Displays

Evolution of the Tibetan Plateau is important for understanding continental tectonics because of its exceptional elevation (~5 km above sea level) and crustal thickness (~70 km). Patterns of long-term landscape evolution can constrain tectonic processes, but have been hard to quantify, in contrast to established datasets for strain, exhumation and paleo-elevation. This study analyses the relief of the bases and tops of 17 Cenozoic lava fields on the central and northern Tibetan Plateau. Analyzed fields have typical lateral dimensions of 10s of km, and so have an appropriate scale for interpreting tectonic geomorphology. Fourteen of the fields have not been deformed since eruption. One field is cut by normal faults; two others are gently folded with limb dips <6^o. Relief of the bases and tops of the fields is comparable to modern, internally-drained, parts of the plateau, and distinctly lower than externally-drained regions. The lavas preserve a record of underlying low relief bedrock landscapes at the time they were erupted, which have undergone little change since. There is an overlap in each area between younger published low-temperature thermochronology ages and the oldest eruption in each area, here interpreted as the transition between the end of significant (>3 km) exhumation and plateau landscape development. This diachronous process took place between ~32.5^o - ~36.5^o N between ~40 and ~10 Ma, advancing northwards at a long-term rate of ~15 km/Myr. Results are consistent with incremental northwards growth of the plateau, rather than a stepwise evolution or synchronous uplift.

How to cite: Allen, M. and Law, R.: Diachronous Tibetan Plateau landscape evolution derived from lava field geomorphology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1270, https://doi.org/10.5194/egusphere-egu2020-1270, 2020.

Knowledge of the geological history of the Tibetan plateau is critical to understanding crustal deformation process, and the plateau’s influence on climate. However, the timing of Tibetan plateau development remains controversial. The Nima Basin along the Jurassic-Cretaceous Bangong Suture in central Tibet provides well-dated records of exhumation in this area. Here, we present detrital zircon U-Pb, apatite U-Th/He (AHe) and apatite fission track data (AFT) from upper Cretaceous and Oligocene red sandstones and conglomerates in the Nima Basin, as well as from the Xiabie granite in the hanging wall of the basin-bounding Muggar Thrust. 4 granite conglomerate clasts from the above yield zircon U-Pb ages ranging between 114-122 Ma, which likely come from the Xiabie granite. 7 granitoid/sandstone conglomerate clasts yield AHe ages ranging from 21-58 Ma, while AFT ages range from 34-83 Ma. Thermal history inversion modelling for five of the above samples show a consistent rapid cooling from 100 ℃ to 30 ℃ between 50-40 Ma, the cooling rate decreased significantly after 40 Ma. Implications of these data, integrated in the context of previously published data for the wider region (e.g. Rohrmann et al. 2012; Haider et al., 2013; Li et al., 2019) will be discussed.

Reference

Rohrmann, A et al., 2012, Thermochronologic evidence for plateau formation in central Tibet by 45 Ma: Geology, v. 40, p. 187-190.

Haider, V. L et al., 2013, Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co, Tibetan Plateau: Journal of Asian Earth Sciences, v. 70-71, p. 79-98.

Li, H. A et al., 2019, The formation and expansion of the eastern Proto-Tibetan Plateau: Insights from low-temperature thermochronology: Journal of Asian Earth Sciences, v. 183, 103975.

How to cite: Xue, W., Najman, Y., Hu, X., Persano, C., Stuart, F. M., Li, W., and Wang, Y.: Early Eocene rapid exhumation record in the region of Nima, central Tibet, as determined by low-temperature thermochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5082, https://doi.org/10.5194/egusphere-egu2020-5082, 2020.

Kunlun Mountains, SW part of the Tarim Basin and S edge of the Bachu Uplift in central Asia collectively form the northernmost segment of the vast Cenozoic deformation zone and associated depositional areas formed in course of the India – Euroasia collision. Five seismic transects from the SW Tarim Basin (Yechang - Hotan area) calibrated by deep wells were used in order to assess lateral variations of a structural style and syn-tectonic sedimentation in this part of the basin. Pre-Cenozoic substratum of SW Tarim Basin is formed by crystalline basement covered by Paleozoic strata, with important mid-Cambrian evaporites (Awatage Formation) that served as first, deep detachment level. Cenozoic sedimentary infill consists of several kilometers of shallow water to terrestrial clastics with Paleogene evaporites of the Bashiblake Formation at their base. Paleogene evaporites acted as a second, shallow detachment. Mid – late Miocene to Quaternary wedging along the front of the Kunlun Mts., associated with formation of a large-scale duplex consisting of slivers built of Precambrian to Permian rocks, resulted in progressive, laterally variable uplift of the S margin of the Tarim Basin documented by well-preserved growth strata that have been also described in the field. Jade Anticline, large intra-basinal structure that is located in the central part of the Tarim Basin, previously interpreted as a regional wrenching zone, was reinterpreted as a thin-skinned syn-depositional “fish tail” structure, detached in the Paleogene evaporites and formed in Quaternary above local basement elevation. Northernmost late Miocene compressional deformations have been recognized along the S edge of the Bachu Uplift in its Western and central segment. They formed due to complex interplay of thick-skinned basement reverse faulting responsible for regional elevation of basement blocks, and two types of thin-skinned thrusting: southward directed thrusting detached within the mid-Cambrian evaporites and northward directed thrusting detached within the Paleogene evaporites. Compressional deformations along the S edge of the Bachu Uplift are diminishing and eventually disappearing towards the East. All these findings point to significant transfer of compressional stresses into the far foreland of the W Kunlun Mountains and laterally variable tectonic coupling between the Tibet Plateau and central part of the Tarim Basin.

Seismic data used in this study was kindly provided by China National Petroleum Corporation (PetroChina). IHS Markit is thanked for providing academic license of Kingdom seismic interpretation software.

How to cite: Liu, W., Krzywiec, P., Mazur, S., Meng, F., Zhuo, Q., and Chen, Z.: Cenozoic growth of West Kunlun Mountains and tectono-sedimentary evolution of adjacent SW Tarim Basin-New spatial model based on seismic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5865, https://doi.org/10.5194/egusphere-egu2020-5865, 2020.

The Pamir Frontal Thrust (PFT) constitutes the northernmost boundary of the Pamir mountain range at the NW edge of the India-Eurasia collision zone. Due to the ongoing collision this active system propagates into and overthrusts the Quaternary deposits of the Alai Valley, an intermontane basin separating the Pamir from the Tien Shan in the north. Geodetic data across the Central Pamir document a shortening rate of 25 mm/yr, with a dramatic decrease of ~10 mm over a short distance across the northernmost Trans-Alai range (250 km aperture); this suggests that almost half of the shortening in the greater Pamir – Tien Shan collision zone is absorbed along the PFT.

Consequently, the frontal thrusts must accommodate a significant amount of slip and may be capable of generating ≥M7 earthquakes in this part of the orogen. In contrast to similar tectonic settings along the Himalayan megathrust, the present-day seismicity in the Pamir apparently does not reflect the long-term deformation history. Despite few studies in the late 20^th century, and an extensive data base of recent earthquakes, the relationships between seismicity and the geometry of the thrust zone are not well understood. In this context our study aims to improve the understanding of the earthquake geology of the PFT by asking two principal questions: (1) How much of the PFT is activated during an earthquake rupture? (2) Does the paleoseismic slip history agree with the geodetically-derived shortening rate?

Here, we present our results of five analyzed paleoseismic trenches that reveal the youngest manifestation of thrusting along the central segment of the PFT. We combined field-based observations with a TanDEM-X data, UAV-based DEMs, and dGPS profiling for an offset analysis along the fault scarp. The interpretation of the trench stratigraphy and event horizons in the context of these tectonic landforms was combined with radiocarbon and luminescence dating to develop an earthquake chronology.

We find robust evidence for at least three surface-rupturing events during the past 6 kyr. At least one event can be recognized in all five trenches separated by ~10 km, indicating a full-length activation of the central fault segment during rupture. Ages obtained from uplifted fluvial terraces coupled with the total cumulative fault offset indicate a Holocene slip rate of up to 3.5 mm/yr. Based on dip-slip motion offsets per event we estimated an average earthquake paleo-magnitude ranging between M6.5-7.0.

Despite the regional extent of the central PFT, and a rather high displacement gradient across it, our results suggest a seismic behavior characterized by strong surface-rupturing earthquakes, short surface ruptures, and low slip rates. Earthquakes along this structure do not cover the total geodetic shortening, which suggests that a strongly segmented PFT system may be linked with active seismogenic deformation in the alluvial-fan covered piedmont regions to the north. However, the preservation potential for fault scarps in the piedmont may be low in this highly dynamic environment due to climate-driven fluvial and glacial processes in the high sectors of the Pamir.

How to cite: Patyniak, M., Landgraf, A., Dzhumabaeva, A., Williams, A. M., Baikulov, S., Arrowsmith, J. R., Abdrakhmatov, K., and Strecker, M. R.: Seismic Behavior Along a Fault Segment in an Active Continental Collision Zone: New Paleoseismic and Structural Data of the Pamir Frontal Thrust in the Alai Valley, Kyrgyzstan, Central Asia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4599, https://doi.org/10.5194/egusphere-egu2020-4599, 2020.

The Cenozoic Pamir comprises the western equivalent of the Tibetan plateau, offset to the north by ca. 300 km. A significant geodynamic question is what controls the lateral extent of the Pamir. Here we suggest that the width of the Pamir is controlled by east-west variations in the rheology of blocks farther to the north. In particular, the rigid, Precambrian-cored Tarim block, directly north of Tibet, apparently does not extend farther west. Indirect evidence for this crustal structure is derived from the late Paleozoic - early Mesozoic evolution of the northern and external Pamir. The northern part of the Western Kunlun comprises Proterozoic Tarim basement; such rocks are unknown on the northern margin of the Pamir. In the late Ordovician or Silurian, the Kudi suture formed, representing the consumption of the Proto-Tethys and the collision of Tarim with the southern part of the Western Kunlun terrain. Although the Western Kunlun has been considered to be the lateral equivalent of the North Pamir, the Kudi suture does not appear to be preserved in the Pamir. In contrast, the North Pamir preserves remnants of a broad Carboniferous ocean which are not recognized in the Western Kunlun. The northern margin of this ocean is unclear; it may have merged with the Turkestan ocean, on the southern margin of the Tian Shan. There are no documented basement units directly north of the Pamir; the basement Garm block lies at the northwest corner of the Pamir and may represent a fragment of Tarim which we suggest must have been rifted away by the Ordovician. The North Pamir Carboniferous deep marine units are unconformably overlain by upper Carboniferous and lower Permian shallow marine units at the eastern and western ends of the North Pamir, suggesting a contractile episode; the contact appears to be conformable in the central part. The lower Permian is overlain by an uppermost Permian - Triassic back-arc basin or rift, which stretches ca. 500 km east-west. There is no evidence that this basin extended into the Western Kunlun. Therefore, the location of the Cenozoic Pamir corresponds to the extent of both Carboniferous oceanic crust and Permo-Triassic extended or oceanic crust. We suggest that the differences between the Western Kunlun Shan and the North Pamir reflect the presence and absence, respectively, of the rigid Tarim block to the north. Although it has been suggested that the geometry of the Pamir reflects the geometry of a promontory at the northwest corner of the Indian indentor; this seems highly improbable given the pre-Cenozoic history. Rather, we suggest that differences in the evolution of the Pamir and Tibet are first-order consequences of the different rheologies of the northern crustal backstops of these two regions.

How to cite: Sobel, E., Rembe, J., Kley, J., Zhou, R., Terbishalieva, B., and Chen, J.: Control of pre-existing crustal architecture on Cenozoic formation of the Pamir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12995, https://doi.org/10.5194/egusphere-egu2020-12995, 2020.

The Tianshan belt is one of the key regions in understanding the tectonics of the Central Asian Orogenic Belt (CAOB), as it presents a typical example of subduction, accretion and collision. Its tectonic evolution is recently in hot debate and draws more and more attention of the international geological society. As a major tectonic segment, the Middle Chinese Tianshan was considered to witness the most significant tectonic events. On the basis of structural and geochronological works, three zones have been recognized namely: 1) the northern zone, composed of weakly metamorphosed sedimentary rocks of Silurian to Carboniferous ages; 2) the central zone, comprised of well sheared amphibolite, marble, quartzo-schist, quartzite, garnet-biotite schist, and orthogneiss; and 3) the southern zone, which consists of amphibolite facies metamorphic rocks whose protolith is considered to be Silurian to Devonian. The most significant deformation was marked on the various schist or gneiss of the central zone. E-W striking, vertical or sub-vertical foliation with horizontal or sub-horizontal mineral and stretching lineations indicate conspicuous strike-slip shearing. Shear criteria indicate a dextral sense of shearand geochronological results indicates it looks like two phase deformation (~290 Ma and ~250 Ma). South-dipping foliation with northward thrusting in the northern zone and north-dipping foliation with southward thrusting in the southern zone show a large-scale flower structure related to the early stage of the dextral strike-slip tectonics of the central zone. The absolute timing of the dextral strike-slip deformation is also discussed in the light of available radiometric dating. Our structural data emphasizes that the post-collisional dextral wrenching has largely modified the architecture of the Tianshan orogenic belt and played a critical role in the tectonic evolution of Central Asia.

How to cite: Lin, W. and Li, L.: A syn-collisional or post-collisional belt? In the view from the middle segment of the Central Tianshan Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6248, https://doi.org/10.5194/egusphere-egu2020-6248, 2020.

The high-grade metamorphic complexes in the Chinese Altai were previously regarded as the Precambrian basement and thus important for unravelling tectonic evolution of the Altai orogen. This study reports detailed filed investigation, zircon U-Pb-Hf isotopic and whole-rock geochemical data for the paragneissic rocks from Northern Fuyun Complex (NFC), southern Chinese Altai. Detrital zircons from the paragneisses have a predominant early Paleozoic age population (ca. 535-435 Ma), with minor Neoproterozoic and sparse Mesoproterozoic to Archean ages. The geochemical analyses together with the euhedral shape of the detrital zircons suggest that their sedimentary protoliths mainly came from felsic-intermediate igneous rocks with low maturity. In combination with the cumulative distribution curves of zircon age spectra, the variable zircon εHf(t) values (-25 to +13), as well as the immature geochemical compositions, we infer that the protoliths were most likely deposited on an active continental margin in the early Paleozoic and sourced mainly from proximal igneous rocks, which are comparable to the Habahe Group. Similar detrital zircon age spectra of early Paleozoic sequences from the Chinese Altai, Mongolia Altai and Khovd Zone support the existence of a giant accretionary wedge developed along the western margin of the Ikh-Mongol Arc system, resulting from continuous northeast-dipping oceanic subduction. This research was financially supported by the National Key R&D Program of China (2017YFC0601205), Hong Kong RGC GRF (17302317 and 17303415) and NSFC Projects (41730213 and 41190075).

How to cite: Cui, X., Sun, M., Zhao, G., Zhang, Y., Yao, J., and Han, Y.: Early-Paleozoic geodynamics of the Altai orogen: Constraints from geochemical and zircon U-Pb-Hf isotopic study of paragneissic rocks from the southern Chinese Altai, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1798, https://doi.org/10.5194/egusphere-egu2020-1798, 2020.

Situated at the northwestern end of the India–Asia collision zone, the northeastern Pamir is an important area to explore intracontinental tectonic processes and geodynamic models. In this study, thermochronology is applied to constrain the Late Cenozoic exhumation history of the northeastern Pamir. A new thermochronological data set, combined with previous thermochronological data, suggests that (1) the Late Cenozoic exhumation of the northeastern Pamir began at ~22–18 Ma; (2) the strong crustal contraction in the hinterland of the northeastern Pamir occurred during ~13–10 Ma and ~8–6 Ma; and (3) the east-west extension along the Kongur Shan dome initiated at ~5–3 Ma, and it has resulted in the exhumation of the core of the dome with an average rate of ~2–4 mm/a. I propose that (1) the Early Miocene exhumation of the northeastern Pamir is related to the initiation of the Main Pamir thrust; (2) the first and second stages of the strong crustal contraction are respectively correlated with the northward propagation of the crustal channel flow in the northeastern Pamir and the initial collision between the northeastern Pamir and the Tian Shan; and (3) the east-west extension is driven by the extrusion of the ductile channel flow.

How to cite: Chen, S.: Thermochronological constraints on the Late Cenozoic evolution of the northeastern Pamir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8576, https://doi.org/10.5194/egusphere-egu2020-8576, 2020.

The western and northern sectors of the northward convex Pamir arc are underlain by a steep Benioff zone dipping east to south, traced by earthquakes to depths of 250 km in the southwest and 150 km in the northeast. This slab has been interpreted to indicate intracontinental subduction. However, the convergence accommodated in thrust belts around the western and northern Pamir margins seems to fall short of the values required to produce the observed slab lengths. Delamination models in which the slab only consists of Asian mantle lithosphere avoid that problem but predict shallow asthenosphere beneath the Pamir, conflicting with geophysical evidence. This contradiction is resolved in a forced delamination scenario (Kufner et al. 2016) where indenting/underplating Indian lithosphere forces down and immediately replaces the delaminating Asian lithosphere. In this scenario the formation of the slab would be largely accommodated by south-directed thrust imbrication at crustal level, unrelated to substantial north-vergent thrusting in the Pamir.

Based on published and our own analyses of foreland thrusting we propose that the formation of the slab does to some extent reflect shortening in the Pamir thrust belts. Thin-skinned shortening in the Tajik basin and the External Pamir further north and east decreases northeastward from 150 to 75 and 30(?) km. The slab lengths show a similar trend. Interpreted mimimum shortening values correspond to 60-50 (20?) percent of the slab length on the same transect. With crustal and lithospheric thicknesses taken from seismological data, 70 km of shortening on a translithospheric thrust fault are sufficient to subduct mafic lower crust to asthenospheric depth and probably induce eclogite formation. Rather than the comparison with slab lengths alone, which may be biased by low estimates of shortening, geometrical relations call for additional slab delamination and rollback towards the foreland. The sedimentary cover stacked in the thin-skinned belts restores to at least tens of km of across-strike (N-S) width, underlain by a subhorizontal to gently dipping basal décollement. Basement-involving faults on the internal borders of the thin-skinned belts such as the Darvaz fault and Main Pamir thrust (MPT) must merge with or flatten into this décollement and thus cannot directly connect to the present-day updip end of the slab via a steeply dipping fault. We hypothesize that the Pamir slab was initiated by a translithospheric thrust fault (MPT and equivalents) around 20 Ma and owes at least half of its length to displacement on these faults and imbrication of the sedimentary cover in their footwalls. Delamination and rollback lengthened the slab and displaced it north- and westward. Mantle lithosphere, not necessarily of Indian affinity, contemporaneously moved in from the southeast, preventing the opening of a lithospheric gap and upwelling of asthenosphere.

Reference:

Kufner, S. K. et al. (2016). Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia). Earth and Planetary Science Letters, 435, 171-184.

How to cite: Kley, J., Sobel, E. R., Rembe, J., Voigt, T., Jie, C., Liu, L., and Thiede, R.: Foreland thrusting and slab formation in the Pamir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12959, https://doi.org/10.5194/egusphere-egu2020-12959, 2020.

Rutile is frequently found in metamorphic and less commonly in igneous rocks, as well as sediments derived from the former rock types. It may contain enough U (typically up to ~100ppm) to be dated by U/Pb geochronology. In detrital studies, rutile U/Pb ages supplement zircon U/Pb data, as zircon age peaks often reflect magmatic activity, while rutile U/Pb age peaks can be connected to metamorphic events. Using Zr-in-rutile thermometry, one could also estimate metamorphic facies of the terrane, from which detrital rutile grains are derived. Zircon U/Pb dating provides usually a crystallization age, while rutile gives cooling ages that are dependent on the size of the diffusion domain and its cooling rate. The closure temperature has been estimated at ca. 600°C. A major challenge of rutile U/Pb geochronology is the variable amount of common Pb present and most rutile dating requires the correction for common Pb. A widely used method is the Stacey & Kramers approach, which estimates a formation age for a group of rutile grains and assigns them an age-dependent initial Pb isotope composition from the terrestrial Pb evolution curve (Stacey and Kramers, 1975). We present detrital rutile U/Pb data measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) from Mesozoic and Cenozoic units in the North Pamir in Central Asia. The laser ablation system obtains a time resolved signal of all required isotopes. Using data reduction schemes in Iolite (Paton et al., 2011) and VizualAge (Petrus and Kamber, 2012), the signal is routinely integrated to a single spot age for each ablation pit. Following a similar approach for apatite (Stockli et al., 2017), we subdivided the signal of each single spot into several time-slices and obtained data that crosses diffusion domains or compositional zones within a single rutile grain. Time slices in most cases are aligned along a Discordia in the Tera-Wasserburg diagram, enabling us to calculate a lower intercept age and initial 207Pb/206Pb ratio. We also discuss similarities and differences between these internally corrected ages and the Stacey & Kramers approach-corrected ages.

Paton, C., Hellstrom, J., Paul, B., Woodhead, J., Hergt, J., 2011. Iolite: Freeware for the visualisation and processing of mass spectrometric data. Journal of Analytical Atomic Spectrometry 26 (12), 2508–2518.

Petrus, J.A., Kamber, B.S., 2012. VizualAge: A Novel Approach to Laser Ablation ICP-MS U-Pb Geochronology Data Reduction. Geostandards and Geoanalytical Research 36 (3), 247–270.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26 (2), 207–221.

Stockli, D.F., Boyd, P., Galster, F., 2017. Intra-grain common Pb correction in apatite by LA-ICP-MS depth profiling and implications for detrital apatite U-Pb dating. EGU General Assembly Abstract Volume.

How to cite: Rembe, J., Zhou, R., Sobel, E. R., Kley, J., and Thiede, R.: Time resolved rutile U/Pb data derived from LA-ICPMS – a case study from the North Pamir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4115, https://doi.org/10.5194/egusphere-egu2020-4115, 2020.

The Nankalayuergun fold zone in the North Tarim Basin, NW China, provides an exceptional opportunity for documenting the structural characteristics and evolution of en echelon folds along transpressional fault zone. However, the genetic mechanism of these en echelon detachment folds remains debatable due to poor understanding of the deep structure. Combined with seismic and borehole data, we characterized the geometries and kinematics of Nankalayuergun fold zone, revealed its Cenozoic evolution, and discussed the formation mechanism. The stratified fold zone was geometrically decoupled by salt structures, and the structural style of three salt-influenced folds had individual characteristics due to differences in salt thickness. The timing and strength of Cenozoic deformation of three en echelon detachment folds has a sequential evolution tendency from northwest to southeast. The structural relief of supra-salt fold is the sum of Cenozoic detachment and sub-salt Paleozoic-Mesozoic transpressional folds, indicating that sub- and supra-salt structures are kinematically coupled. Segmentation of Deep Nankayuergun Transpressional Fault (DNTF) can be observed by gravity and seismic data. The supra-salt detachment folds differ from classic echelon structures in that it is only located on the active side of the DNTF. Furthermore, the hinges of the supra-salt folds located right above the sub-salt transpressional fold scarps, corresponding to the reactivation of three DNTF segments. The transpressinoal regimes, sub-salt structures, and the heterogeneity of salt rock are major factors forming the polygenetic echelon detachment folds. The case presented in this study displayed a specific pattern of salt-influenced en echelon structures along transpressional faults and highlighted the influence of pre-exiting structures on the geometry and kinematics of shallow folds, even though salt can decouple sub- and supra-salt deformation.

How to cite: Chen, J. and He, D.: Stratified and polygenetic en echelon detachment folds: Cases for Nankalayuergun fold zone, North Tarim Basin, NW China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8221, https://doi.org/10.5194/egusphere-egu2020-8221, 2020.

Locating Tarim during assembly and breakup of Supercontinent Rodinia remains enigmatic, with different models advocating a Tarim-Australia linkage or a location between Australia and Laurentia at the heart of unified Rodinia. In this study, zircon U-Pb dating results first revealed middle Neoproterozoic sedimentary rocks in the Altyn Tagh orogen, southeastern Tarim. These sedimentary rocks were deposited between ca. 880 and 750 Ma in a rifting-related setting slightly prior to breakup of Rodinia at ca. 750 Ma. A compilation of Neoproterozoic geological records indicates that the Altyn Tagh orogen in southeastern Tarim underwent ca. 1.0-0.9 Ga collision and ca. 850-600 Ma rifting related to assembly and breakup of Rodinia, respectively. In order to place Tarim in Rodinia, available detrital zircon U-Pb ages and Hf isotopes from Meso- to Neoproterozoic sedimentary rocks in relevant Rodinia blocks are compiled. Comparable detrital zircon ages (at ca. 0.9, 1.3-1.1, and 1.7 Ga) and Hf isotopes indicate a close linkage among southeastern Tarim, Cathaysia, and North India, but rule out a North or West Australian affinity for Tarim. In addition, detrital zircons from northern Tarim exhibit a prominent age peak at ca. 830 Ma with minor spectra at ca. 1.9 and 2.5 Ga but lack Mesoproterozoic ages, which are comparable to those from northern and western Yangtze. Together with comparable geological responses to assembly and breakup of Rodinia, a new Tarim-South China-North India connection is inferred in the periphery of Rodinia.

How to cite: Liu, Q.: A Tarim-South China-North India connection in the periphery of Rodinia: Constraints from provenance of middle Neoproterozoic sedimentary rocks in the Altyn Tagh orogen, southeastern Tarim, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2108, https://doi.org/10.5194/egusphere-egu2020-2108, 2020.

The Talas Range forms the northwest part of the Caledonides of the Northern Tian Shan. Based on differences in the structural style, metamorphism and sedimentary successions, three thrust sheets have been identified – the Uzunakhmat, Talas, and Kumyshtag thrust sheets. The Talas and Kumyshtag thrust sheets consist of Neoproterozoic-Ordovician terrigenous and carbonate rock units, whereas the Uzunakhmat thrust sheet consists of Neoproterozoic terrigenous rocks metamorphosed up to greenschist facies. The Uzunakhmat thrust sheet is separated from the Talas and Kumyshtag thrust sheets by the southwest-dipping Central Talas thrust (CTT). The dextral strike-slip Talas-Fergana Fault bounds the Uzunakhmat thrust sheet in the southwest. The main deformation events occurred in the Middle-Late Ordovician.

Structural and strain studies were done along profiles normal to the strike of folds and faults and located in the northwest and southeast parts of the Uzunakhmat thrust sheet. We also incorporate in our study structural profile in the central part of the Uzunakhmat thrust sheet, documented by Khudoley (1993) and Voytenko & Khudoley (2012).

The main strain indicators were detrital quartz grains in sandstones. Rf/φ and Normalized Fry methods were used to identify the amount of strain. Oblate ellipsoids predominate with Rxz values varying mostly from 1,6 to 2,4. Long axes of strain ellipsoids are sub-horizontal with the southeast to east-southeast trend. Similar trends have long axes of the anisotropy magnetic susceptibility ellipsoid being parallel to fold axes, cleavage-bedding intersection and mineral lineation as well as the trend of the major thrusts, including CTT.

The modern shape of the Uzunakhmat thrust sheet is similar to an elongated triangle, pinching out northwest and expanding southeast. Cross-section balancing corrected for the amount of strain shows along-strike decreasing of shortening in the southeast direction. Total shortening varies from 35% to 55% between sections located about 15 km from each other. Such significant variation in shortening corresponds to variation in structural style with much more tight folds and more numerous thrusts for cross-sections with a higher amount of shortening. However, the restored length of all cross-sections is quite similar pointing to the approximately rectangular initial shape of the Uzunakhmat thrust sheet. Our interpretation is that during the Caledonian tectonic events, the Uzunakhmat thrust sheet was displaced in the northwest direction with accompanied thrusting and folding of rock units within the thrust sheet. These deformations formed the modern shape of the thrust sheet in accordance with the amount of shortening detected by cross-section balancing. This interpretation also implies that modern erosion did not significantly affect shape of the Uzunakhmat thrust sheet formed after the Caledonian deformation.

Khudoley, A.K., 1993. Structural and strain analyses of the middle part of the Talassian Alatau ridge (Middle Asia, Kirgiystan). J. Struct. Geol. 6, 693–706.

Voytenko N.V., Khudoley A.K. Structural evolution of metamorphic rocks in the Talas Alatau, Tien Shan, Central Asia: Implication for early stages of the Talas-Ferghana Fault. // C. R. Geoscience. 2012. V. 344. P. 138–148.

How to cite: Kushnareva, A., Moskalenko, A., and Pasenko, A.: Structure, strain and AMS of the Uzunakhmat thrust sheet (Talas Range, Kyrgyz North Tian Shan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-726, https://doi.org/10.5194/egusphere-egu2020-726, 2020.

Uplifting mechanisms for the Tien Shan, an active intracontinental orogenic belt, have been under debate for decades, a key issue being how the convergence has been accommodated at depth. Here we investigate the Moho structure across the central Tien Shan by common-conversion-point imaging and H-k-c stacking of receiver functions from a dense array. The observed Moho exhibits distinct characteristics among sub-blocks. Southward-dipping diffuse Moho is imaged in the Southern Tien Shan (STS), in contrast with the relatively flat and sharp Moho beneath the Tarim Basin. This feature along with the large Moho offset beneath the South-Boundary Fault suggests that the shortening and thickening of Tien Shan crust rather than the underthrusting of the Tarim Basin are responsible for the uplift of the STS. In the Northern Tien Shan, however, the imaged Moho doublet provides direct evidence for the underthrusting of the Kazakh Shield accommodating the convergence there.

How to cite: Bao, X., Zhang, B., and Xu, Y.: Distinct Orogenic Processes in the South- and North-Central Tien Shan from Receiver Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20754, https://doi.org/10.5194/egusphere-egu2020-20754, 2020.

      The Tibetan Plateau, resulting from the active Eurasian-India collision, presents a major scientific challenge in understanding its growth and propagation. One key region is the Longmen Shan mountain belt in western Sichuan, which forms the steepest margin of the plateau and has been active as demonstrated by the Mw 7.9 Wenchuan (2008) and Mw 6.6 Lushan (2013) earthquakes. Tectonic history of the Longmen Shan belt and the neighboring Songpan-Garze terrane, however, began in the Triassic Indosinian orogenesis, which complicates the geologic records. But the major thickening of Tibet was formed in Himalayan orogenesis. Therefore, quantitative constraints on the pre-Tertiary tectonic evolution of the region are crucial in delineating Himalayan geodynamics. In this study, the raman spectroscopy of carbonaceous material (RSCM) geothermometer is applied to the metasediments of the Longmen Shan and Sonpan-Garze terrane to obtain their peak metamorphic states. Combining existing metamorphic, geochronologic and thermochronologic data, better rock thermal histories may be reconstructed, providing insights to the structure and development of the orogenic system.

      In this study, 50 samples were collected in eastern margin of Tibetan Plateau along several transects in NW-SE direction, perpendicular to the structural grain of the Longmen Shan and into the Songpan-Garze terrane. Together with existing data, distribution of the peak temperatures from RSCM analyses is not correlated to later igneous intrusions, ruling out significant contact metamorphism overprint. Along the WenChuan Fault, the Songpan-Garze terrane is of higher grade than the Longmen Shan, indicating it is a major reverse shear zone. The rather high RSCM temperatures (over 500 °C) acquired from Songpan-Garze metasediments are inconsistent with past models as remnants of a classical accretionary prism; the complex wedge kinematics involving significant basal accretion observed in the slate belt of Taiwan orogen may give clues in reconstructing the structure and evolution of eastern Tibetan Plateau.

Keywords: Tibetan Plateau; Longmen Shan; RSCM geothermometer

How to cite: Chang-Chien, W.-C., Chen, C.-T., and Tan, X.-B.: Perliminary thermal metamorphic constraints on tectonic evolution in the eastern margin of Tibetan Plateau: lessons from the slate belt of Taiwan?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2114, https://doi.org/10.5194/egusphere-egu2020-2114, 2020.

The Beishan Orogenic Belt plays an important role in understanding the Paleozoic tectonic evolution of the Central Asian Orogenic Belt and the final closure time of the Paleo-Asian Ocean. However, although numerous geochronologic, geochemical, and isotopic data have been obtained, no consensus has been reached yet on the Early Permian tectonic setting for this region and, thus, the final closure time of the Paleo-Asian Ocean, mainly because of the nonuniqueness of the interpretations deduced from such data base. Therefore, other methods are urgently needed to provide more constraints from different perspectives. We present here a paleomagnetic study on the Gubaoquan doleritic dike swarm in the South Beishan area. Thermo-magnetic experiments and room-temperature hysteresis loops reveal that single-domain and multi-domain magnetite is the principal carrier of remanence. Anisotropy of magnetic susceptibility of studied dikes shows a horizontal magnetic foliation with a magnetic lineation generally parallel to the dikes’ strike. Plagioclase ⁴⁰Ar/³⁹Ar dating result of one dolerite sample collected from the margin of a 10m-thick dike provides a cooling age at 300~284 Ma. Scanning electronic microscope observation coupled with energy-dispersive X-ray spectrometry shows that the remanence carrier is mainly euhedral without evident chemical alteration nor secondary mineral formation. Characteristic remanent magnetizations are successfully isolated from twenty dikes, and pass baked contact test. According to Deenen et al. (2011) statistical criteria, the distribution of the remanence directions reflects the contribution from paleosecular variation of the geomagnetic field. Taking all data together, the Gubaoquan doleritic dike swarm probably preserves a primary remanence. Consequently, an Early Permian paleomagnetic pole for the South Beishan can be calculated at λ = 80.2°N, φ = 300.3°E, A₉₅ = 5.3° and N = 20. Comparisons of this new result with published ones from neighboring blocks bring us following implications for the tectonic evolution of the SW CAOB: 1. Neither relative latitudinal movement nor relative rotation can be paleomagnetically detected among Yili, Turpan-Hami, and South Beishan since the Early Permian. 2. Significant relative rotations have taken place between South Junggar and Tarim with respect to South Beishan-Turpan-Hami-Yili, respectively, since the Early Permian, corresponding to large-magnitude strike-slip displacements along mega-shear zones. 3. No obvious relative latitudinal movement has occurred between South Beishan and its neighboring blocks (Tarim, South Junggar, Yili, Turpan-Hami, and Dunhuang) since the Early Permian, combined with other evident, suggesting that the Paleo-Asian Ocean probably have closed before the Early Permian, and South Beishan was in a rift setting in the Early Permian.

How to cite: Zhu, X., Chen, Y., Wang, B., Scaillet, S., Faure, M., and Ni, X.: Early Permian Paleomagnetic Result from the South Beishan (NW China) and Its Implications for the Tectonic Evolution of the SW Central Asian Orogenic Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20677, https://doi.org/10.5194/egusphere-egu2020-20677, 2020.

Craton is the stable unit of the lithosphere. The cratonic basin is thus the sedimentary basin developed upon craton. It has long been recognized as a kind of basin characterized by minor tectonic deformation and stable architecture. With the increasing evidences in the recent years, it is noticed that it has much more mobility, and is controlled not only by the lithospheric plate movements but also by the deep mantle activation. To explore the mobile behaviour of cratonic basin is an important window to address the intra-continental deformation mechanism. Taking the Ordos basin as an example, based on the new deep boreholes, the high-resolution seismic reflection profiles, cores, and the outcrops around the basin, the paper establishes the chronology of tectonic movement around the Ordos basin utilizing the integrated method of the isotopic dating, the bio-stratigraphy, and the sequence stratigraphy. It shows that, the basin developed the ten regional unconformities, underwent multi-period volcanic activities during the Middle Proterozoic, the late Early Paleozoic, the Late Triassic, and the Early Cretaceous. It was subjected to multi-stage compression, such as the Late Ordovician to Devonian, the Late Triassic, the Late Jurassic to Early Cretaceous, and the Neogene to Quaternary. Upon the crystalline basement of the Archaean and the Lower Proterozoic, the basin underwent five distinct extension-compression cycles, such as the extension in middle Proterozoic and compression in late Proterozoic, the extension in Cambrian to early Ordovician and compression in late Ordovician to Devonian, the extension in Carboniferous to middle Triassic and compression in late Triassic, the extension in early to middle Triassic and compression in late Jurassic to Cretaceous, and the extension in Paleogene and compression in Neogene to Quaternary, with a charter of a much longer period of the earlier cycle and a shorter period of the later cycle, and a longer period of extension and a shorter period of contraction in each cycle. The extension-compression cycle controlled the formation and evolution of the Ordos oil and gas super basin.

How to cite: He, D.: Chronology of Tectonic Movement of Cratonic Basin: Insight from New Evidences from Ordos Basin, North China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6404, https://doi.org/10.5194/egusphere-egu2020-6404, 2020.