TS7.7

EDI
Central Asian Tectonics - Pamir, Tian Shan and Tibet from Paleozoic to Present

The mountain ranges of the Pamir, Tian Shan, and the Himalaya-Tibetan orogen form the most prominent morphological features in central Asia. Much of this morphology results from uplift related to the Cenozoic India-Asia collision. However, this is built upon a complex pre-Cenozoic history of ocean closures (Proto- and Paleo-Tethys, Paleo-Asian), terrane accretions and the related reorganization of Asia's southern margin. This long-lasting history of consecutive accretionary events left behind a complex mosaic of high- and low-strain domains, magmatic arcs, allochthonous blocks (terranes) and intervening suture zones. A significant challenge is to correlate and date those domains, which are often used as large-scale structural markers for quantifying large structural offsets. Quantifying pre-collisional topography and crustal thickness is crucial. Both the pre-Cenozoic history and the timing and kinematics of young deformation must be well-constrained in order to reconstruct the orogenic evolution in time and space and to understand how pre-existing structures influenced Cenozoic deformation. To promote discussion on this topic, we invite contributions from geoscientists who are working on various aspects of the geologic evolution of Central Asia, including structural geology, geochemistry, sedimentology, detrital studies, as well as geophysical or modeling studies.

Co-organized by GD6/GM9/GMPV11/SSP2
Convener: Johannes RembeECSECS | Co-conveners: Jonas Kley, Yani Najman, Ed Sobel, Rasmus Thiede
vPICO presentations
| Thu, 29 Apr, 11:00–12:30 (CEST)

Session assets

Session materials

vPICO presentations: Thu, 29 Apr

Chairpersons: Johannes Rembe, Rasmus Thiede, Jonas Kley
11:00–11:05
11:05–11:07
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EGU21-4086
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ECS
Anastasia Kushnareva, Andrey Khudoley, Dmitriy Alexeiev, and Eugeny Petrov

The Mesoproterozoic Karadjilga pluton is a poorly studied fragment of the North Tianshan microcontinent located in the western Central Asian Orogenic Belt. Metasedimentary rocks surrounding the pluton consist of marbles and mica schists of the Mesoproterozoic Ortotau Group. These rocks constitute a major west-northwest trending syncline with steep to subvertical limbs. The hinge of the fold is well expressed in the west part of the syncline and plunges east with 30-40° angle of plunge. Eastern termination of the syncline is cut by faults. Granitoid gneisses and granites of the Karadjilga pluton crop out in the core of the syncline. The contacts of the pluton are sub-parallel to bedding and schistosity in surrounding rocks. Primary magmatic contacts are locally reworked by reverse faults and thrusts. Our detailed mapping and structural study revealed inhomogeneous deformation of rocks of the Karadjilga pluton. The following rock types are identified: 1) undeformed granite 2) foliated granite 3) granite-gneiss and 4) mylonite. Undeformed granites form <25-30% of total volume of the pluton and are most widespread in the northeast part of the pluton. On some geological maps they are shown as Ordovician or Devonian. However, U-Pb dating of 9 zircon grains by SHRIMP-II (VSEGEI, St. Petersburg, Russia) yielded a 1125±5 Ma concordant age. It agrees with previously reported U-Pb SHRIMP ages for deformed granites and gneisses (Degtyarev et al., 2011; Kröner et al., 2013) and indicates that undeformed granites belongs to the same Mesoproterozoic magmatic complex. Foliated granites and gneisses prevail and constitute up to 60-70% of total volume. They form west-northwest trending zones alternating with mylonites or undeformed granite. Mylonites are subordinate and occur mainly along the contacts of the pluton. Shear zones seem to be approximately parallel to the schistosity of deformed granites, but their geometry needs more study and mapping. Shear-sense indicators were studied in the oriented thin sections and are represented mainly by sigma and delta structures and oblique foliation with rare folds and other indicators. In all but one sample only strike-slip displacement has been identified. In the northern part of the pluton sinistral displacement predominates, whereas dextral displacement prevails in the southern part of the pluton. Shear zones are most widespread on the margins of the Karadjilga pluton, but locally also occur in the central part of the pluton, where they form narrow west-northwest trending zones. According to shear-sense indicators, displacement within the Karadjilga pluton occurred mainly in the approximately west-east direction that strongly differs from the north-south sense of displacement in the Paleozoic thrust and fold belts of Tianshan.

The study was supported by the RFBR project 20-05-00252.

How to cite: Kushnareva, A., Khudoley, A., Alexeiev, D., and Petrov, E.: Structure and shear-sense indicators of the Mesoproterozoic basement of the North Tianshan microcontinent: Example of granitoid gneisses of the Karadjilga pluton, NW Kyrgyzstan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4086, https://doi.org/10.5194/egusphere-egu21-4086, 2021.

11:07–11:09
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EGU21-10663
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ECS
Yujia Song, Xijun Liu, Zhiguo Zhang, Pengde Liu, and Yao Xiao

The Central Asian Orogenic Belt (CAOB), also known as the Altay orogenic belt, is the largest accretionary orogenic belt in the world. It is situated between the Eastern European, Siberian, Tarim, and North China cratons. The CAOB is a large and complex suture zone formed by amalgamation of diverse geologic units including several microcontinents, ophiolites, island arcs, seamounts and accretionary wedges. The evolution of the Precambrian basement in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB as well as the evolution of Rodinia supercontinent. The Tianshan block, an important part of the CAOB, is located in the southwestern CAOB, and subdivided from north to south into North Tianshan, Central Tianshan-Yili blocks, and South Tianshan. The Central Tianshan block, located between the Tarim block, the Junggar block and the Kazakhstan block, is one of numerous microcontinental block within the CAOB that overlie Precambrian basement rocks. Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB, and its place within the Rodinia supercontinent. However, to date, the timing and tectonic settings in which the basement rocks in the Central Tianshan formed are poorly constrained, with only sparse geochemical and geochronological data from granitic rocks within the central segment of the belt. Here, we present a systematic study combining U-Pb geochronology, whole-rock geochemistry, and the Sr-Nd isotopic compositions of newly-identified granites from the Bingdaban area of Central Tianshan. The analyzed samples yield a weighted mean Neoproterozoic 206Pb/238U ages of 975-911 Ma. All have affinities with calc-alkaline, weakly-peraluminous, magnesian I-type granites. The samples are enriched in LREE, display relatively flat HREE patterns with negative Eu anomalies, and show a depletion in the high field strength elements (HFSEs) Nb, Ta, and Ti and enrichment in large ion lithophile elements (LILEs) Rb, U, Th and Nd geochemical characteristics indicative of subduction-related magmatism. All samples show initial (87Sr/86Sr)(t) ratios between 0.705136 and 0.706745. Values for ƐNd(t) in the granites are in the range -1.2 to -5.7, corresponding to Nd model ages of 1.6-2.1 Ga, indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths. The documented geochemical features indicate the protoliths for the granites had a similar petrogenesis and magmatic source, which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material. The Tianshan Block probably constituted part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the early Neoproterozoic, and which underwent a transition from subduction to syn-collision compression at 975-911 Ma. This study reveals that crustal reworking may played a key role in Neoproterozoic crustal evolution in the Central Tianshan block and this block has a tectonic affinity to the Yili block.

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).

How to cite: Song, Y., Liu, X., Zhang, Z., Liu, P., and Xiao, Y.: Neoproterozoic I-type granites geochronology and geochemistry of the Chinese Central Tianshan Block, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10663, https://doi.org/10.5194/egusphere-egu21-10663, 2021.

11:09–11:11
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EGU21-10786
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ECS
Yao Xiao, Xijun Liu, Zhiguo Zhang, Yujia Song, and Pengde Liu

The Central Asian Orogenic Belt (CAOB), is the largest proliferative orogenic belt in the phanerozoic, located between Siberia and the Tarim north China plate. Its tectonic evolution is closely related to the evolution of the ancient Asian Ocean. The CAOB has an intimate connection with the evolution of Paleo-Asian Ocean (PAO)  which experienced geodynamic processes like seamounts accretion, ridge-trench interaction, the constitution of back-arc basins. Since the Paleozoic era, the PAO has undergone expansion, subduction and closure, and finally formed the current Central Asian orogenic belt. The West Junggar, located in the southwest of the Central Asian orogenic belt, is an accretive Mosaic body on the southern edge of the Siberian Craton. It is an important part of the Palaeozoic orogenic collage of the CAOB, and a composite terrane composed of island arcs, ophiolites, seamounts and a key area for the study of the tectonic evolution of The Central Asian orogenic belt during the Paleozoic era. The ophiolite mélange zone in Karamay and the carboniferous siliceous calcite with great thickness jointly indicate the existence of the late Paleozoic residual ocean basin in Junggar area. This paper presents new zircon geochronolgy and whole rock major and element, and Sr-Nd isotope data for mafic rocks in the Baijiantan ophiolitic mélanges.

The studying area is located in the northeast part of Karamay city, In the substratum of metamorphic peridotite serpentine, the pyroxenite, gabbro, jasper and radiolarite blocks of different sizes are distributed, and the edge of the blocks is fragmented and in contact with the matrix structure. The Baijiantan ophiolitic mélange is covered by a set of late Carboniferous volcanic-sedimentary tectonic unconformities .

The magmatic zircons from a anorthosite in Baijiantan ophiolite yield concordia U–Pb isotope age of 370.1±1.2Ma, which is interpreted as the crystallization age of the anorthosite. The mafic rocks of Baijiantan ophiolite are geochemically belong to tholeiitic basalts with low SiO2 contents as well as relatively depleted in light rare earth element (LREE) and flat in heavy rare earth element (HREE), while the high-field strength elements (Nb and Ta) display a weak depletion. thus they have a N-MORB-type characteristics. which is similar to those of basalts from back-arc basin. The (87Sr/86Sr)i of Baijiantan ophiolite range from 0.704567 to 0.705172, and they have positive εNd(t) with from +8.23 to +8.81, indicating they were derived from a depleted MORB-type mantle source.

To sum up, the Baijiantan ophiolite in the western Junggar was formed in the late Devonian. The mafic rocks are characterized by MORB type of basaltic magma. Their Sr-Nd isotopic compositions indicate they were derived from a depleted asthenospheric mantle, all of these features are similar to the back-arc basin basalts. Thus, we suggest the Baijiantan ophiolite was possibly formed in the back arc oceanic basin in the late Devonian.

Acknowledgments:This work is granted by the National Natural Science Foundation of China (Grant No. 41772059), CAS "Light of West China" Program (2018-XBYJRC-003), Guangxi National Natural Science Foundation (Nos. 2018GXNSFFA281009) and Bagui Scholar Innovation Project of Guangxi Province.

How to cite: Xiao, Y., Liu, X., Zhang, Z., Song, Y., and Liu, P.: Tectonic setting and geochronology of  Paleo-Asian Baijiantan Ophiolite in West Junggar, NW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10786, https://doi.org/10.5194/egusphere-egu21-10786, 2021.

11:11–11:13
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EGU21-11906
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ECS
Pengde Liu, Xijun Liu, Zhiguo Zhang, Yujia Song, Yao Xiao, and Dechao Li

    The subduction and closure of the Paleo-Asia Ocean generated the Central Asian Orogenic Belt (CAOB), which extends from the Urals in the west through Kazakhstan, northwestern China, Mongolia, and northeastern China to the Russian Far East. It is generally accepted that the CAOB comprises a complicated and varied collage of terranes, including island arcs, ophiolites, accretionary prisms, seamounts, and microcontinents. The CAOB is the world’s largest accretionary orogen and is also considered a type area for studying Phanerozoic continental growth. The accretionary processes of the orogen might have resulted from either the progressive duplication of a single and long-lived island-arc system or the collision of several island arcs and micro-continents, similar to the complex archipelago systems in the modern southwestern Pacific. West Junggar is located in a key area of the CAOB, has been a focus of studies of the tectonic evolution and crustal growth of the orogenic belt. West Junggar has been considered by some geologists as a paleo-Asian intra-oceanic subduction system, whereas others have variously argued that West Junggar was formed by single subduction, arc–arc collision, or ridge subduction, or by post-collisional processes after the early Carboniferous. An understanding of the Carboniferous tec-tonic setting is critical for determining the evolution of West Junggar. A series of early Carboniferous volcanic and intrusive rocks occur in the southern West Junggar. Our new zircon U–Pb geochronological data reveal that diorite intruded at 334.1 ± 1.1 Ma, and that basaltic andesite was erupted at 334.3 ± 3.7 Ma. These intrusive and volcanic rocks are calc-alkaline, display moderate MgO (1.62–4.18 wt.%) contents and Mg# values (40–59), low Cr (14.5–47.2 ppm) and Ni (7.5–34.6 ppm) contents, and are characterized by enrichment in light rare-earth elements and large-ion lithophile elements and depletion in heavy rare-earth elements and high-field-strength elements, meaning that they belong to typical subduction-zone island-arc magma. The rocks show low initial 87Sr/86Sr ratios (0.703649 to 0.705008), positive ƐNd(t) values (+4.8 to +6.2, mean +5.4), and young TDM Nd model ages ranging from 1016 to 616 Ma, indicating a magmatic origin from depleted mantle involving partial melting of 10%–25% garnet and spinel lherzolite. Combining our results with those of previous studies, we suggest that these rocks formed as a result of northwestward subduction of the Paleo-Asian Junggar oceanic plate, which caused partial melting of sub-arc mantle. We conclude that intra-oceanic arc magmatism was extensive in southern Paleo-Asian Ocean during the early Carboniferous.

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).

How to cite: Liu, P., Liu, X., Zhang, Z., Song, Y., Xiao, Y., and Li, D.: Early Carboniferous Paleo-Asian oceanic plate subduction: Implications from geochronology and geochemistry of early Carboniferous magmatism in southern West Junggar, NW China , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11906, https://doi.org/10.5194/egusphere-egu21-11906, 2021.

11:13–11:15
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EGU21-10555
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ECS
Zhiguo Zhang, Xijun Liu, Pengde Liu, Yujia Song, Yao Xiao, and Dechao Li

Sanukitoid is a type of high-Mg andesite that is distinct from typical andesite in being characterized by elevated MgO contents and/or Mg#[=100* Mg/(Mg + Fe)]. They represent rare mantle-derived rocks that are preserved in both modern and Archean subduction settings, as well as in accretionary orogenic belts. The Central Asian Orogenic Belt (CAOB) is a giant accretionary orogen and the most important area of Phanerozoic continental growth around the world. It is evolved through a long-lived orogeny involving multiple episodes of subductions and accretions marking a major phase of continental growth during the Paleozoic. The West Junggar is an important component within the core of the CAOB, and is located at the junction between the Siberian, Kazakhstan and Tarim blocks. The rocks in West Junggar preserve the amalgamation of the southern CAOB, and are subdivided into northern and southern parts by the Xiemisitai Fault. The study of Carboniferous magmatism in northern West Junggar plays an important role in understanding the tectonic evolution of that part of the Central Asian Orogenic Belt. In this study, we present petrology, zircon U–Pb geochronology, mineral and whole-rock geochemistry, and the Sr–Nd–Hf–Pb isotope compositions of volcanic rocks from the Hamutusi area of northern West Junggar. LA–ICP–MS zircon U–Pb analysis of a representative andesite yielded an early to late Carboniferous age of 324.4±6.9Ma. The volcanic rocks are calc-alkaline, with high SiO2 (58.10–59.01 wt%), MgO (6.09–6.99 wt%), Mg# (60.7–62.2), Cr (147–403 ppm), and Ni (29–119 ppm) contents, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE), These characteristics are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. All samples have radiogenic initial Sr and Pb isotopic compositions, and low εNd(t) and εHf(t) values, indicating the sanukitoids were generated by partial melting of subducting sediments in which the melts interacted with the mantle. Geochemical modeling calculations indicate a proportion of 3-10% sediment melt and slab-derived fluids were mixed with the depleted mantle to produce the bulk of the Hamutusi rocks. We conclude that the studied rocks from Northern West Junggar record the transition from normal subduction to subduction of young and hot oceanic lithosphere between the early and late Carboniferous. 

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003)

How to cite: Zhang, Z., Liu, X., Liu, P., Song, Y., Xiao, Y., and Li, D.: Petrogenesis of late Carboniferous sanukitoids from northern West Junggar of China in the Central Asian Orogenic Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10555, https://doi.org/10.5194/egusphere-egu21-10555, 2021.

11:15–11:17
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EGU21-9516
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ECS
Johannes Rembe, Edward R. Sobel, Jonas Kley, Renjie Zhou, Rasmus Thiede, and Jie Chen

A lateral continuity between belts of mafic and ultramafic Paleozoic rocks found in the West Kunlun of Northern Tibet and comparable rocks, known from an outcrop in the Chinese North Pamir, has long been proposed. This led to the concept of an originally generally straight, E–W trending Oytag–Kudi suture zone. In turn, this paleogeographic model formed a key constraint for the hypothesis, that the Pamir has indented 300 km northward with respect to Tibet during the Cenozoic. We show, that the arc volcanic rocks found in the North Pamir are distinguishable from the units known from the West Kunlun.
The North Pamir is dominated by Paleozoic arc volcanic rocks. We present new geochemical and geochronological data to give a holistic view of an early to mid-Carboniferous arc complex. This belt was previously identified as an intraoceanic arc in the northeastern North Pamir. Our data yields evidence for a gradual lateral change towards the west into a Cordilleran-style arc in the Tajik North Pamir. Large leucocratic granitoid intrusions are hosted in part by Devonian to Carboniferous oceanic crust and the metamorphic Kurguvad basement block of Ediacaran age (maximum deposition age) in Tajikistan. LA-ICP-MS U-Pb dating of zircons, together with whole rock geochemistry derived from tonalitic to granodioritic intrusions, reveal a major Visean to Bashkirian intrusive phase between 340 and 320 Ma ago.
The West Kunlun experienced two major intrusive phases, connected with arc-volcanic activity — a first phase during Proto-Tethys closure in Ordovician and Silurian times and a second phase connected to the Triassic Paleo-Tethys closure. The Carboniferous arc-volcanic phase in the North Pamir clearly postdates Paleozoic arc-magmatic activity in the West Kunlun by ~100 Ma. This observation, along with geochemical evidence for a more pronounced mantle component in the Carboniferous arc-magmatic rocks of the North Pamir, disagrees with the common model of a continuous Kunlun belt from the West Kunlun into the North Pamir. Moreover, Paleozoic oceanic units younger than and west of Tarim cratonic crust challenge the idea of a continuous cratonic Tarim-Tajik continent beneath the Pamir.

How to cite: Rembe, J., Sobel, E. R., Kley, J., Zhou, R., Thiede, R., and Chen, J.: No continuous suture between Kudi and Oytag: new evidence from geochronology and geochemistry data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9516, https://doi.org/10.5194/egusphere-egu21-9516, 2021.

11:17–11:19
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EGU21-12484
Jonas Kley, Thomas Voigt, Edward R. Sobel, Johannes Rembe, and Chen Jie

The ca. 35 km long, N-S-trending Altyn Darya valley in Kyrgyzstan exposes a nearly complete cross-section of the External Pamir thrust belt (EP), extending from the active Pamir Frontal Thrust in the north to the Main Pamir Thrust (MPT) and some distance into its hanging-wall. The EP comprises a northward imbricated stack of Carboniferous to Late Neogene rocks. From north to south, young clastics of the Alai Valley foreland basin are overthrust by an intensely folded and thrust-repeated frontal stack of Upper Cretaceous to Paleogene limestone, shale and evaporite. Lower Cretaceous red sandstones first emerge above north- and south-verging thrusts forming a triangle zone whose core comprises spectacular isoclinal folds in Upper Cretaceous strata. Towards the south, another thrust imbricate of Lower Cretaceous is overthrust by Late Triassic-Jurassic sandstones and mafic volcanics which are themselves overthrust by an internally deformed, Carboniferous to Triassic succession of, from bottom to top, greywacke and shale, limestone, volcanoclastic conglomerates, variegated sandstone-shale and pink conglomerates. The Carboniferous units in the south are truncated by the MPT which emplaces a succession of greenschist, marble and chert overlain by a km-thick sequence of metamorphosed and deformed, pillow-bearing lavas of Carboniferous age. Structural geometries and fault preference indicate that the basal detachment of the EP deepens southward very gently, stepping down from a detachment in Upper Cretaceous shale to another one near the base of the Lower Cretaceous and eventually a third one in Triassic shale. Cross-section balancing suggests minimum shortening of 75 km for units in the MPT´s footwall. The displacement on the MPT is poorly constrained due to eroded hanging-wall cutoffs, but must exceed 15 km. The basal detachment cuts into basement no earlier than 100 km from the present thrust front, too far south to link up with the top of the Pamir slab.

The stratigraphic succession exposed in Altyn Darya can be readily correlated with less deformed and less metamorphosed transects in westernmost China (Qimgan and Kawuke), some 250 km to the east. A marble-greenschist sequence similar to that carried on the MPT in Altyn Darya has been identified there as a tectonic nappe of the Karakul-Mazar unit, emplaced from the south already in an Upper Triassic to Lower Jurassic (Late Cimmerian) event. If the correlation is correct, then the MPT had a Mesozoic precursor structure extending over much of the E-W striking segment of the Northern Pamir.

How to cite: Kley, J., Voigt, T., Sobel, E. R., Rembe, J., and Jie, C.: Transect across the External Pamir thrust belt and Main Pamir Thrust along the Altyn Darya valley, Kyrgyzstan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12484, https://doi.org/10.5194/egusphere-egu21-12484, 2021.

11:19–11:21
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EGU21-10405
Edward R. Sobel, Rasmus Thiede, Paolo Ballato, Konstanze Stübner, Jonas Kley, Johannes Rembe, Mustafo Gadoev, Ilhomjon Oimahmadov, and Manfred Strecker

The Pamir forms the northwestern tail of the Tibetan plateau and is a first-order tectonic feature of the Cenozoic Indo-Eurasian collision. The nature of the topographic uplift and orogenic growth of the entire northwestern margin of the Pamir is poorly constrained; however, this history can provide important constraints that are required to test geodynamic models of the tectonic evolution of the Pamir. Here we focus on the uplift history of the western and northwestern unglaciated margin of the Northern Pamir, the Darvaz and the Peter-the-First Ranges. These three ranges were formed by three major fault systems: the Main Pamir Thrust (MPT), the Darvaz and the Vakhsh fault zones (DFZ, VFZ). To assess the impact of tectonic uplift on the geomorphic evolution, we analyzed geomorphic characteristics of the topography, the longitudinal river profiles and the relief. To better constrain the regional crustal cooling history and uplift, we obtained thermochronologic cooling ages from the three regions.

We present 19 new zircon (U-Th-Sm)/He (ZHe) ages, 7 apatite fission track (AFT) ages, and 4 apatite (U-Th-Sm)/He (AHe) ages, ranging between >200 and 4 Ma, 14 and 4 Ma, and 15 and 3 Ma, respectively. The three units are characterized by unique Neogene cooling pathways, suggesting that they exhumed independently.

We discovered extensive low-relief landscapes with Neogene sedimentary cover uplifted ~2 km in elevation above the present-day regional base level. Our analysis indicates that the Panj and Vakhsh rivers form the regional base levels for the river network draining the entire northern and western margin of the Pamir. In the hanging wall of DFZ, the Paleozoic bedrock is characterized by significant relief (>1 km), the Neogene cover onlaps directly onto this Paleozoic bedrock. The tributary rivers crossing these landscapes are characterized by gentle, concave upstream longitudinal profiles at high elevation. These are interrupted by major knickpoint zones and steep downstream segments draining towards the deeply incised Panj and Vakhsh rivers. This indicates that the Darvaz Fault hanging wall had been uplifted and eroded prior to deposition of upper Neogene sediments, suggesting that the DFZ has a prolonged Neogene slip history. In contrast to the northeastern Pamir, here, the MPT-hanging-wall is characterized by reset late Oligocene-Early Miocene ZHe cooling ages ranging between 26 and 17 Ma. AFT and AHe-ages between 15 and 13 Ma suggest that exhumation suddenly terminated during the middle Miocene. In contrast, Jurassic sandstones exposed near the DFZ yield mostly un-reset Triassic-Jurassic ZHe ages (~250-170 Ma), a reset AFT age of ~5 Ma and a 2.5 Ma AHe age. Within the Peter-the-1st-Range, we obtained fully reset ~ 5 Ma ZHe ages, and ~4 Ma AFT ages. The rapid cooling trends since at least ~5 Ma suggest that deformation and a significant portion of crustal shortening propagated into the Tadjik foreland basin, causing enhanced uplift and erosion of the hanging wall of the VFZ and related faults. This deformation triggered ~2 km uplift of the entire northwest Pamir, recorded in uplifted paleo-landscapes and dissected tributaries of the Panj and Vakhsh rivers.

How to cite: Sobel, E. R., Thiede, R., Ballato, P., Stübner, K., Kley, J., Rembe, J., Gadoev, M., Oimahmadov, I., and Strecker, M.: Uplift and growth of the northwest Pamir, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10405, https://doi.org/10.5194/egusphere-egu21-10405, 2021.

11:21–11:23
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EGU21-9901
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ECS
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Highlight
Lin Li, Guillaume Dupont-Nivet, Pierrick Roperch, Yani Najman, Mustafa Kaya, Niels Meijer, and Jovid Aminov

Contrasting models have been proposed to explain the formation of the Pamir salient: either largely inherited from a Mesozoic arcuate structure or recently formed by Indian northward indentation and possibly related to syn-orogenic lateral extrusion. The vertical-axis counterclockwise rotations observed in the Tajik Basin are key constraints on testing these models, but the timing of these rotations remains hindered by poor age control on the basin sediments. We report a combined analysis of vertical-axis rotation and magnetostratigraphic dating of a long sedimentary section in the eastern Tajik Basin, which yields strong counterclockwise rotations (~56°) in early Late Cretaceous to late Miocene strata. This result suggests that rotation in the Tajik Basin occurred after ~8 Ma, much later than previously suggested. Combining with a regional compilation of previous paleomagnetic studies as well as structural and GPS constraints including Pamir and Tarim, we explore potential implications on models of the Pamir salient. We infer that after 8 Ma (probably even later), the Pamir (North, Central, and South) began to overthrust west- and northwest-ward, causing counterclockwise rotations in the Tajik Basin. This reconstruction allows for ~150 km of post-8 Ma northwestward indentation into the Tajik Basin, in agreement with coeval underthrusting of the Indian mantle lithosphere into Asia.

How to cite: Li, L., Dupont-Nivet, G., Roperch, P., Najman, Y., Kaya, M., Meijer, N., and Aminov, J.: Large recent counterclockwise rotations in the Tajik Basin and implications on the Pamir salient formation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9901, https://doi.org/10.5194/egusphere-egu21-9901, 2021.

11:23–11:25
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EGU21-2552
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ECS
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Sabrina Metzger, Łukasz Gągała, Lothar Ratschbacher, Bernd Schurr, Milan Lazecky, and Yasser Maghsoudi Mehrani

Embedded between the South Tian Shan in the north, the Pamir in the east, and the Hindu Kush in the south, the Tajik basin is a remnant of the Mesozoic-Miocene Tajik-Tarim basin. Since ~12 Ma, ~E-W shortening has been dominating due to the westward collapse of the north-advancing Pamir-plateau, inverting the basin into a thin-skinned, W-convex fold-and-thrust belt detached on Upper Jurassic evaporites. The detachment depth is ~6-8 km b.s.l. under most of the basin, shallowing north towards the Tian Shan. Geologic cross sections yield a maximum of 150 km of E-W shortening, distributed between foreland- and hinterland-vergent fold and thrusts. From the eastern to the western rim of the basin, sparse global positioning (GNSS) rates decay from ~15 mm/yr WNW to 2 mm/yr NNW. Seismicity highlights dextral shear along the ~E-striking Ilyak fault – bounding the basin in the north –, and distributed E-W shortening in the central and eastern Tajik basin and in the foothills of the Hindu Kush. The majority of seismic events occurs below the evaporitic detachment. In 1907, the region was struck by a Ms7.6±0.3 earthquake with a poorly-constrained epicenter, either at the northwestern rim of the basin or more than 200 km farther east at the Pamir’s rim.

We present rate maps of the region obtained from Sentinel-1 radar interferometric (InSAR) time-series. The underlying data-base comprises 900+ radar scenes, acquired over 2-4.5 years in two view angles (LOS) on 13 frames. The initial LiCSAR interferograms1) and tropospheric delay maps2) were created automatically. The LOS rate maps resulting from a small-baseline inversion (LiCSBAS) were Gaussian-filtered both in space and time. Before decomposition to east and vertical rates, the rate maps were tied to a Eurasian-stable GNSS reference frame. The final products span from the western basin to the eastern Pamir, and from the southern edge of the Tian Shan to the northern Hindu Kush, covering an area of 270 000 km2 with a spatial sampling of ~400 m.

The most reliable results were obtained in the Tajik basin, where the rate maps unveil a combination of basin-scale tectonics, localized halokinesis, effects of extensive irrigation, and seasonal precipitation. Our key findings are: (1) The Tajik basin infill is largely being displaced west as a result of the western collapse of the Pamir. The westward rates decrease away from the Pamir, reflecting dissipated shortening on thin-skinned structures. (2) A bulk of E-W shortening of ~6 mm/yr is absorbed by the most external Babatag (back)thrust with >20 km of past displacement evidenced by borehole data. (3) The Ilyak fault accommodates ~5-8 mm/yr of dextral slip with eastward increasing values; sharply decaying rates suggest a locking depth of ≤1 km. (4) A strong (>10 mm/yr) uplift and westward motion is associated with the sinistral-transpressive Darvaz fault, bounding the basin against the western Pamir. (5) The highest displacement rates >300 mm/yr are demonstrated over the Hoja Mumin salt fountain.

1) See LiCSAR data portal: https://comet.nerc.ac.uk/comet-lics-portal/
2) See Generic Atmospheric Correction Online Service for InSAR: http://www.gacos.net/