentological record. To enhance our understanding of river response to tectonic faulting, we used the physics-based, two-dimensional morphodynamic model Nays2D to simulate the responses of a laboratory-scale alluvial river to various faulting and offset scenarios. Our model focusses on the morphodynamic responses at the scale of multiple meander bends around a normal fault zone. Channel sinuosity increases as the downstream meander bend expands as a result of the faulting-enhanced valley gradient, after which a chute cutoff reduces channel sinuosity to a new dynamic equilibrium that is generally higher than the pre-faulting sinuosity. Relative uplift of the downstream part of the river due to a fault leads to reduced fluvial activity upstream, caused by a backwater effect. The position along a meander bend at which faulting occurs has a profound influence on channel sinuosity; fault locations that enhance flow velocities over the point bar result in a faster sinuosity increase and subsequent chute cutoff than locations that cause increased flow velocity directed towards the outer floodplain. Our study shows that inclusion of process-based reasoning in the interpretation of geomorphological and sedimentological observations of fluvial response to faulting improves our understanding of the natural processes involved and, therefore, contributes to better prediction of faulting effects on river morphodynamics.

How to cite: Woolderink, H., Weisscher, S., Kleinhans, M., Kasse, C., and Van Balen, R.: Modelling the effects of normal faulting on alluvial river morphodynamics., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1108, https://doi.org/10.5194/egusphere-egu21-1108, 2021.

Sedimentary breccias formed during extensional tectonics are spatially associated with large-throw normal faults. They result from the creation of a steep topography that becomes unstable, producing major rockfalls. The studied breccias, in Crete and in the Pyrenees, are up to 300 meters thick and are characterized by poorly sorted polygenic deposits of pebbles to boulders composed of highly angular plurimillimetric to plurimetric carbonate clasts. A lateral evolution is observed, with pebble-size clasts found near the normal fault and boulder-size clasts away from the fault. This evolution is related to the rockfall process as the total kinetic energy acquired by the small clasts during the fall is lower than that acquired by the bigger ones; as a result, the latter are able to travel farther. Interestingly, the fact that the smallest clasts are proximal while the bigger ones are more distal is contrary to the distribution found in alluvial fan systems, making it possible to differentiate from one another. The studied breccias commonly show disorganized layers and/or no noticeable layering across large distances. We interpret this feature as related to the movement on the normal fault, which progressively tilts the breccia layers and favours their gliding along the slope. Gliding is an important internal process to take into account in rockfall systems because it may disorganize the layering, create specific geometries like onlap around olistoliths, and produce deformation inside the breccia layers; the latter feature could be mistakenly interpreted as resulting from post-deposition regional deformation.

According to our observations, active normal faults with large throws provide the conditions for the formation and preservation of great volumes of sedimentary breccias through the following processes: i) footwall uplift, creating a pronounced topography with steep slopes, giving rise to major rockfalls, ii) hangingwall rapid subsidence, which allows the accumulation and preservation of the breccias without clast reworking by drainage systems. The latter is reinforced by the fact that, during the early stages of extension, the main watersheds point in a direction opposite to the fault slope whereas only small, discontinuously distributed watersheds flow in the direction of the fault slope. Upon ongoing extension, the size of these small watersheds increase. At one point, the sedimentary flow coming from these watersheds becomes more important than rockfall processes. Part of the breccia body is then eroded, reworked, and replaced by conglomerates of an alluvial fan deposited unconformably above the breccias.

Summing up, sedimentary breccias are readily formed as thick syn-tectonic deposits during early stages of extensional basin development. Thus, they may be considered as a typical lithology, and a marker, of continental extension.

How to cite: Kernif, T., Nalpas, T., Bourquin, S., Gautier, P., and Poujol, M.: Sedimentary breccias formed during extensional tectonics: facies organization and processes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5681, https://doi.org/10.5194/egusphere-egu21-5681, 2021.

Dilatant fissures are a common feature at the Earth’s surface in active rift systems where faults cut mechanically-strong rocks, such as igneous rocks, metamorphic basement or carbonates. Much attention has focused on modern examples of large-aperture fissures in basaltic rocks, where in most cases, only the near-surface-expression is accessible to depths of ~100 m. Numerous mechanisms have been proposed for the formation of such dilatant fractures, including near-surface tensile fracturing along active normal faults at depth, geometric mismatch along faults, and fault-block rotation. However, fissure system architecture and connectivity in the subsurface, and the depth to which dilatant sections can grow are less well understood, as are the ways in which such structures may interact with surface processes.

In this presentation, we focus on dilatant faults and fractures from the ancient rock record, including examples hosted in rocks below regional erosional unconformities, commonly on the upfaulted flanks of nearby sedimentary basins. Such fissures are typically sub-vertical Mode I fractures that can be kilometres long, tens of metres wide and can extend to depths of 1 km or more below the palaeosurface. They are filled with a remarkably diverse range of high porosity, high permeability fills which act as natural proppants holding fractures open for tens to hundreds of million years. Fills include: wall rock collapse breccias; clastic or carbonate sediment; fossiliferous materials, and a variety of epithermal mineral deposits with characteristically vuggy forms and cockade-like textures. Alteration related to weathering and/or near-surface epithermal mineralization may extend down fissure systems to depths of many hundreds of metres. The subterranean clastic fills are commonly water-lain and preserve a unique record of the stratigraphic or fossil record that may be missing due to erosion at the overlying unconformity. Fissures can form along active normal faults at depth, as later-stage reactivations of pre-existing exhumed fault zones and along regional joint sets associated with folds. Some fissures form along the margins or interior of pre-existing mafic dykes or may act as sites of subsequent dyke emplacement – or both. Sub-unconformity fissure systems and their associated fills are likely to be a major influence on both the fluid storage capacity and flow behaviour in subsurface reservoirs including those hosting hydrocarbons, geothermal resources, and in aquifers worldwide.

How to cite: Holdsworth, B., Hardman, K., Walker, R., Bubeck, A., Greenfield, C., Lee, J., McCaffrey, K., and Dempsey, E.: Geological fissures: linking sub-surface structures to surface processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-96, https://doi.org/10.5194/egusphere-egu21-96, 2020.

Here we present new results and findings from an analogue modelling series using an extension gradient to simulate continental rifting in rotational settings. We study the effect of a pressure-gradient driven, rift-axis parallel lower crustal flow on rift propagation. The dynamically scaled two-layer models represent a brittle upper and a ductile lower crust. To simulate different crustal set-ups, we use variable ductile/brittle ratios R_DB, where increasing values indicate a hotter crust with the brittle-ductile transition at relatively shallower depth. An additional package of sand on one part of the model simulates tectonic loading to provoke a pressure-gradient driven lower crustal flow.

Several factors play a role in dynamic rift propagation such as extension rates, fault evolution and the interplay of vertical motions at the surface as well as model-internal rift-axis parallel horizontal flow. We combine surface and internal deformation analysis using stereoscopic Digital Image Correlation and Digital Volume Correlation applied on surface stereo images and XRCT images, respectively to obtain the fully quantified model deformation.

Our results show that rift propagation occurs in two consecutive stages: (i) bidirectional step-wise growth in fault length by linkage and (ii) unidirectional linear fault growth. Strain partitioning of bulk extension causes episodic alternative fault growth on conjugate rift margin faults. Over time, fault activity abandons rift boundary faults and migrates inward creating intra-rift faults. This process occurs segment-wise along the rift axis, where different fault generations are simultaneously active. We quantify increasing lower crustal flow parallel to the rift axis with increasing R_DB as the result of tectonic loading. In return, such lower crustal flow causes vertical and horizontal motions at the surface expressed by dynamic topography and deformation features.

These results give insights into deformation processes of rifting and highlight the important role of extension gradients on fault growth and strain partitioning in segmented rotational rift systems. Rift-axis parallel lower crustal flow in rotational rift settings may be of relevance when dealing with restorations of 2D crustal seismic sections across rifts.

How to cite: Schmid, T., Schreurs, G., Adam, J., and Hollis, D.: Interplay of near surface rift evolution and deep-seated lower crustal flow: New findings from fully quantified crustal-scale analogue models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1027, https://doi.org/10.5194/egusphere-egu21-1027, 2021.

Mantle plume-lithosphere interactions modulated by surface processes across extensional tectonic settings give rise to outstanding topographies and sedimentary basins. However, the nature of these interactions and the mechanisms through which they control the evolution of continental rifts are still elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely proportional to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple model is validated against petro-thermo-mechanical numerical experiments where a rheologically stratified lithosphere above a mantle plume is subject to fluvial erosion proportional to stream power during extension. These findings are consistent with Eocene mantle upwelling and flood basalts in Ethiopia synchronous with distal initiation of lithospheric stretching in the Red Sea and Gulf of Aden as well as asymmetric topography and slip along extensional structures where orography sets an erosional gradient in the Main Ethiopian Rift (MER). I conclude that, although inherently related to the lithosphere rheology, the evolution of continental rifts is even more seriously conditioned by the mantle and surface dynamics than previously thoughts.

How to cite: Sternai, P.: Mantle flow and orography: the effect of basal lithospheric shearing and lateral erosion gradients on continental rifting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1726, https://doi.org/10.5194/egusphere-egu21-1726, 2021.

The Danakil depression in the northern part of the Afar is the only modern example of a rift undergoing the active transition from continental to marine settings, a crucial stage in rift and passive margin development. Thick evaporite deposits in its central part, and fringing Pleistocene coralgal reef terraces along its margins evidence at least four Red Sea incursions in to the basin and subsequent desiccation. The two youngest coralgal reef terraces were dated as respectively MIS 5e and MIS 7. Recent field expeditions measuring the paleo-shorelines' elevation provide a precious record of neotectonic activity in the basin. The margins show varied uplift while outcrops situated closer to the rift axis subsided below sea level. MIS 7 sediments at the northern, western margin, were uplifted up to 170 masl. Neotectonic movements are smaller on the eastern margin of the Danakil depression but moderate uplift was sufficient to avoid flooding of the depression during the Holocene. Syn-rift sedimentary patterns in the Danakil basin illustrate that the transition from continental to marine conditions is not gradual but marked by alternating marine and continental episodes. This alternation is controlled by the interaction between eustatic, tectonic and volcanic processes. Significant increase in accommodation space and sediment deposition can happen at very short time intervals.

How to cite: Rime, V., Foubert, A., Perrochet, L., Jaramillo-Vogel, D., Negga, H., Atnafu, B., and Kidane, T.: Tectonic controls on the sedimentation patterns in the Danakil Depression, Afar, Ethiopia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6135, https://doi.org/10.5194/egusphere-egu21-6135, 2021.

The East African Rift Systems (EARS) is a modern example of a divergent plate boundary at early stages of development. In Tanzania, the rift has evolved in two branches since the Early Miocene. In addition, recent studies have proposed the existence of a marine branch of the rift in the western Indian Ocean, corresponding to the Kerimbas Graben – Davie Ridge (DR) system offshore northern Mozambique and southern Tanzania. North of this region, putative passive margin structures are present: the islands of Zanzibar and Pemba, and the troughs that separate them from the mainland. Although different theories for their formation have been proposed, a clear understanding of how the islands relate to the regional tectonic regime and the effect on the deep-water sediment routing system is lacking. 

In this study, we use 2D seismic reflection profiles and exploration wells to investigate the Oligocene to recent stratigraphy offshore northern Tanzania to examine the following two questions: When did the Pemba and Zanzibar islands form? And how does the evolution of deep-water depositional systems record rift tectonics? Regional correlation of dated seismic horizons, integrated with 3D reconstruction of canyons/channels network through time, allow understanding of the main depositional events and their timing. A net decrease in the number of slope channels is visible offshore Pemba during the middle-late Miocene, which we interpreted to mark the onset of the uplift of the island. At the same time, deep-water channels were still aggrading offshore Zanzibar, indicating that the uplift of this island occurred later, likely during the late Miocene to early Pliocene. The uplift of the islands promoted the formation of a newly discovered giant canyon, characterized by a modern width of > 30 km and depth of > 485 m at > 2,200 m water depth.

The timing of the islands’ uplift indicates a potential relation with the EARS tectonics. While the structures which form the anticlines of Pemba and Zanzibar Islands may be related to Tertiary (EARS) inversion of Mesozoic-aged rift faults,  numerous high-angle normal faults, both antithetic and synthetic, dissect the post-Oligocene stratigraphy. These create horsts and grabens on a variety of scales, some of which (e.g. Kerimbas Graben and Zanzibar/Pemba trough) show comparative shape and size respect to onshore rift basins. The stratigraphic evolution of deep-water channel systems provides a tape-recorder with which to determine the modification of EARS’ tectonics on sedimentation of the older Tanzania margin.

Supported by these new results, we propose a new alternative conceptual model for the evolution of the central East African margin during the Neogene and Quaternary, highlighting the main tectonic structures and their timing of formation.

How to cite: Dottore Stagna, M., Maselli, V., Grujic, D., Reynolds, P., Reynolds, D., Iacopini, D., Richards, B., Underhill, J., and Kroon, D.: Effects of the tectonics of the East African Rift System on the evolution of the Tanzania margin offshore Zanzibar and Pemba Islands., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2456, https://doi.org/10.5194/egusphere-egu21-2456, 2021.

Little is still known about the structural fabric of a potential continuation of the East African Rift System (EARS) offshore Tanzania in the West Somali Basin. This continuation has been established mostly through sparse GPS measurements, earthquake slip vector data, spatial distribution of teleseismically detected earthquake focal mechanisms, and some recent seismic reflection data. West of the Davie Ridge (which part of a larger structure named the Davie Fracture Zone) and across its northern extension, regional seismic reflection profiles indicate the occurrence of continental - oceanic crust transition, which is characterized by early Cretaceous reverse faulting localized along deformation corridors. After the Aptian, the seafloor spreading ceased and the Tanzania margin evolved into a passive margin dominated by clastic deep-water deposition. In this contribution, we describe some results obtained from structural mapping of a 3D seismic dataset, calibrated by few explorations well, covering an area located between the Davie Ridge and the continent, south of Mafia Island.  The seismic data maps suggest a major structural style change across the Neogene that is still active today. The recent structures are represented by two main interacting fault trends: some NS boundary faults corridors and a NW-SE internal arcuate segmented fault, both depicting a widely and diffused distribution of normal fault (with an overall cumulative amount of horizontal brittle extension ranging between 5 to 10 km). Some of the largest faults appear to reactivate older extensional structures but the general absence of growth faults cutting across the Paleo-Neogene depositional units suggest very recent rift re-activation. The recent rift system appears to show a component of obliquity with respect to the orientation of the Davie Ridge, and to the onshore structure related to the EARS tectonics.

How to cite: Ruocco, S., Iacopini, D., Tavani, S., Ebinger, C., Dottore Stagna, M., Reynolds, D., and Maselli, V.: Neogene rift tectonic activity in the West Somali Basin, offshore Tanzania: example of a segmented oblique rift structure., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9066, https://doi.org/10.5194/egusphere-egu21-9066, 2021.

Formation of the Phanerozoic basins of Madagascar coincided with the initial stages of Permian break-up of Gondwana. The sources of the sediments in these basins are from the seven major Precambrian terranes that border them. The tectonic history of rifting, drifting, and uplift of the Madagascan terrane over the last 300 million years is recorded within the sedimentary strata that comprise the Morondava Basin, the largest of these basins located along the west coast. In this study, we have applied detrital zircon U-Pb geochronology and (U-Th)/He low-temperature thermochronology to resolve the sedimentation patterns and thermal history of the Morondava Basin as Madagascar separated from Africa and subsequently India. Nine coarse-grained siliciclastic samples were taken along two transects parallel to the Morondava River in the central Morondava Basin. Karoo sandstones and shales were deposited directly atop the basement during Permo-Triassic rifting. Two samples from each transect were taken in the uppermost Jurassic Karoo sandstones. Overlying the Karoo are carbonates that were deposited as part of a carbonate platform as the basin experienced Middle Jurassic subsidence due to successful rifting during the separation of Madagascar and Africa. A Late Jurassic unconformity suggests tectonic quiescence. As the passive margin subsidence renewed, changes in eustatic sea level resulted in several cycles of sedimentation, and two Cretaceous samples in each transect were collected from this interval. Separation of India from Madagascar during the Turonian resulted in uplift of the central highlands and tilting of the Morondava Basin accompanied by extensive volcanic activity throughout the basin. Previously published apatite fission track studies mark this as the final stage of cooling. Above a Paleocene unconformity, deposition occurred in the Eocene with a package of sandstones and shales represented by a single sample in the southern transect. The detrital zircon U-Pb age distributions include common Neoarchean and Neoproterozoic populations which suggests input from the basement terranes of the Madagascan central highlands (Antananarivo domain). A subset of samples contain a Paleo- to Mesoarchean population linked to the metasedimentary Anosyen domain and a Cambrian population associated with metamorphic zircon formed during the Pan-African Orogeny the source of which occurs in the southwestern basement terranes. Spatial variations within the detrital zircon U-Pb age populations indicate two distinct sedimentation patterns separating the north and south parts of the basin and a likely post-Jurassic sediment recycling history within the Morondava Basin. Initial zircon (U-Th)/He ages range from 500 to 80 Ma with effective uranium (eU) values ranging from 35 to 1760, which exhibit a strong negative eU-age relationship and indicate partial resetting of zircon throughout the basin. The combined data will be utilized to construct the low-temperature thermal history of the basin.

How to cite: Schetselaar, W., Schneider, D., Tari, G., Raharisolofo, H., Rahajarivelo, S., and Ramboasalama, F.: Detrital zircon U-Pb and (U-Th)/He geochronology of the central Morondava Basin, Madagascar, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5698, https://doi.org/10.5194/egusphere-egu21-5698, 2021.

The Congo basin (CB), considered as a typical intracratonic basin, due its slow and long-lived subsidence history and the largely unknown formation mechanisms, occupies a large part of the Congo craton, derived from the amalgamation of different cratonic pieces. It recorded the history of deposition of up to one billion years of sediments, one of the longest geological records on Earth above a metamorphic basement. The CB initiated very probably as a failed rift in late Mesoproterozoic and evolved during the Neoproterozoic and Phanerozoic under the influence of far-field compressional tectonic events, global climate fluctuation between icehouse and greenhouse conditions and drifting of Central Africa through the South Pole then towards its present-day equatorial position. Since Cretaceous, the CB has been subjected to an intraplate compressional setting due to ridge-push forces related to the spreading of the South Atlantic Ocean, where most of sediments are being eroded and accumulated only in the center of the basin.

In this study, we first reconstructed the stratigraphy, the depths of the main seismic horizons, and the tectonic history of the CB, using geological and exploration geophysical data. In particular, we interpreted about 2600 km of seismic reflection profiles and well log data located inside the central area of the CB (Cuvette Centrale). We used the obtained results to constrain the gravity field data that we analyzed, in order to reconstruct the depth of the basement and investigate the shallow crustal structure of the basin. To this purpose, we used a gravity inversion method with two different density contrasts between the surface sediments and crystalline rocks.

The results evidence NW-SE trending structures, also revealed by magnetic and seismic data, corresponding to the alternation of highs and sediments filled topographic depressions, related to rift structures, characterizing the first stage of evolution of the CB. They also show a general good consistency between the seismic and gravity basement along the seismic profiles and evidence the presence of possible high-density bodies in the shallow to deep crust. The identified structures are prevalently the product of an extensional tectonics, which likely acted in more than one direction.

Therefore, we performed 3D numerical simulations to test the hypothesis of the formation of the CB as multi-extensional rift in a cratonic area, using the thermomechanical I3ELVIS code, based on a combination of a finite difference method applied on a uniformly spaced Eulerian staggered grid with the marker-in-cell technique. To this purpose, the numerical tests have been conducted considering a sub-circular weak zone in the central part of the cratonic lithosphere and applying a velocity of 2.5 cm/yr in two orthogonal directions (N-S and E-W). We repeated these numerical tests by increasing the size of the weak zone and varying its lithospheric thickness. The results show the formation of a circular basin in the central part of the cratonic lithosphere, characterized by a series of highs and depressions, consistent with those obtained from geophysical/geological reconstructions.

How to cite: Maddaloni, F., Delvaux, D., Tesauro, M., Gerya, T., and Braitenberg, C.: The Congo basin: an example of failed rift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5950, https://doi.org/10.5194/egusphere-egu21-5950, 2021.

Tectonic controls on landscape evolution are well documented globally. In actively extending areas, tectonic geomorphology is typically represented by uplifted footwalls, downthrown hanging walls, distinct bounding escarpments, and characteristic drainage patterns.

In onshore parts of the NE Atlantic margin, several studies suggest that some present-day landforms are inherited from rifting and margin formation in the Late Paleozoic, Mesozoic and Early Cenozoic. Such inheritance can be difficult to recognize because much of the pre-existing landscapes are obscured by erosional features related to post-rift Cenozoic uplift and repeated glaciations during the Quaternary. Interpretations of these landscapes vary considerably; some have suggested the preservation of vast Mesozoic erosion surfaces, whereas others argue that most present-day landforms are Quaternary in origin with little pre-Quaternary inheritance. However, some remnants of Late Paleozoic and Mesozoic rifting are demonstrably preserved directly inboard of the NE Atlantic margin, in the form of sedimentary basins.

In this study we use structural and geomorphological field observations and DTM (Digital Terrain Model) analyses to investigate the landscape surrounding three half-graben basins. Detailed landscape classification and analysis is used to systematically review present-day landscape distribution and bounding faults in and around the remnant basins, in order to distinguish extensional tectonic landforms from other geomorphological features. The half-grabens considered in this study are the Carboniferous Billefjorden half-graben on central Spitsbergen, Svalbard; the Jurassic Sortlandsundet half-graben in Vesterålen, northern Norway; and the Jurassic Beitstadfjorden half-graben in Trøndelag, mainland Norway.

Preliminary results reveal major topographic contrasts between footwall and hanging wall in all three half-grabens, with generally higher topographical elevations and deeper incision in the footwalls compared to the hanging walls. Additionally, the three study areas have very distinct landscape signatures, suggesting a difference in the post-rift landscape evolution. These differences appear to be dependent on a number of factors related to unique post-rift events. The inherited half-grabens display profoundly different degrees of erosional exploitation of pre-rift structures, glacial incision, and possible late-Cretaceous or younger reactivation of the basin-bounding normal faults. This study will provide insight into the relationships between inherited, tectonically controlled landforms, and incising Cenozoic and Quaternary landforms.

How to cite: Haaland, L. C., Osmundsen, P. T., Redfield, T., Svendby, K., and Senger, K.: Investigating tectonic geomorphology of three half-graben basins onshore the NE Atlantic margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1498, https://doi.org/10.5194/egusphere-egu21-1498, 2021.

The Mentelle Basin is a large and deep-water sedimentary basin located on the southwest Australian rifted margin. The basin lies west of the Perth Basin, east of the Naturaliste Plateau and south of the Perth Abyssal Plain. The rifted margin formed when the Greater Indian plate separated from the Australian-Antarctic plate during the Jurassic to early Cretaceous. Based on seismic reflection data, several km thick sediments infilling the basin have been interpreted. However, due to lack of geological and geophysical data, the basin has not been studied enough to understand its evolution. In 2017, International Ocean Discovery Program (IODP) Expedition 369 drilled four sites, U1513–U1516, in the Mentelle Basin and recovered important cores including late Jurassic to Early Cretaceous sections. At Site U1515 on the eastern margin of the basin, drilling penetrated below the seismically imaged breakup unconformity into the middle Jurassic to earliest Cretaceous syn-rift strata. Holes at Site U1513 on the western margin cored the syn-rift volcanic sequence, the Hauterivian to early Aptian volcaniclastic-rich sandstone sequence spanning the syn- to post-rift phase, and the Aptian to Albian post-rift claystone sequence. Drilling at Sites U1514 and U1516 in the central part reached the Albian post-rift sequence. Using a combination of shipboard and post-expedition data, we interpret the lithological, paleontological and geochemical characteristics of the syn- to post-rift sequences. The results allowed us to reconstruct the Early Cretaceous stratigraphy, tectonics, paleo-environment, and basin evolution of the Mentelle Basin. During the syn-rift phase, the middle Jurassic to lower Cretaceous non-marine sediments were deposited in the eastern Mentelle Basin, while volcanic rocks were emplaced in the western part. The 82 m thick volcanic sequence consists of alternating basalt flows and volcaniclastics with dolerite dikes, which indicate multiple volcanic eruption events in subaerial to shallow water environments. It was overlain by the 235 m thick volcaniclastic-rich sequence consisting of massive or laminated sandstone layers, deposited in shelf to upper bathyal depths. The deposition period spans the syn- to post-rift phase of the basin but decreasing sedimentation rate and shallow marine setting suggest that the post-rift thermal subsidence did not immediately follow the final continental breakup. We interpret that the delayed thermal subsidence was likely to be induced by adjacent mantle plume activities. Deep marine claystone sequences blanketing most of the basin indicate Aptian to Albian post-rift thermal subsidence.

How to cite: Lee, E. Y., Wolfgring, E., Tejada, M. L. G., Chun, S. S., Yi, S., Schentger, B., Brumsack, H.-J., Riquier, L., and Meszar, M.: Early Cretaceous syn- to post-rift evolution of the Mentelle Basin on the southwest Australian rifted margin (IODP Expedition 369 Sites U1513–U1516), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3751, https://doi.org/10.5194/egusphere-egu21-3751, 2021.

Before Break-Up, the opening of the South China Sea Passive Margin (SCS) was characterized by a wide rift mode during Cenozoic rifting. Such wide extensional margin (>600 km wide) is controlled by a set of hyper-extended sub-basins separated by basement highs.

These basins infill recorded a polyphased extensional deformation hence resulting in complex 3D sedimentary evolution. Based on a recent industrial 3D seismic reflection survey along the Sabah area (southern margin of the SCS), this contribution aims to investigate the detailed 3D geometries of extensional structures as well as their control on the overlying successive sedimentary sequences and relation to crustal deformation.

We mapped and analyzed several crustal-scale rolling hinge structures controlled by a series of low-angle normal faults. Deeper crustal levels are likely exhumed along the core of these rolling hinge structures, separated by extensional allochthones blocs of upper continental crust. Our structural analysis enables us to identify three main extensional phases corresponding to distinct sedimentary packages: (1) a synrift sequence 1 controlled by small offset normal faults formed during incipient rifting; (2) an intermediate synrift sequence 2 recording the development of extensional detachment faults. (3) a thick syn-rift sequence 3 recording a continuation of extension along the detachment faults resulting in the dismembering of the syn-
rift sequence 2. Intra-basement seismic reflectors dipping towards the north-west are observed, onto which extensional structures often seem to root. Some of these reflectors are interpreted as interleaved thrust sheets from a dismantled accretionary wedge of the former Mesozoic active margin (Yenshanian magmatic Arc).

Our results provide new key observations on the 3D mechanisms of detachment faulting and its control on sedimentary evolution as well as coeval crustal deformation. 3D approach throw some light on the detailed geometries of a metamorphic core-complex in relation with crustal boudinage, shear zones and lower/middle crust exhumation below the syn- rift sediments. These geometries can be compared to those described in the Basin and Range province or the Aegean Sea. Consequently, our results have implications for our understanding of rift and breakup mechanisms of marginal basins as a whole.

How to cite: Legeay, E., Mohn, G., Ringenbach, J.-C., and Vetel, W.: 3D structure of detachment faulting and related tectono-sedimentary processes in the SE South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14568, https://doi.org/10.5194/egusphere-egu21-14568, 2021.