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.
vPICO presentations: Thu, 29 Apr
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.
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.
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.
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.
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.
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.
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.
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.
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.
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/
How to cite: Metzger, S., Gągała, Ł., Ratschbacher, L., Schurr, B., Lazecky, M., and Maghsoudi Mehrani, Y.: High-resolution InSAR rate maps showcase tectonic and anthropogenic processes in the Tajik Basin, Central Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2552, https://doi.org/10.5194/egusphere-egu21-2552, 2021.
The vast Tarim basin is surrounded by Tian Shan Mountains in the north, West Kunlun Mountains in the southwest, and the Altyn Mountains in the southeast. The southwestern Tarim Basin developed within the foreland of the West Kunlun Mountains and cumulated up to 10 km of Cenozoic strata. Despite several decades of geological studies its structural styles and details of its geological evolution are still being debated. In this study, we used seven regional seismic transects from the Yecheng - Hotan area calibrated by deep wells to assess lateral variations of a structural style and syn-tectonic sedimentation in this part of the basin.
The basement of the SW Tarim Basin is covered by Paleozoic and Cenozoic strata, as revealed by several deep calibration wells. The regional north-directed basement thrust together with two evaporitic detachments including the Middle Cambrian evaporites (Awatage Formation) and Paleogene evaporites (Aertashi Formation) controlled the overall tectonic framework and structural evolution of this part of the basin. The visible growth strata on seismic data indicate progressive development of the structural wedge within the frontal W Kunlun Mountains from the Late Miocene to the Present day.
Four main Cenozoic evolutionary stages of the W Kunlun Mountains and adjacent SW Tarim Basin have been determined. At the end of Paleogene, evaporites of the Aertashi Formation have been deposited in SW Tarim Basin; their thickness, as indicated by seismic data, increases towards the Kunlun orogenic wedge which suggests their deposition within the flexural foreland basin. Then, during the Early to Middle Miocene, about 4000m of sediments have been deposited in rapidly subsiding foreland basin. Towards the end of Late Miocene-Pliocene, tectonic wedging along thrust front led to significant uplift of the Kunlun Mountains that presently form S margin of the Tarim Basin. Quaternary migration of compressional deformations towards the North, towards the basin interior led to formation of the intra-basinal Jade anticline that was re-interpreted as a thin-skinned syn-depositional “fish tail” structure detached within the Paleogene evaporites. Present-day activity along some deeply buried thrusts of the Kunlun Mts. tectonic wedge might be related to current earthquakes.
How to cite: Liu, W., krzywiec, P., Mazur, S., Meng, F., and Chen, Z.: The Cenozoic structural evolution of the southwestern Tarim Basin, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3721, https://doi.org/10.5194/egusphere-egu21-3721, 2021.
Quantitative constraints on the Cenozoic deformation of the northwestern edge of the Tibetan Plateau remain limited, in particular in terms of shortening rates and of their possible evolution over time. This is indeed the case for the Western Kunlun Range, along the southwestern rim of the Tarim Basin, even though surface geological data and an extensive database of seismic profiles allow to explore the sedimentary record of Cenozoic deformation. Here, we take advantage of these data to document the structural geometry and Cenozoic kinematics of the large scale east-west striking Hotan anticline along the mountain front. Four balanced cross-sections are constructed, and the temporal evolution of deformation is deciphered from the exceptionally seismically well imaged growth strata on the forelimb of the anticline.
The fold results from a broad unfaulted basement ramp anticline, subsequently deformed by a duplex structure that developed in the footwall units. The total shortening of the Hotan thrust system is relatively constant along strike, from ~40 to ~32 km. The shortening accommodated by the duplex varies laterally from west to east, from ~50-40 % to 0 % of the total shortening.
Two distinct successive patterns of growth strata are recognised in the forelimb, and are interpreted to be representative of deformation on the basement ramp, followed by deformation related to the growth of the underlying duplex. Deformation on the basement ramp initiated by ~17 Ma, when calibrating growth seismic reflectors on surface magnetostratigraphic sections. Deformation of the underlying duplex began at ~12 Ma to the west and subsequently propagated eastward.
From these results on shortening and timing of deformation, we determine a shortening rate of 4-3 mm/yr from ~17 to ~7 Ma across the Hotan anticline. We find a significant subsequent decrease in shortening rates, possibly down to <0.5 mm/yr since the uppermost Miocene. These rates are compared to existing values and their regional significance is discussed.
How to cite: Baby, G., Simoes, M., Barrier, L., Guilbaud, C., Van der Woerd, J., and Li, H.: Kinematics of Cenozoic shortening across the foothills of the Western Kunlun Range (Xinjiang, China): the case of the Hotan anticline, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7443, https://doi.org/10.5194/egusphere-egu21-7443, 2021.
Shear fault-bend folds are characterized by a long back-limb that dips more gently than the fault ramp . During the folding growth, the back limb rotates and widens progressively through a combination of limb rotation and kink-band migration. Two end-member models of shear fault-bend folding theories, including simple-shear fault-bend folding (C=0.5) and pure-shear fault-bend folding (C=1), have been developed and widely applied. Mixtures of pure and simple shear (0.5<C<1) are possible and have been found in the natural. Few quantitative methods to limit the shear-index (C) of the shear fault-bend folds so far. The incremental shortening can be calculated based on a simplified equation that assumes the linear relationship between the shortening and the limb rotation angle of the back limb . However, the relationship of these two parameters is nonlinear according to the shear fault-bend folding theory . Calculation results of the linear model have large uncertainty.
Here, we develop a method to calculate the shear-index (C), providing an idea to improve the mixed-shear fault-bend fold models, and establishing a formula to calculate the incremental shortening based on the nonlinear relationship between the back-limb dip angle and the shortening. It is a more general method to calculate the incremental shortening of the shear fault-bend folds.
This model has been applied to the Tugulu anticline in the northern foreland of Chinese Tian Shan, which is a mixed-shear fault-bend fold based on previous studies . Through an analysis of deformed fluvial terraces and growth strata, we develop the shortening history of the Tugulu anticline along the Hutubi River using our developed nonlinear model. Our results show that the Tugulu anticline has a shear-index of ~0.91 and a steady shortening rate of ~1.5mm/yr over the last 500ka.
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How to cite: Zhang, W., Chen, J., and Li, T.: Incremental shortening calculation of the mixed-shear fault-bend folds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15075, https://doi.org/10.5194/egusphere-egu21-15075, 2021.
Slip partitioning along the northern boundary of the Tibetan Plateau is essential for understanding regional deformation and the seismic potential of the different faults that accommodate it. Within this framework the Altyn Tagh Fault (ATF) is commonly viewed as the primary structure that separates the Tibetan Plateau from the stable Gobi-Alashan block to the north. Late Quaternary sinistral slip rates of 8-12 mm/yr across the central ATF between 86° and 93°E decrease eastwards to zero as the fault approaches its mid-continental termination at ~97°E. To better understand how late Quaternary slip is partitioned along the ATF’s eastern termination we obtained new slip-rate measurements for the ~200-km-long left-lateral ENE striking Sanweishan Fault (SSF) located ~60 km north of the ATF between 94°-96°E near the town of Dunhuang.
Multiple sinistral offsets ranging up to 600 m were identified by linking the clast assemblage of offset alluvial fan remnants with their provenance upstream of the fault. Luminescence dating revealed depositional ages ranging from 100 - 200 ka for the offset features and time-invariant minimum sinistral slip of 2.5±1 mm/yr during the last ~200 ka, which is approximately an order of magnitude higher than previously reported slip-rates for the SSF. Our results indicate that the SSF and the eastern segment of the ATF accommodate comparable magnitudes of late Quaternary slip. Considering this substantial transfer of lateral slip as far as 60 km north of the eastern ATF we propose that the SSF may represent juvenile northeastward expansion of the Tibetan Plateau into previously stable parts of the Gobi-Alashan block.
How to cite: Wieler, N., Mushkin, A., Shelef, E., Zhang, H., Sagy, A., Porat, N., Ren, Z., Huang, F., and Liu, J.: New constraints on Quaternary slip partitioning near the eastern termination of the Altyn Tagh Fault , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12517, https://doi.org/10.5194/egusphere-egu21-12517, 2021.
Flexural basins are the common geological feature in convergent settings, and usually regarded as the result of flexural subsidence of the margins of under-thrusting cratons in response to the gravitational load of over-riding orogens. This process usually causes the fastest tectonic subsidence and thickest orogenic-related deposits in the basin margins adjacent to the orogens, such as India Foreland Basin in front of the Himalaya. The Qaidam Basin, which is the largest sedimentary basin within the Tibetan Plateau interior, was once interpreted to belong to this type and form by flexural subsidence on its south and north margins in response to loading of the Qiman Tagh and the South Qilian Shan orogenic belts, respectively. However, the latest studies present sedimentary and structural features that contrast to a typical foreland basin. These features include (1) depocenters being located along the central axis, rather than the margins, with thickest sediments up to 15 km, and (2) development of many high-angle reverse faults, rather than thin-skinned thrusts, to generate upper-crustal shortening as low as 10-15% (20 – 30 km), indicating that the widths of the orogenic belts juxtaposed atop the basin margins are limited. These features cannot be explained by the flexural subsidence of basin margins and/or sediment load. Herein, we investigate the impact of lithospheric buckling, which has been ignored in most studies of basin formation in compressional settings, on the tectonic subsidence of the Qaidam Basin through numerical simulation based on finite elastic plate model. We first use the flexural backstripping method to calculate the tectonic subsidence of the Cenozoic basement across the Qaidam Basin. And then, we simulate the tectonic subsidence caused by (1) gravitational load of orogenic belts alone, and (2) combined gravitational load and lithosphere buckling. The result shows that the simulated tectonic subsidence curve fits well with the real one only when considering the effect of lithospheric buckling that accounts for >90% tectonic subsidence. Our finding indicates for the first time that lithospheric buckling is also an important mechanism for the subsidence of intramountain basins like the Qaidam Basin, and should not be ignored when studying lithospheric-scale deformation across large orogenic belts.
How to cite: Xiaoyi, H. and Lei, W.: Lithospheric buckling dominates the Cenozoic subsidence of the Qaidam Basin, NE Tibetan Plateau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3633, https://doi.org/10.5194/egusphere-egu21-3633, 2021.
Central Asia is one of the most tectonically active and orographically diverse regions in the world and is the location of the highest topography on Earth resulting from major plate tectonic collisional events. Yet the role of tectonics versus climate on erosion remains one of the greatest debates of our time. We present the first regional scale analysis of 2526 published low-temperature thermochronometric dates from Central Asia spanning the Altai-Sayan, Tian Shan, Tibet, Pamir, and Himalaya. We compare these dates to tectonic processes (proximity to tectonic boundaries, crustal thickness, seismicity) and state-of-the-art paleoclimate simulations in order to constrain the relative influences of climate and tectonics on the topographic architecture and erosion of Central Asia. Predominance of pre-Cenozoic ages in much of the interior of central Asia suggests that significant topography was created prior to the India-Eurasia collision and implies limited subsequent erosion. Increasingly young cooling ages are associated with increasing proximity to active tectonic boundaries, suggesting a first-order control of tectonics on erosion. However, areas that have been sheltered from significant precipitation for extensive periods of time retain old cooling ages. This suggests that ultimately climate is the great equalizer of erosion. Climate plays a key role by enhancing erosion in areas with developed topography and high precipitation such as the Tian Shan and Altai-Sayan during the Mesozoic and the Himalaya during the Cenozoic. Older thermochronometric dates are associated with sustained aridity following more humid periods.
How to cite: Jepson, G., Carrapa, B., Gillespie, J., Feng, R., DeCelles, P., Tabor, C., and Zhu, J.: Climate as the great equalizer of continental-scale erosion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13718, https://doi.org/10.5194/egusphere-egu21-13718, 2021.
The cut-and-fill terrace sequence, resulted from aggradation and incision cycles of alluvial rivers on the piedmont of active orogens, is a common tectonic geomorphological feature observed across different continents under different climatic regimes. Such aggradation and incision cycles are identified on both the orbital and sub-orbital cycles, which poses a question about their origins. Efforts have been put into disentangling the contributions from tectonics, climate and other autogenic sources. In this study, we investigated it by exploring the cut-and-fill terrace sequences along the Jingou River on the northern piedmont of Chinese Tianshan, where numerous terraces are seen along tens of alluvial rivers flowing through the fold-and-thrust belt. More than ten terrace flights, are observed where Jingou River flows across the active Huoerguosi anticline. We collected sediment samples for OSL dating to decipher the building and abandonment processes of these terraces and made topographic measurements to evaluate the paleo-slope of this section of Jingou River. Detailed field survey of sedimentary structure and luminescence dates unambiguously unveil the aggradation and incision cycles on sub-orbital cycles since the last interglacial. Down-cutting of no less than 80 meters is identified during the last ten thousands of years. We tentatively evaluated the possible roles of regional climatic variation, anticline deformation and the autogenic processes. Of all these factors, we detailedly investigated the role of longitudinal contrast of lithologies along the river due to the deformation of the Huoerguosi anticline, which always promotes the channel incision.
How to cite: Qin, J., Li, K., Chen, J., and Li, S.: Role of Longitudinal Lithological Contrast On the Channel Evolution of Piedmont Rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13963, https://doi.org/10.5194/egusphere-egu21-13963, 2021.
In this study, we investigate the ongoing crustal deformation in the Haridwar-Kotdwar piedmont zone of the Northwestern Himalaya, India. The Himalayan mountain front has been actively deforming along the Himalayan Frontal Thrust (HFT) which marks the conjunction between the Siwalik hills and the Indo-Gangetic Plains. We report NNE-SSW trending left lateral strike-slip fault towards the west of the study area namely Haridwar Fault (HF) and it offsets the HFT sinistrally by ~ 9 Km. Using the satellite imagery (Cartosat-1 stereo pairs) flat-lying uplifted river terrace have been identified, which is at an elevation of ~80 m from the flood plain of Mitthawali River. Along with uplifted terraces, the HF offsets various structural features, the rivers flowing across it and manifests itself as a series of scarps and slope breaks visible in the satellite imagery. The Khoh River Fault (KRF) trends N-S and offsets HFT dextrally by ~12 Km, this controls the course of the Khoh River and forms a lateral ramp perpendicular to HFT. The KRF manifests itself geomorphically as uplifted terraces at an elevation of ~50 m from the flood plain of the Khoh River which is conspicuous in the DEM and the Cartosat-1 imagery of the area. The Haridwar-Kotdwar piedmont zone has been surrounded in the north by HFT, in the south by Najibabad Fault (NF), towards east by KRF and the western margin has been dissected by HF. The KRF and HF show signatures of neotectonic activity and offsets HFT at two locations forming two ramps in the region. The piedmont zone has been showing signatures of upwarping which causes sudden migration of the rivers flowing into the piedmont zone on a decadal scale, mainly caused by an E-W trending NF. NF is a blind fault and manifests itself geomorphically by series of knee turn bending of the rivers in the study area. The deformation caused by NF has been comprehended using the satellite imageries and Gradient Length Anomalies (GLA). The GLA results show signatures of upliftment in the piedmont zone along the NF. The Haridwar-Kotdwar piedmont zone is surrounded by neotectonically active faults from four sides, making this block a potential seismic hazard in near future.
How to cite: Kralia, A. and Thakur, M.: Geomorphic signatures of the neotectonic activity in Haridwar-Kotdwar region, Northwestern Himalaya, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14055, https://doi.org/10.5194/egusphere-egu21-14055, 2021.
The Southeast Tibet is characterized by extensive low-relief high-elevation surfaces that have been interpreted as “relict surfaces”, where thermochronological data generally show old ages and very little exhumation during the India-Asia collision. Those relict surfaces are proposed either to be formed at low elevation and then uplifted and dissected by large rivers since middle Miocene, or to inherit a pre-existing low-relief landscape by or prior to the collision, as revealed by stable-isotope paleoaltimetry. Among these relict surfaces, the BaimaXueshan low-relief (<600 m), moderate-elevation (~4500 m) massif is the closest to the Eastern Himalayan Syntaxis (EHS) in the Three Rivers Region, where Salween, Mekong and Yangtze rivers flow southward parallelly and closely, showing large-scale shortening during the collision.This region represents a transition between the strongly deformed zone around EHS and the less deformed southeast Tibetan plateau margin in Yunnan and Sichuan, and is an appropriate zone to examine the relief development and the interaction between pre-existing structures, Cenozoic tectonics and river incision during the Tibetan plateau growth.
We compile and model published thermochronometric ages for BaimaXueshan massif, east of the Mekong River, to constrain its exhumation and relief history using the thermo-kinematic code Pecube. Modelling results show regional rock uplift at a rate of 0.25 km/Myr since ~10 Ma, following slow exhumation at a rate of 0.01 km/Myr since at least 22 Ma. Estimated Mekong River incision accounts for a maximum of 30% of the total exhumation since 10 Ma. We interpret moderate exhumation of the BaimaXueshan massif since 10 Ma as a response to a regional uplift due to the continuous northward indentation of NE India in a zone around the Eastern Himalayan Syntaxis (EHS) and delimited by Longmucuo-Shuanghu suture in the north. Thus BaimaXueshan massif with significant exhumation could not be classified as “relict surface”, as proposed by previous studies and its low relief results from in part glacial “buzzsaw-like” processes at high elevation, enhancing since ~2 Ma. In contrast, modelling results for the high-relief, high-elevation Kawagebo massif to the west of the Mekong River, facing the BaimaXueshan massif, imply a similar contribution of Mekong River incision (25%) to exhumation, but much stronger local rock uplift at a rate of 0.45 km/Myr since at least 10 Ma, accelerating to 1.86 km/Myr since 1.6 Ma. We show that the thermochronometric ages are best reproduced by local rock uplift related to late Miocene reactivation of a kinked westward-dipping thrust, striking roughly parallel to the Mekong River, with a steep shallow segment flattening out at depth. Thus, the strong differences in elevation and relief that characterize both massifs are linked to variable exhumation histories due to a strongly differing tectonic imprint.
How to cite: Ou, X., Replumaz, A., and van der Beek, P.: Contrasting exhumation histories and relief development within the Three Rivers Region (Southeast Tibet), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15346, https://doi.org/10.5194/egusphere-egu21-15346, 2021.
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