East Asia experienced compressional deformation in the early Mesozoic, across the South China Block, North China Craton (NCC) and the part of the Central Asian Orogenic Belt to the north of the NCC. Deformation and magmatism resulted from Triassic collisions that accreted the continental blocks, and also Izanagi (Paleo-Pacific) Plate subduction from the east. We suggest that there was a single East Asian orogenic plateau by the Middle Jurassic, from NE Russia to SW China, with a length of ~4000 km. The causes and timings of the destruction of this plateau are unclear, especially loss of the lower lithosphere of the NCC. Here, we synthesize evidence for late Mesozoic and early Cenozoic crustal thinning via extension and denudation, to quantify the previous crustal thickness. We find that there was a ~50 km thick crust by the Middle Jurassic across much of the area between NE Asia and SW China, which has since undergone ~30% thinning. A force balance indicates that the buoyancy force produced by the gravitational potential energy of this thick crust drove extension from the latest Jurassic - Early Cretaceous (~145 Ma), when a rapid switch from orthogonal to oblique subduction at the Asia-Izanagi plate margin reduced the compressive boundary force by ~30%. Mantle lithosphere thinning of the NCC exceeds crustal thinning by a factor of ~2; extensional collapse cannot be the only cause of cratonic destruction, but played a major role, and potentially triggered mantle instability. Early Cretaceous extension was accompanied by a flare-up in volcanism along East Asia, which we speculate contributed to the Cretaceous hothouse climate.

How to cite: Allen, M., Song, S., Jean-Arthur Olive, J.-A., Chu, Y., and Wang, C.: East Asian orogenic collapse caused by oblique subduction and reduced boundary force, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-708, https://doi.org/10.5194/egusphere-egu21-708, 2021.

Email: pengfeili@gig.ac.cn; pengfeili2013@gmail.com

The pre-Mesozoic subduction history of the Mongol-Okhotsk oceanic plate has been poorly understood. Here we conducted geochronological and geochemical studies on four granitic plutons in the westernmost Mongol-Okhotsk Orogen (Hangay Range), with an aim to understand their petrogenesis and role in the Paleozoic tectonic evolution of the Mongol-Okhotsk Orogen. Our geochronological results constrain four granitic plutons to be emplaced from middle Ordovician to early Devonian. Geochemically, the Ordovician pluton belongs to A2-type granites, and three Silurian to Devonian plutons show the characteristics of I-type granites. These granitic plutons were probably generated by partial melting of basaltic rocks in the lower crust given the high contents of Na₂O and K₂O. The negative ε_Nd(t) values (-4.7 to -0.9) and variable ε_Hf(t) values (-2.6 to +6.1) for the four granitic plutons suggest that ancient basement materials were possibly involved in the magma source. We further investigate the geodynamic origin of these plutons in the context of the Paleozoic tectonics of the Mongol-Okhotsk Orogen, and we conclude that they were probably formed in response to the Ordovician to Devonian subduction of the Mongol-Okhotsk oceanic plate.

How to cite: Ling, J. and Li, P.: Paleozoic subduction of the Mongol-Okhotsk oceanic plate: insight from the petrogenesis of Ordovician to Devonian granitic plutons in the Hangay Range, central Mongolia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2045, https://doi.org/10.5194/egusphere-egu21-2045, 2021.

Oceanic subduction and its last underthrusted part can both triggers arc-like magmatism. As the existence of multi-subduction zones in the Central Asian Orogenic Belt, controversy still surrounds on when and especially how the subduction of the (Paleo-Asian Ocean) PAO terminated. We present geochronological, geochemical, and Lu-Hf isotopic data for a suite of basalt-andesites, dacite-rhyolites and later trachyandesite-mugearitic dykes from the Khan-Bogd area in the Gobi Tianshan Zone (GTZ) of the southern Mongolia. U-Pb dating of zircons indicate the basalt-andesites and dacite-rhyolites were formed at ~334-338 Ma, and the dykes at ~300 Ma. These Early Carboniferous volcanic rocks display high U/Th, Ba/Th, low La/Sm and variable Zr/Nb ratios, implying the involvement of subduction fluids or sediment melt. They display arc geochemical features such as calc-alkaline and metaluminous nature and positive Ba and U and negative Nb, Ta and Ti anomalies. Moreover, their continental geochemical signals (e.g. positive Pb, K anomalies) and some old captured zircons implying a continental arc setting. Comparatively, the ~300 Ma dykes are characterized by high alkaline contents, which are common for coeval (~320-290 Ma) and widespread post-subductional granites there. Given a mainly crust-derived magma source for those granites, these dykes likely reflect a mantle disturbance due to: (1) their relative low SiO₂ (51.71-55.85 wt. %) and high Mg# (40.3-67.3) values, and (2) positive zircon Ɛ_Hf(t) (most > 12). Considering a slab rollback model during the Carboniferous and Triassic, the mantle disturbance was possibly induced by the oceanic slab breakoff. Combined with previous work, this ~320-290 Ma slab breakoff-induced extension marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) north of the main ocean basin of the PAO. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).

How to cite: Zhou, H., Zhao, G., and Zhang, D.: Magmatic evidence for Late Carboniferous-Early Permian slab breakoff and extension of the southern Mongolia collage system in Central Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16411, https://doi.org/10.5194/egusphere-egu21-16411, 2021.

Debates of the Permo-Carboniferous paleogeography of the eastern Central Asian Orogenic Belt (CAOB) mainly focus on the existence, extent, and thereby evolutionary history of the Paleo-Asian Ocean (PAO) in this period. South Mongolia locates at a key position that denotes the southernmost margin of the Mongolia block. Here, we present a paleomagnetic study on the earliest Permian dykes near the Khanbogd of South Gobi Province in Mongolia to better constrain the paleo-position of the Mongolia block. Zircon U-Pb dating results of the studied dykes indicate an emplacement age of 299 ± 3 Ma. Magnetites are the dominant magnetic carriers as revealed by the synthesized rock magnetic experiments. A likely primary high coercivity/temperature component was isolated from 66 of 125 samples and displays consistent reverse polarity, which coincides with the Kiaman Reverse Superchron that overlapping the emplacement age of our studied dykes. Accordingly, a ~299 Ma paleomagnetic pole is calculated at λ/φ = −4.1°N/146.3°E (dp = 3.8, dm = 5.8, n = 66). Potential influence from Paleo-Secular Variation (PSV) is excluded following the Deenen et al. (2011) procedure. Our new results present a ~30.9°N paleolatitude for the Mongolia block, which differs from the lower paleolatitude of the North China and Xilinhot blocks as well as the much higher paleolititude of Siberia. Surrounded by these blocks of different paleolatitude, the PAO and Mongol-Okhotsk Ocean both remained wide open at least by the earliest Permian.

Acknowledgments
This research was funded by the Natural Science Foundation of China (NSFC) (41902229, 41730213, 42072264, 41902229, 41972237), China Postdoctoral Science Foundation funded project and Hong Kong RGC GRF (17307918).

References

Deenen, M. H. L. , Langereis, C. G. , Van, H. D. J. J. , & Biggin, A. J. . (2011). Geomagnetic secular variation and the statistics of palaeomagnetic directions. Geophysical Journal International(2), 509-520.

How to cite: Zhang, D., Zhao, G., Huang, B., Zhao, Q., Zhou, H., and Orsoo, E.-O.: New paleomagnetic results of the earliest Permian dykes in South Mongolia and their implications for the paleogeography of the Eastern CAOB, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15599, https://doi.org/10.5194/egusphere-egu21-15599, 2021.

The western Central Asian Orogenic Belt (CAOB) underwent the prolonged accretion from Neoproterozoic to latest Paleozoic, and evolved into an intracontinental orogenic environment in the Mesozoic to Cenozoic, which was accompanied by significant changes of climatic environments. To constrain earlier accretion mechanisms and processes of the CAOB is fundamentally important given its control on the orogenic architecture and paleogeography, which inevitably affects the subsequent intracontinental orogeny. Here, I focus on the late Paleozoic tectonic reconstruction of the western CAOB with an aim to understand the role of oroclinal bending, arc amalgamation, and large-scale transcurrent tectonics in shaping the orogenic architecture of the western CAOB. My results show that the development of the U-shaped Kazakhstan Orocline in the western CAOB may have been controlled by the along-strike variation of the trench retreat, which was accompanied by the consumption of the Junggar Ocean in the core area of the orocline. The subsequent amalgamation of multiple arcs in the western CAOB may further amplify the oroclinal structure, and I emphasize that the orogen-parallel extension plays a significant role in arc amalgamation of the western CAOB. In the Permian, the large scale of strike-slip faults characterized the western CAOB with sinistral shearing in the north (Chinese Altai) and dextral kinematics in the south (Tianshan), which together indicates the eastward migration of orogenic materials (current coordinate). Following the termination of accretionary orogeny, the western CAOB was in an intracontinental environment with relatively arid climate in the early to middle Triassic as indicated by the widespread occurrence of red beds, which may mark the initiation of aridification in Central Asia.

Acknowledgements: this study was financially supported by the Hong Kong Research Grant Council (HKU17302317), the international partnership program of the Chinese Academy of Sciences (132744KYSB20200001), the National Key Research and Development Program of China (2017YFC0601205), the National Natural Science Foundation of China (41872222) and a project from Guangdong Province (2019QN01H101).

How to cite: Li, P.: Late Paleozoic oroclinal bending, arc amalgamation, and large-scale transcurrent tectonics in the western Central Asian Orogenic Belt: termination of accretionary orogenesis and initiation of aridification in Central Asia?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1010, https://doi.org/10.5194/egusphere-egu21-1010, 2021.

As the largest accretionary orogen, the Central Asian Orogenic Belt (CAOB) involved episodic accretion/collision of arc terranes or microcontinental blocks from Neoproterozoic to late Paleozoic. Understanding the time and processes of such collisional events is crucial for the tectonic reconstruction of the CAOB. Here we focus on the Irtysh Shear Zone that represents the suture of the Peri-Siberian orogenic system (Chinese Altai Orogen) with the Kazakhstan orogenic system/East Junggar Terrane. On a basis of a combined structural and chronological study along the eastern segment of the Irtysh Shear Zone (Qinghe area), we reconstructed the collisional processes of the Chinese Altai Orogen with an intra-oceanic island arc of the East Junggar Terrane. Our results show that the oceanic basin between the Chinese Altai Orogen and the East Junggar Terrane was completely consumed in the late Carboniferous. The following arc-arc collision was characterized by early stage of orogen-perpendicular contraction, followed by orogen-parallel extension and transpressional deformation. The orogen-parallel extension, which is demonstrated by originally sub-horizontal foliation and associated orogen-parallel stretching lineation, may have be responsible for Permian high-temperature metamorphism and extensive magmatism in the southern Chinese Altai. On a scale of the western CAOB, the sinistral kinematics of the Irtysh Shear Zone, together with dextral shearing farther south in the Tianshan, suggests eastward tectonic wedging in the Permian, possibly in response to the coeval convergence of the Siberian, Baltic, and Tarim cratons.

E-mail addresses: pengfeili@gig.ac.cn, pengfeili2013@gmail.com (P. Li).

Acknowledgements: this study was financially supported by the National Natural Science Foundation of China (41872222), the National Key Research and Development Program of China (2017YFC0601205), Hong Kong Research Grant Council (HKU17302317) and a project from Guangdong Province (2019QN01H101).

How to cite: Hu, W. and Li, P.: Arc-arc collision in the Central Asian Orogenic Belt: insight from the eastern segment of the Irtysh Shear Zone, NW China., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1653, https://doi.org/10.5194/egusphere-egu21-1653, 2021.

Amalgamation of northern Gondwana involves a wealth of present-day East Asian blocks (e.g., South China, North China, Alxa, Tarim, Indochina, Qiangtang, Sibumasu, Lhasa, etc.) due to consumption and closure of the Proto-Tethys Ocean. Locating the Tarim craton during assembly of northern Gondwana remains enigmatic, with different models separating Tarim from Gondwana by a paleoceanic domain throughout the Paleozoic, advocating a long-term Tarim-Australia linkage in the Neoproterozoic to the early Paleozoic, or suggesting a Tarim-Arabia connection in the early Paleozoic.

This study carried out field-based zircon U-Pb dating and Hf isotopic analyses for early Paleozoic sedimentary rocks in the Altyn Tagh orogen, southeastern Tarim. New dating results revealed that the early Paleozoic sedimentary rocks were deposited from ca. 494 to 449 Ma. Provenance tracing indicates the ca. 494-477 Ma sedimentary rocks were primarily sourced from the local Altyn Tagh orogen to the south of the North Altyn Ocean (one branch of the Proto-Tethys Ocean between southeastern Tarim and northern Gondwana). In contrast, the ca. 465-449 Ma sedimentary rocks have remarkably increasing ca. 840-780 Ma, 2.0-1.7 Ga, and 2.7-2.4 Ga detrital zircons, indicating an augmented supply of detritus from the Tarim craton to the north of the North Altyn Ocean. Such a significant provenance shift between ca. 477 and 465 Ma marks the timing of the final closure of the North Altyn Ocean. Combined with the timing of the final closure of other branches of the Proto-Tethys Ocean, the entire Proto-Tethys Ocean might have been progressively closed at ca. 500-420 Ma, resulting in the connection of most East Asian blocks with northern Gondwana. Based on detrital zircon U-Pb-Hf isotopic comparison, Tarim most likely shared a North Indian affinity with many East Asian blocks (such as North Qilian, North Qinling, South China, Indochina, South Qiangtang, etc.). This new finding argues against an Australian or Arabian affinity for the Tarim craton.

This work was financially supported by National Natural Science Foundation of China Projects (grants 41730213, 42072264, 41902229, 41972237, and 41888101), Hong Kong Research Grants Council General Research Fund (grant 17307918), and Grant-in-Aids for Scientific Research from Japan Society for the Promotion of Science (JSPS) to Prof. Toshiaki Tsunogae (No. 18H01300) and to Dr. Qian Liu (No. 19F19020). JSPS fellowship is also much appreciated.

How to cite: Liu, Q., Tsunogae, T., Zhao, G., Han, Y., Yao, J., Li, J., and Wang, P.: A Tarim-North India connection in northern Gondwana: Constraints from provenance of early Paleozoic sedimentary rocks in the Altyn Tagh orogen, southeastern Tarim, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8111, https://doi.org/10.5194/egusphere-egu21-8111, 2021.

A long-lasting orogenic process often generates vast complexity of deformation and metamorphism. Understanding the time scales of these processes is essential for the reconstruction of the finite architecture of a fossil orogenic belt, which, nevertheless, is not always straightforward. This is because multiple episodes of tectonic events would lead to multiple growth periods of accessory minerals and deformation of rock-forming minerals, which brings challenges for conventional dating methods such as U–Pb, K/Ar, and ⁴⁰Ar/³⁹Ar step-heating. Fortunately, the emplacement of syn-tectonic quartz veins witness the deformation process and potentially, the associated metamorphism. They, therefore, have the potential to provide vital age information for regional crustal evolution. These veins, especially those in metapelitic terranes, usually contain andalusite, a fluid inclusion bearing K-poor pure aluminosilicate, which stands a good chance for directly dating syn-tectonic veining events by the fluid inclusion ⁴⁰Ar/³⁹Ar stepwise crushing technique.

Combined with detailed petro-structural investigation, this study applies the fluid inclusion ⁴⁰Ar/³⁹Ar geochronology, for the first time, on andalusite minerals in syn-tectonic quartz veins from the Chinese Altai Orogenic Belt, Central Asia, to explore a new way for dating deformation and metamorphism. ⁴⁰Ar/³⁹Ar stepwise crushing on three andalusite samples yielded well-defined Early Permain ages of 282–274 Ma. These ages are consistent with previously published emplacement ages of regional syn-tectonic leucosome/pegmatite/granite veins and metamorphic ages for local and region schist/gneiss from the same metamorphic series. These results collectively suggest that the fluid inclusion ⁴⁰Ar/³⁹Ar geochronology of andalusite in syn-tectonic quartz veins has the potential to constrain the timing of fluid-present deformation and potentially contemporaneous metamorphism. This work, therefore, provides a novel way for the age constraints of regional tectonic-thermal evolution of metapelitic terranes in general.

Acknowledgements

This project was supported by the Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515012190), the International Partnership Program of Chinese Academy of Sciences (No. 132744KYSB20190039) and the Projects funded by China Postdoctoral Science Foundation (No. 2019M663133). A Guangdong Special Support Program to Y.D. Jiang is also acknowledged.

How to cite: Xiao, M., Jiang, Y.-D., Qiu, H.-N., and Zhao, G.-C.: Fluid inclusion 40Ar/39Ar geochronology of andalusite from syn-tectonic quartz veins: perspectives on dating regional deformation and metamorphism events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16501, https://doi.org/10.5194/egusphere-egu21-16501, 2021.

The South Tianshan Orogenic Belt in NW China marks the suturing site between the Tarim Craton and the Central Asian Orogenic Belt (CAOB) during late Paleozoic-Mesozoic time. Despite numerous investigations, the amalgamation history along the South Tianshan Orogen remains controversial, especially on the timing and process of the final continental collision between the Tarim Craton and the Central Tianshan (CTS)-Yili Block. We inquire into this issue on the basis of a compiled dataset across the Tarim, South Tianshan and CTS-Yili regions, comprising elemental and isotopic data of magmatic rocks and radiometric ages of regional magmatism, detrital zircons, (ultra-)high pressure metamorphism and tectonothermal events. The data support a continental collision along the South Tianshan belt in 310-300 Ma, in accord with a contemporaneous magmatic quiescence and a prominent decrease of εNd(t) and εHf(t) values of magmatic rocks in the CTS region, and a main exhumation stage of (U)HP rocks in the South Tianshan region. The collisional orogeny along the South Tianshan have most likely been influenced by a mantle plume initiated at ca. 300 Ma underneath the northern Tarim Craton, as evidenced by temporal and spatial variations of geochemical proxies tracing magma source characteristics. The new model of plume-modified collision orogeny reconciles the absence of continental-type (U)HP rocks in the orogen and the insignificant upper-plate uplift during continental collision. In the mid-Triassic (ca. 240 Ma), the Chinese western Tianshan underwent intense surface uplift and denudation, as indicated by sedimentary provenance analysis and tectonothermal events. Paleocurrent and detrital zircon age data from Triassic strata in northern Tarim suggest a provenance change from a single source of the Tarim Craton to multiple sources including the CTS-Yili Block to the north and the Western Kunlun Orogen to the south. We suggest that the mid-Triassic uplifting in Chinese western Tianshan was an intracontinental orogeny caused by far-field effects of the collision between the Tarim Craton and the Qiangtang Block. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).

How to cite: Han, Y. and Zhao, G.: Collision and reactivation along the South Tianshan Orogen (NW China) through late Paleozoic to Mesozoic time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15241, https://doi.org/10.5194/egusphere-egu21-15241, 2021.

The Chinese North Tianshan (CNTS) extends E-W along the southern part of the Central Asian Orogenic Belt and has undergone complicated accretion-collision processes in the Paleozoic. This study attempts to clarify the late Paleozoic tectonism in the region by investigating the provenance of the Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS by U-Pb dating and Lu-Hf isotopic analyses of detrital zircons. Detrital zircon U-Pb ages (N=519) from seven samples range from 261 ± 4 Ma to 2827 ± 32 Ma, with the most prominent age peak at 313 Ma. There are Precambrian detrital zircon ages (~7%) ranged from 694 to 1024 Ma. The youngest age components in each sample yielded weighted mean ages ranging from 272 ± 9 Ma to 288 ± 5 Ma, representing the maximum depositional ages. These and literature data indicate that some previously-assumed “Carboniferous” strata in the Bogda area were deposited in the Early Permian, including the Qijiaojing, Julideneng, Shaleisaierke, Yangbulake, Shamaershayi, Liushugou, Qijiagou, and Aoertu formations. The low maturity of the sandstones, zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East ­Junggar Arc and the Harlik-Dananhu Arc in the CNTS. The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc. Zircon ɛ_Hf(t) values have increased since ~408 Ma, probably reflecting a tectonic transition from regional compression to extension. This event might correspond to the opening of the Bogda intra-arc/back arc rift basin, possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean. A decrease of zircon ɛ_Hf(t) values at ~300 Ma was likely caused by the cessation of oceanic subduction and subsequent collision, which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous. This research was financially supported by the Youth Program of Shaanxi Natural Science Foundation (2020JQ-589), the NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).

How to cite: Wang, Q., Zhao, G., Han, Y., and Yao, J.: Late Paleozoic tectonism of the Bogda region in  Chinese North Tianshan: Insights from sedimentary provenance analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16422, https://doi.org/10.5194/egusphere-egu21-16422, 2021.

We present two carbonate oxygen and carbon isotope records from late Miocene – early Pleistocene stratigraphic sections from the southern flank of the Issyk Kul basin, Kyrgyz Tien Shan. The two sections are 700 and 500 m thick and composed of fluvial and lacustrine sediments. They were dated using magnetostratigraphy (Roud et al., G-Cubed, in review) and ²⁶Al/¹⁰Be isochron burial dating (presented here).

Carbonate stable isotope data is useful for reconstruction of climate in Asia over the Cenozoic. Oxygen isotopes are commonly used to detect moisture sources and their interaction with topography. Pedogenic carbon isotopes are used to reconstruct past atmospheric CO² levels or the spread of C4 vegetation.

The environment of Central Asia is primarily affected by the northern mid-latitude westerlies − winds transporting moisture eastward across Eurasia. Issyk Kul basin is situated on the windward side of the northern Tien Shan. Published data suggest that the Tien Shan mountain ranges interacted with the westerlies since late Oligocene and reorganized Central Asian climate during Neogene (Caves et al., 2017; Charreau et al., 2012; Macaulay et al., 2016; Wang, et al., 2020). The amount of existing published paleoclimate data from northern Central Asia is scarce compared to interior China, and therefore the influence of the Tien Shan uplift on climate in Asia during the Cenozoic is poorly reconstructed.

Our data provide new insight into the role of the range and its interaction with the westerlies in forming climate on the windward side of the northern Tien Shan in the late Neogene. We combine our data with published stratigraphically-older sections nearby (Macaulay et al., 2016) to complete the Neogene stable isotope record of the Issyk Kul basin and study how the evolution of the basin influenced regional climate.

Our d18O and d13C values show slightly positive trends, unlike stratigraphically-older data from the Issyk Kul basin. The preliminary interpretation suggests that the circulation pattern within the range was changed in late Miocene possibly reflecting active tectonic uplift northward of the basin and an increase in aridification.

How to cite: Kudriavtseva, A., Sobel, E., Codilean, A., Roud, S., Wack, M., Gilder, S., Hoke, G., Mulch, A., Mikolaichuk, A., Fink, D., Fülöp, R., and Wilcken, K.: Late Neogene climatic features recorded by isotope records in the Issyk Kul basin, Kyrgyzstan. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12907, https://doi.org/10.5194/egusphere-egu21-12907, 2021.

How to cite: Meijers, M. J. M., Brocard, G. Y., Kaya, F., Pehlevan, C., Başoğlu, O., Bibi, F., Krsnik, E., and Mulch, A.: Tectonics–Earth surface processes interactions of the Central Anatolian Plateau during the late Miocene to Pliocene revealed by ecosystem and paleotemperature reconstructions , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14582, https://doi.org/10.5194/egusphere-egu21-14582, 2021.

The crustal structure of the Iranian Plateau bears important information about the details of the tectono-magmatic processes associated with the Neo-Tethys subduction and subsequent Arabia-Eurasia collision. Using a newly developed method of joint inversion of multi-frequency waveforms around and horizontal-to-vertical (H/V) ratios of the direct P arrivals in teleseismic P-wave receiver functions, we construct the shear-wave velocity image of the shallow crust (from surface up to 10-km depth below sea level) along a dense seismic array across the Zagros suture in the northwest Iranian Plateau. The most striking structural feature of the study region is the presence of low- and high-velocity anomalies (LVAs and HVAs) beneath the Zagros fold-and-thrust belt and the Iranian continent, respectively, indicating strong structural differences on the two sides of the suture. Systematic analysis on the velocity estimates and comparison with laboratory measurements and regional geology suggest that the LVAs and HVAs are representatives of Zagros sedimentary rocks and arc to intraplate magmatic rocks, respectively. The LVAs (1.3-2.0 km/s) are characterized by a series of faulted anti-form structures at ~1-7 km depths beneath Zagros. They are likely dominantly composed of shales and mudstones, and could have acted as mechanically weaknesses to accommodate different deformations of surroundings and give rise to the present-day depth-dependent seismicity. The HVAs beneath the central domain and Alborz in the Iranian continent present large ranges in both velocity (3.2-3.9 km/s) and depth (0-10 km), probably suggesting strong lithological variations in these areas. Most of the HVAs above 5-km depth have shear-wave velocities of 3.2 to 3.6 km/s, comparable to those of andesites and basalts dominated in the northwestern Iranian plateau. The deeper HVAs (below 5-km depth), which generally have greater velocities ~3.6-3.9 km/s falling into the velocity range of intrusive rocks such as granodiorites, diorites and diabases, appear to have much larger volumes at depth than that exposed on the surface in the study region. Moreover, the surface projections of the HVAs are spatially coincident with the major faults or tectonic boundaries of the region, suggesting a causal link. Our observations provide evidence for not only the lithology-controlled layering in both sedimentary structure and deformation in the Zagros passive margin but also the much more substantial magma generation and emplacement at depth than faulting-facilitated eruption and exposure on the surface in the Iranian active margin during the subduction and collision processes.

How to cite: Wang, X., Chen, L., Talebian, M., Ai, Y., Jiang, M., Yao, H., He, Y., Ghods, A., Sobouti, F., Wan, B., Chu, Y., Hou, G., Chen, Q., Xiao, W., Wu, F., Zhu, R., and Chung, S.-L.: Shallow crustal structure in the northwestern Iranian Plateau and its tectonic implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9330, https://doi.org/10.5194/egusphere-egu21-9330, 2021.

Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed crust of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Owing to its complicated opening history, the Indian Ocean comprises a mosaic of such features that is an excellent illustration of the degree of geographic complexity that must have occurred in now-subducted oceanic realms of the geologic past and provides the ideal natural laboratory to validate interpretations of present-day orogenic architecture in terms of paleogeography. Current classification schemes of orogens divide between settings associated with termination of subduction (continent-continent collision, continent-ocean collision (obduction)) and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave like accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motion of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis’ that link the rules of rigid plate tectonics with the reality of plate deformation. We illustrate the use of these rules with a thought experiment, in which we predict two contrasting orogenic architectures that may result from the closure of the Indian Ocean and subsequent collision of the Somali, Malagasy and Indian Margins in a global continental drift scenario for a future supercontinent. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of
plate tectonics.

How to cite: Schouten, T. and van Hinsbergen, D.: Deciphering paleogeography from orogenic architecture: constructing orogens by a future closure of the Indian Ocean as thought experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7270, https://doi.org/10.5194/egusphere-egu21-7270, 2021.

Understanding the Tibetan Plateau (TP) topographic history is essential to determining its building mechanisms and its role in driving regional climate, environments and biodiversity. The Lunpola Basin (central-southern Tibet) is the key place to constrain the Tibet building because it deposits the most complete Cenozoic stratigraphy sequence in the central TP and bears many layers of tuffs, abundant fossil plants and mammals and paleosols. It is also the first place that stable isotope based paleoaltimetry was applied to, which suggested that similar to present elevation was attained in the central TP at least 35 Ma ago, implying a much earlier uplift of the TP than before. This view was soon widely accepted by international society but was challenged by recent discoveries of low elevations tropical fossil apparently deposited at 25.5 Ma. However, we use magnetostratigraphic and radiochronologic dating to robustly revise the chronology of regional elevation estimates both from the stable isotope and fossils in the Lunpola Basin. The results indicate that both ages estimated for the stable and fossil based elevations are wrong with the former from ~40 Ma revising to ~26-21 Ma and the later from ~26 Ma to ~40 Ma. Thus this revised chronology demonstrates that central Tibet was generally low (<2.3 km) since at least ~40 Ma and became high (3.5-4.5 km) since at least ~26 Ma. This supports the Eocene existence of a lowland between the Gangdese Shan and Tanggula Shan until their early Miocene uplift. This later uplift of central-southern Tibet has important implications for Tibetan Plateau (TP) growth mechanisms and agrees well with recently updated studies of the TP-imposed impacts on Asian atmospheric circulations, surface processes and biotic evolution and diversification differentiation.

How to cite: Fang, X., Dupont-Nivet, G., Wang, C., Song, C., Meng, Q., Zhang, W., Nie, J., Zhang, T., and Mao, Z.: Revised chronology of central Tibet uplift and its implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14050, https://doi.org/10.5194/egusphere-egu21-14050, 2021.

The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. We analyse the link between climate and tectonics in the central and eastern Tibetan Plateau using geomorphic indices of surface roughness (SR) hypsometric integral (HI) and elevation-relief ratio (ZR) and mean annual precipitation, thermochronology and erosion rate data. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (“channel flow”). Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. We identify a broad ˜WSW-ENE trending transition in the landscape where changes in landscape and precipitation are grouped and in alignment. It represents, from east to west, a sharp decline in precipitation (interpreted as the western extent of the East Asian monsoon), a change to a low relief landscape at 4500-5000 m elevation, an increase in ZR and a transition to low HI and SR. This zone cuts across structural boundaries and is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau. We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Modern erosion rates are lower in the non-incised region, west of the monsoon extent (mean 0.02 mm/yr), than the incised region (mean 0.26 mm/yr). Compiled thermochronology data shows an increase in exhumation from ˜25 Ma in the incised area but no evidence of this increased exhumation in the non-incised area. This pattern supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by Miocene monsoon intensification. Our results do not support the channel flow model, which would predict an eastwards wave of surface uplift and therefore erosion and exhumation during the Miocene, which are not present in the data.

How to cite: Groves, K., Allen, M., Saville, C., Hurst, M., and Jones, S.: Monsoon-driven incision and exhumation of the Eastern Tibetan Plateau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1137, https://doi.org/10.5194/egusphere-egu21-1137, 2021.

The Himalaya is the highest and steepest mountain range on Earth and an efficient north-south barrier for moisture-bearing winds. The close coupling of changes in topography, erosion rates, and uplift has previously been interpreted as an expression of a climatic control on tectonic deformation. Here, we present 17 new zircon U/Th-He (ZHe) bedrock-cooling ages from the Sutlej Valley that – together with >100 previously published mica ⁴⁰Ar/³⁹Ar, zircon and apatite fission track ages – allow us to constrain the crustal cooling and exhumation history over the last ~20 Myr. Using 1D-thermal modeling, we observe a rapid decrease in exhumation rates from >1 km/Myr to <0.5 km/Myr that initiated at ~17-15 Ma across the entire Greater and Tethyan Himalaya, as far north as the north-Himalayan Leo Pargil gneiss dome. This decrease is recognized both in the hanging and footwall of major Miocene shear zones and suggests that cooling is associated to surface erosion rather than to tectonic unroofing. We explain the middle Miocene deceleration of exhumation with major reorganization of Himalayan deformation and the onset of the growth of the Lesser Himalayan duplex. This resulted in accelerated uplift of the Greater Himalaya above a mid-crustal ramp, and thus forming a new efficient orographic barrier. The period of slow exhumation in the upper Sutlej Valley coincides with a period of internal drainage in the south-Tibetan Zada Basin further upstream, which we interpret to be a consequence of tectonic damming of the upper Sutlej River. External drainage of the Zada Basin was re-established ~1 Ma, when we observe exhumation rates in the upper Sutlej Valley to accelerate again. Our new finding document that the location of tectonic deformation processes control the first order spatial pattern of both climatic zones and erosion across the orogen.

How to cite: Thiede, R., Scherler, D., and Glotzbach, C.: Middle Miocene rise of the Greater Himalaya establishing a new orographic barrier, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4934, https://doi.org/10.5194/egusphere-egu21-4934, 2021.

As an important driver of global climate change during the Cenozoic, the uplift of the Tibetan Plateau (TP) has strongly influenced the origination and evolution of the Asian monsoon system, and therefore the aridification of central Asia. Over the last two decades, the application of stable isotope paleoaltimeters and the discoveries of mammal and plant fossils have greatly promoted the understanding of the uplift history of the TP. However, paleoaltitudinal reconstructions based on different paleoaltimeters have suggested differing outcomes and therefore remain controversial. Novel paleoaltimeters have therefore needed to be developed and applied to constrain the uplift history of the TP more accurately and effectively by comparing and verifying multi-proxies. Paleothermometers based on glyceryl dialkyl glycerol tetraethers (GDGTs) are widely used in terrestrial and ocean temperature reconstructions. In this study, GDGT-based paleothermometers were tentatively applied to the Gyirong Basin on the southern TP, and the Xining Basins on the northern TP, in an attempt to quantitatively reconstruct their paleoaltitudes.

Both soil and aquatic-typed branched GDGTs have been identified from Late Miocene to Mid-Pliocene (7.0-3.2 Ma) samples taken from the Gyirong Basin; their reconstructed paleotemperatures were 7.5±3.3°C and 14.2±4.5°C, respectively. The former temperature may represent the mean temperature of the terrestrial organic matter input area, while the latter may represent the lake surface temperature. The results would suggest that the lake surface of the Gyirong Basin during the Late Miocene to Mid-Pliocene was 2.5±0.8 km and that the surrounding mountains exceeded 3.6±0.6 km, implying that the central Himalayas underwent a rapid uplift of ~1.5 km after the Mid-Pliocene.

GDGT-based paleotemperature reconstructions using MBT'_5ME values show that the Xining Basin dropped in temperature by ~10°C during the ~10.5-8 Ma period, exceeding that in sea surface temperatures and low-altitude terrestrial temperatures during these periods. By combining these results with contemporaneous tectonic and sedimentary records, we infer that these cooling events signaled the regional uplift with the amplitude of ~1 km of the Xining basins. Our results support that the TP was still growing and uplifting substantially since the Late Miocene, which may provide new evidence for understanding the growth, expansion and uplift patterns of the TP.

How to cite: Chen, C., Bai, Y., Fang, X., Guo, H., Zhang, W., Meng, Q., Xu, Q., Zhang, T., Deng, T., He, J., and Chen, Q.: Paleoaltitudinal histories for the northern and southern margins of the Tibetan Plateau during the Late Cenozoic: revealed by GDGTs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14073, https://doi.org/10.5194/egusphere-egu21-14073, 2021.

High elevation orogenic plateaus are formed by a complex interplay of deep and surficial processes yet understanding of the deeper processes is limited by few recognized exposures of the lower levels of plateaus. We present evidence for the existence of an orogenic plateau during and after the Devonian Acadian orogeny (sensu lato), the mid-crustal roots of which are exposed in the New England Appalachians. The four-dimensional crustal evolution of this paleo-plateau is constrained by the integration of petrochronology, petrologic and geochronologic databases, and geophysical imaging. Doubly thickened crust, widespread amphibolite to granulite-facies metamorphic conditions, a paleo-isobaric surface, and protracted mid-crustal anatexis all indicate the presence of a high elevation (~5 km), low relief plateau by 380 Ma. ⁴⁰Ar/³⁹Ar thermochronology shows a distinct signature with very slow cooling rates of 2-4°C/m.y. following peak metamorphic conditions. Thermochronologic data, trace element and Nd isotope geochemistry, and monazite and xenotime petrochronology suggest a 50 m.y. lifespan of the plateau (380-330 Ma). Orogen parallel ductile flow and extrusion of gneiss domes resulted in plateau collapse, crustal thinning, and block-like exhumation at ca. 330-300 Ma. Thinning of the plateau crust may have led to the sharp 12-15 km step in Moho depth in western New England, possibly by reactivating the suture between Laurentia and accreted Gondwanan-derived terranes. The formation of the Acadian altiplano may have influenced Li-pegmatite genesis and Paleozoic paleoclimate, while its recognition may provide a window into the deeper processes of orogenic plateaus including partial melting, plutonism, and collapse by ductile extension.

How to cite: Hillenbrand, I., Williams, M., Li, C., Gao, H., and Jercinovic, M.: Rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in the northern Appalachian Orogen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1618, https://doi.org/10.5194/egusphere-egu21-1618, 2021.

The Andes are the case example of an active Cordilleran-type orogen. It is generally admitted that, in the Bolivian Orocline (Central Andes at ~20°S), mountain-building started ~50–60 Myr ago, close to the subduction margin, and then propagated eastward. Though suggested by some early geological cross-sections, the structures sustaining the uplift of the western flank of the Altiplano have often been dismissed, and the most common view theorizes that the Andes grow only by east-vergent deformation along its eastern margin. However, recent studies emphasize the significant contribution of the West Andean front to mountain-building and crustal thickening, in particular at the latitude of Santiago de Chile (~33.5°S), and question the contribution of similar structures elsewhere along the Andes.  Here, we focus on the western margin of the Altiplano at 20–22°S, in the Atacama desert of northern Chile. We present our results on the structure and kinematic evolution on two sites where the structures are well exposed. We combine mapping from high-resolution satellite images with field observations and numerical trishear forward modeling to provide quantitative constraints on the kinematic evolution of the western front of the Andes. Our results confirm two main structures: (1) a major west-vergent thrust placing Andean Paleozoic basement over Mesozoic strata, and (2) a west-vergent fold-and-thrust-belt deforming primarily Mesozoic units. Once restored, we estimate that both structures accommodate together at least ~6–9 km of shortening across the sole ~7–17 km-wide outcropping fold-and-thrust-belt. Further west, structures of this fold-and-thrust-belt are unconformably buried under much less deformed Cenozoic units, as revealed from seismic profiles. By comparing the scale of these buried structures to those investigated previously, we propose that the whole fold-and-thrust-belt has most probably absorbed at least ~15–20 km of shortening. The timing of the recorded main deformation can be bracketed sometime between ~68 and ~29 Ma – and possibly between ~68 and ~44 Ma – from dated deformed geological layers, with a subsequent significant slowing-down of shortening rates. This is in good agreement with preliminary modeling of apatite and zircon (U-Th)/He dates suggesting that basement exhumation by thrusting started by ~70–60 Ma, slowed down by ~50–40 Ma, and tended to cease by ~30–20 Ma. Minor shortening affecting the mid-late Cenozoic deposits indicates that deformation continued after ~29 Ma along the western Andean fold-and-thrust-belt, but remained limited compared to the more intense deformation that occured during the Paleogene. Altogether, the data presented here will provide a quantitative evaluation of the contribution of the western margin of the Altiplano plateau to mountain-building at this latitude, in particular at its earliest stages.

How to cite: Habel, T., Lacassin, R., Simoes, M., Carrizo, D., Aguilar, G., and Margirier, A.: Deformation of the western flank of the Andes at ~20–22°S: a contribution to the quantification of crustal shortening, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9466, https://doi.org/10.5194/egusphere-egu21-9466, 2021.