Pamir Thrust Research Articles

Recent Block Kinematics and Fault Slip Rates in the Pamir, Central Asia, From an Integrated GNSS Velocity Field

AbstractContemporary kinematic characteristics of the Pamir plateau are characterized by (a) significant NS‐shortening across the northern front, (b) EW‐extension within the Pamir and (c) gravitationally driven westward mass‐outflux of the western Pamir into the Tajik basin. The 2015 Mw 7.2 Sarez and 2023 Mw 6.8 Murghab strike‐slip earthquakes highlight the crucial role of shear deformation in the Pamir's interior, but the detailed secular kinematics of the remote Plateau are still elusive. Here, we employ elastic block models to fit GNSS velocities compiled from our own reprocessed velocity solution and from previous studies to determine slip rates along major active faults within and bounding the Pamir plateau. Our favored model separates the Pamir's interior into three micro‐blocks along the NNE‐trending sinistral Sarez‐Karakul fault system and the SSE‐trending Kongur Shan extensional system. The model confirms significant crustal shortening along the Pamir's northern boundary and strike‐slip motion along its western boundary, namely the Pamir thrust system and the Darvaz fault, respectively. In contrast, strike‐slip motion on the eastern boundary is minor. In the interior of the Plateau, we find an upper limit of sinistral shear of 8.8 ± 0.3–9.5 ± 0.2 mm/yr along the Sarez‐Karakul fault system and dextral shear of 6.3 ± 0.3 mm/yr along the WNW‐trending Muji fault. Our model also confirms recent observations of an active, SW‐continuation of the Sarez‐Karakul fault system. In the eastern Pamir, crustal extension mainly occurs across the northern segment of the Kongur Shan extensional system. The recent large strike‐slip earthquakes on the Pamir plateau suggest that shear motion on the plateau is accommodated on a much wider range than only by the Sarez‐Karakul fault system.

Stress triggering and future seismic hazards implied by four large earthquakes in the Pamir from 2015 to 2023 revealed by Sentinel-1 radar interferometry

SUMMARY The Mw 6.8 Murghob earthquake is the third earthquake in an Mw 6.4+ sequence occurring in the Pamir initiated by the 2015 Sarez Mw 7.2 earthquake. It is of great significance to investigate their interactions and to assess future seismic hazards in the region. In this paper, we use Sentinel-1 radar interferometric data to retrieve coseismic deformation, invert for the slip distributions of the four events, and then investigate their interactions. The cumulative Coulomb failure stress changes (ΔCFS) suggest that the 2023 Murghob earthquake was promoted by the three prior earthquakes in the sequence. Pre-stress from historical earthquakes is a key factor in explaining the triggering mechanism of the two 2016 Mw 6.4+ earthquakes. Stress loading and unloading effects on major faults in the region indicate that future attention should be paid in (1) the segment of the Sarez-Karakul fault north of the Kokuibel Valley, (2) the segment of the Sarez-Murghab thrust fault west of the Sarez-Karakul fault and (3) the east segments of the Pamir thrust fault system, all with a large positive ΔCFS.

Application of calcite U–Pb geochronology in sandstone-hosted Pb-Zn deposits: A case study of the giant Uragen deposit, NW China

The age of the giant Uragen sandstone-hosted Pb-Zn deposit in the western margin of the Tarim Basin in Northwest China has long been questioned due to the general paucity of minerals unequivocally linked to Pb-Zn deposition that can be precisely dated using conventional radiogenic isotopic techniques. This paper describes the petrographic characteristics of Pb-Zn ores and presents the results of in situ calcite U–Pb analysis using laser ablation sector field inductively coupled plasma mass spectrometry (LA-ICP-MS) for this deposit. Calcite from the early, main and late ore-forming stages yielded low intercept ages of 11.80 ± 0.4 Ma, 10.9 ± 0.3 Ma and 10.6 ± 0.2 Ma, respectively, which constrain the Uragen Pb-Zn mineralization to the Late Miocene . Combined with the existing isotopic ages in the region, these new ages lead us to propose that the Uragen deposit formed during two hydrothermal episodes. The first refers to the oil-gas migration that occurred during the Middle Eocene to Early Miocene, which may be associated with the India–Eurasia collision and subsequent activity of the Main Pamir Thrust. The second refers to the Pb-Zn mineralization that occurred in the Late Miocene, which may be associated with the activity of the foreland thrust fault belt of South Tianshan and the Pamir Frontal Thrust (PFT). This study demonstrates the potential of using LA-ICP–MS U–Pb dating on calcite to constrain the ages of sandstone-hosted Pb-Zn deposits.

Cyclic Fault Slip Under the Magnifier: Co‐ and Postseismic Response of the Pamir Front to the 2015 Mw7.2 Sarez, Central Pamir, Earthquake

AbstractThe constant increase of geodetic instrumentation over the past decades enables us to not only detect ever smaller tectonic signals but also to monitor their evolution in time and space. We present spatial and temporal slip variations observed on a fault affected by a large, intermediate‐field earthquake: the 2015 Mw7.2 Sarez, Central Pamir, earthquake ruptured the sinistral, NE‐trending Sarez‐Karakul fault system. 120–170 km North of the main rupture, the thin‐skinned, E‐trending Pamir thrust system bounding the Pamir to the North was co‐seismically activated. We derived co‐seismic offsets and post‐seismic rates observed by two dense, high‐rate Global Positioning System (GPS) profiles crossing the Pamir thrust system at different longitudes. The continuous GPS observations of the western profile focus on the dextral, NW‐striking Aramkungey fault segment that connects two thrust faults with opposite dip. We compare inter‐, co‐ and post‐seismic displacement rates by complementing the continuous data with survey‐mode GPS data and East rates derived from satellite radar interferometric displacement time‐series. All the GPS stations were shifted toward the epicenter against the direction of the interseismic load with an increased gradient in the Aramkungey fault segment. During the postseismic stage, the fault‐parallel and fault‐perpendicular rates were affected differently, suggesting gradual re‐locking of the Aramkungey fault after its unlocking by right‐lateral co‐seismic slip.

The late Cenozoic expansion of the northeastern Pamir: Insights from the stratigraphic architecture of the Wupoer Piggyback Basin

• Onset time of the basin was the late Miocene to early Pliocene (∼5.8–4.6 Ma). • Multiple depocenters existed in the basin and intergraded to one in the Quaternary. • Eastward migration of the depocenter records the collision between the Pamir and South Tian Shan. The Pamir salient is the preeminent example of an intracontinental collision and defines the western end of the Himalayan-Tibetan orogen. Previous work has paid much attention to reveal the internal evolution of the Plateau. However, the structural growth and expansion of the Northern Pamir still have several unknowns. To address this process of the indentation, we describe the architecture of the Wupoer Piggyback Basin (WPB), the westernmost tip of the Tarim Basin, in between the northeastern Main Pamir Thrust (MPT), and the Pamir Frontal Thrust (PFT). The geometry of the WPB growth strata and its relationship with the compressive structures were explored through six seismic sections, five cutting the WPB along dip and one along strike. According to the sense prevailing in the literature, we define the onset of a piggyback basin with the first evidence of tectonic deformation within the basin fill. Interpreted from the seismic sections and field work, an unconformity between the Atushi Fm. and the underlying strata marks the base of the WPB, dated the onset of the basin at the latest Miocene to early Pliocene (∼5.8–4.5 Ma). The growth strata were separated into 6 units above the unconformity (G1-G6). The relative isopach maps reveal the existence of three main depocenters, evolving to a single one in the Quaternary. The structural interpretation documents the PFT is significantly segmented by transfer faults and lateral ramps. The eastward migration of the depocenter since the Quaternary points to the oblique collision between the Pamir and the South Tian Shan and gradual encroachment of deformation towards the east.

The Crust in the Pamir: Insights From Receiver Functions

AbstractThe Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (<1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system.

Crustal structure beneath Tien Shan orogenic belt and its adjacent regions from multi-scale seismic data

As one of the world’s most active intracontinental mountain belts, Tien Shan has posed questions for researchers regarding the formation of different tectonic units and active shallow seismicity. Here, we used a huge data set comprising of 7094 earthquakes from local, regional and teleseismic seismic stations. We used waveform modeling and multi-scale double-difference earthquake relocation technique to better constrain the source parameters of the earthquakes. The new set of events provided us with better initial earthquake locations for further tomographic investigation. We found that reverse-faulting earthquakes dominate the whole study area while the fault plane solutions for earthquakes beneath the northwestern Tarim Basin and the Main Pamir Thrust are diverse. There is a low-velocity anomaly beneath Bashkaingdy at depth of 80 km, and high-velocity anomalies beneath central Tien Shan at shallower depths. These observations are the keys to understand the mechanism of Tien Shan’s formation because of Tarim Basin northward and Kazakh Shield’s southward subduction in the south and north respectively. Velocities beneath western Tien Shan are relatively high. We thus infer that the Western Tien Shan is relatively less deformed than the eastern Tien Shan primarily due to a relatively brittle mantle.

Cenozoic intracontinental deformation and exhumation at the northwestern tip of the India-Asia collision-southwestern Tian Shan, Tajikistan, and Kyrgyzstan

Along the Ghissar-Alai Range of the southwestern Tian Shan (southwestern Kyrgyzstan, northern Tajikistan), the deformation front of the India-Asia collision—the Pamir-Tibet orogen—is interacting with the intra-continental Tian Shan orogen without the intervening Tarim Craton. Apatite fission-track (n = 33, ~3.3–145.6 Ma, 27% <10 Ma) and (U-Th)/He (n = 32, ~1.9–26.1 Ma, 56% <10 Ma) thermochronologic ages suggest approximate isothermal holding (very slow cooling to weak reheating) during relative tectonic quiescence between ~150–15 Ma. Accelerated exhumation (~0.2–1.0 km/Myr, median ~0.5 km/Myr) and cooling (11–16 °C/Myr) occurred over the last ~10 Myr. Geomorphologic parameters—incision, and river steepness and concavity—confirm the youth of the southwestern Tian Shan's mountain building. High exhumation/cooling rates correlated with pronounced local relief, produced by Cenozoic faults reactivating inherited (Late Paleozoic) structures. Regions with similarly young exhumation are centered along rims of rigid crustal blocks in the central and eastern Tian Shan. Structurally, the Ghissar-Alai Range is a broad, ~east-trending zone of dextral transpression that includes the northern Tajik Basin (Illiak Fault Zone) and the Pamir Thrust System of the frontal northern Pamir. It is the particular deformation field at the northwestern tip of the India–Asia collision—the interaction of the westward gravitational collapse of the Pamir Plateau into the Tajik Basin with the bulk northward motion of the Pamir—that transformed the southwestern Tian Shan into a dextral transpression belt. The dextral transpression in the southwestern Tian Shan contrasts with sinistral strike-slip shear localized along inherited fault zones, accommodating dominant ~ north–south shortening, in the central and eastern Tian Shan. The deformation field influenced by the Pamir and the associated young exhumation make the Ghissar-Alai Range a unique feature in the Tian Shan orogen.

The effect of foreland palaeo-uplift on deformation mechanism in the Wupoer fold-and-thrust belt, NE Pamir: Constraints from analogue modelling

Palaeo-uplifts often exist in fold-and-thrust belts. However, their effects on the deformational process have not yet been well understood. To evaluate such effects, six analogue models were systematically run based on geological features of the Wupoer fold-and-thrust belt (FTB), NE Pamir, where the Wulagen palaeo-uplift with overlying gypsum bed has been clearly identified. Our analogue results demonstrated that the palaeo-uplift (its location and inhomogenous distribution), accompanied with overlying gypsum bed that serves as ductile décollement, plays a critical role in localizing the front thrust fault and shaping it into arc form. The results indicate that the front thrust fault slides along the ductile décollement (gypsum bed), and breaks through onto the surface at the region where the palaeo-uplift develops, forming a piggy-back basin in the hanging wall. It suggests that the palaeo-uplift with consequent topographic variation of the overlying ductile décollement localizes the breakthrough point of the front thrust fault. In addition, the results indicate that the front thrust fault (Pamir Front Thrust, PFT) initially broke through in the location where it develops the Wulagen palaeo-uplift and propagated aside. This resulted in the distance between the PFT and the basement-involved fault (Main Pamir Thrust, MPT) to decrease from the region with palaeo-uplift to the areas aside without palaeo-uplift, thereby forming the arc-shaped PFT in map view. The results of this study also provide a revised geological model, which emphasizes the effect of décollement layer on absorbing the slip along the PFT. Our results provide a new mechanical interpretation of the deformation of Wupoer FTB, NE Pamir.

Exhumation history of the West Kunlun Mountains, northwestern Tibet: Evidence for a long-lived, rejuvenated orogen

How and when the northwestern Tibetan Plateau originated and developed upon pre-existing crustal and topographic features is not well understood. To address this question, we present an integrated analysis of detrital zircon U–Pb and fission-track double dating of Cenozoic synorogenic sediments from the Kekeya and Sanju sections in the southwestern Tarim Basin. These data help establishing a new chronostratigraphic framework for the Sanju section and confirm a recent revision of the chronostratigraphy at Kekeya. Detrital zircon fission-track ages present prominent Triassic–Early Jurassic (∼250–170 Ma) and Early Cretaceous (∼130–100 Ma) static age peaks, and Paleocene–Early Miocene (∼60–21 Ma) to Eocene–Late Miocene (∼39–7 Ma) moving age peaks, representing source exhumation. Triassic–Early Jurassic static peak ages document unroofing of the Kunlun terrane, probably related to the subduction of Paleotethys oceanic lithosphere. In combination with the occurrence of synorogenic sediments on both flanks of the Kunlun terrane, these data suggest that an ancient West Kunlun range had emerged above sea level by Triassic–Early Jurassic times. Early Cretaceous fission-track peak ages are interpreted to document exhumation related to thrusting along the Tam Karaul fault, kinematically correlated to the Main Pamir thrust further west. Widespread Middle–Late Mesozoic crustal shortening and thickening likely enhanced the Early Mesozoic topography. Paleocene–Early Eocene fission-track peak ages are presumably partially reset. Limited regional exhumation indicates that the Early Cenozoic topographic and crustal pattern of the West Kunlun may be largely preserved from the Middle–Late Mesozoic. The Main Pamir–Tam Karaul thrust belt could be a first-order tectonic feature bounding the northwestern margin of the Middle–Late Mesozoic to Early Cenozoic Tibetan Plateau. Toward the Tarim basin, Late Oligocene–Early Miocene steady exhumation at a rate of ∼0.9 km/Myr is likely related to initial thrusting of the Tiklik fault and reactivation of the Tam Karaul thrust. Thrusting together with upper crustal shortening in the mountain front indicates basinward expansion of the West Kunlun orogen at this time. This episode of exhumation and uplift, associated with magmatism across western Tibet, is compatible with a double-sided lithospheric wedge model, primarily driven by breakoff of the Indian crustal slab. Accelerated exhumation of the mountain front at a rate of ∼1.1 km/Myr since ∼15 Ma supports active compressional deformation at the margins of the northwestern Tibetan Plateau. We thus propose that the West Kunlun Mountains are a long-lived topographic unit, dating back to Triassic–Early Jurassic times, and have experienced Middle–Late Mesozoic to Early Cenozoic rejuvenation and Late Oligocene–Miocene expansion.

The 2008 Nura Mw6.7 earthquake: A shallow rupture on the Main Pamir Thrust revealed by GPS and InSAR

The 2008 Nura Mw6.7 earthquake occurred in front of the Trans-Alai Range, central Asia. We present Interferometric Synthetic Aperture Radar (InSAR) measurements of its coseismic ground deformation that are available for a major earthquake in the region. Analysis of the InSAR data shows that the earthquake ruptured a secondary fault of the Main Pamir Thrust for about 20 km. The fault plane striking N46°E and dipping 48°SE is dominated by thrust slip up to 3 m, most of which is confined to the uppermost 2–5 km of the crust, similar to the nearby 1974 Mw7.0 Markansu earthquake. The elastic model of interseismic deformation constrained by GPS measurements suggests that the two earthquakes may have resulted from the failures of two high-angle reverse faults that are about 10 km apart and rooted in a locked décollement at depths of 5–6 km. The elastic strain is built up by a freely creeping décollement at about 16 mm/a.

Late Miocene northward propagation of the northeast Pamir thrust system, northwest China

©2015. American Geophysical Union. Piggyback basins on the margins of growing orogens commonly serve as sensitive recorders of the onset of thrust deformation and changes in source areas. The Bieertuokuoyi piggyback basin, located in the hanging wall of the Pamir Frontal Thrust, provides an unambiguous record of the outward growth of the northeast Pamir margin in northwest China from the Miocene through the Quaternary. To reconstruct the deformation along the margin, we synthesized structural mapping, stratigraphy, magnetostratigraphy, and cosmogenic burial dating of basin fill and growth strata. The Bieertuokuoyi basin records the initiation of the Pamir Frontal Thrust and the Takegai Thrust ~5-6Ma, as well as clast provenance and paleocurrent changes resulting from the Pliocene-to-Recent uplift and exhumation of the Pamir to the south. Our results show that coeval deformation was accommodated on the major structures on the northeast Pamir margin throughout the Miocene to Recent. Furthermore, our data support a change in the regional kinematics around the Miocene-Pliocene boundary (~5-6Ma). Rapid exhumation of NE Pamir extensional domes, coupled with cessation of the Kashgar-Yecheng Transfer System on the eastern margin of the Pamir, accelerated the outward propagation of the northeastern Pamir margin and the southward propagation of the Kashi-Atushi fold-and-thrust belt in the southern Tian Shan. This coeval deformation signifies the coupling of the Pamir and Tarim blocks and the transfer of shortening north to the Pamir frontal faults and across the quasi-rigid Tarim Basin to the southern Tian Shan Kashi-Atushi fold-and-thrust system.

Oceanic-style subduction controls late Cenozoic deformation of the Northern Pamir orogen

The northern part of the Pamir orogen is the preeminent example of an active intracontinental subduction zone in the early stages of continent–continent collision. Such zones are the least understood type of plate boundaries because modern examples are few and of limited access, and ancient analogs have been extensively overprinted by subsequent tectonic and erosion processes. In the Pamir, it has been assumed that most of the plate convergence was accommodated by overthrusting along the plate-bounding Main Pamir Thrust (MPT), which forms the principal northern mountain and deformation front of the Pamir. However, the synopsis of our new and previously published thermochronologic data from this region shows that the hanging wall of the MPT experienced relatively minor amounts of late Cenozoic exhumation. The Pamir orogen as a whole is an integral part of the overriding plate in a subduction system, while the remnant basin to the north constitutes the downgoing plate, with the bulk of the convergence accommodated by underthrusting. Herein, we demonstrate that the observed deformation of the upper and lower plates within the Pamir–Alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback, subduction erosion, subduction accretion, and marginal slab-tear faults. We suggest that the curvature of the North Pamir is genetically linked to the short width and rollback of the south-dipping Alai slab; northward motion (indentation) of the Pamir is accommodated by crustal processes related to this rollback. The onset of south-dipping subduction is tentatively linked to intense Pamir contraction following break-off of the north-dipping Indian slab beneath the Karakoram.

Focused Pliocene–Quaternary exhumation of the Eastern Pamir domes, western China

The Kongur Shan and Muztagata domes are the most prominent topographic features in the Eastern Pamir indenter corner. In spite of several previous bedrock thermochronology studies, the extent, processes and mechanisms of dome exhumation remain unclear. We present 1158 detrital zircon fission-track (DZFT) ages from modern glacier and river sediments along the domes to better constrain the spatial distribution of exhumation. Our thermochronologic dataset from the Chakragil massif shows a sharp increase in the proportion of DZFT ages >10Ma from basins that drain the peak region to those further to the north, which suggests that the Kalagile fault forms the northern boundary of the rapidly exhuming dome structure. A cluster of Middle Miocene DZFT ages in these samples documents pre-doming cooling, possibly associated with motion on the Main Pamir thrust and/or Oytag fault. Large fractions of DZFT grain ages <10Ma in samples from catchments east of the divide likely originate from the Chakragil, Kongur Shan and Muztagata massifs, implying focused exhumation consistent with DZFT age distributions in samples from catchments draining the western flank of the range and bedrock geo- and thermochronologic data from the core of the domes. Furthermore, the majority of the samples contain two distinct DZFT age groups at ∼6–4Ma and ∼3–1Ma respectively. These ages are synchronous with major fluvial sedimentation phases at the outlets of the rivers draining northeastward into the Tarim Basin and suggest rapid Pliocene–Quaternary exhumation of the domes. We interpret the ∼6–4Ma age group to record the onset of extension along the entire Kongur Shan fault; its timing with respect to compressional deformation supports a model of gravitational collapse of over-thickened crust. The fraction of ∼3–1Ma DZFT ages is larger than what would be inferred from the bedrock data alone. We interpret this to reflect recent focused denudation in response to glaciation and fluvial incision of the domes and their possible feedback with extensional faulting.

Equivalency of geologic and geodetic rates in contractional orogens: New insights from the Pamir Frontal Thrust

Across contractional orogens, the equivalency between decadal convergence rates from geodetic GPS data and geologic shortening rates at time scales of thousands or millions of years has rarely been documented. Here, we present an example from the northern margin of Chinese Pamir, where the Main Pamir Thrust is tectonically quiescent, and recent deformation is concentrated on the Pamir Frontal Thrust (PFT). Based on dated and faulted fluvial terraces, magnetostratigraphy, and mapping, the horizontal shortening rate of the PFT is ∼6–7 mm/a at time scales of both ∼18.4 ka and ∼0.35 Ma, comparable to the geodetic rate of ∼6–9 mm/a across the same zone, implying that modern geodetic rates are a reasonable proxy for geologic rates since ∼0.35 Ma. Comparing this example with studies in other contractional orogens, we conjecture that a match or mismatch of geologic‐geodetic rates typically depends on the time scale of observation, fault geometry, and fault mechanics.

Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data

SUMMARY Utilizing seismic refraction/wide-angle reflection data from 11 approximately in-line earthquakes, 2-D P- and S-velocity models and a Poisson's ratio model of the crust and uppermost mantle beneath the southern Tien Shan and the Pamir have been derived along the 400-km long main profile of the TIPAGE (TIen shan—PAmir GEodynamic program) project. These models show that the crustal thickness varies from about 65.5 km close to the southern end of the profile beneath the South Pamir through about 73.6 km under Lake Karakul in the North Pamir, to about 57.7 km, 50 km south of the northern end of the profile in the southern Tien Shan. Average crustal P velocities are low with respect to the global average, varying from 6.26 to 6.30 km s−1. The average crustal S velocity varies from 3.54 to 3.70 km s−1 along the profile and thus average crustal Poisson's ratio (σ) varies from 0.23 beneath the central Pamir in the south central part of the profile to 0.265 towards the northern end of the profile beneath the southern Tien Shan. The main layer of the upper crust extending from about 2 km below the Earth's surface to 27 km depth below sea level (b.s.l.) has average P velocities of about 6.05–6.1 km s−1, except beneath the south central part of the profile where they decrease to around 5.95 km s−1. This is in contrast to the S velocities which range from 3.4 to 3.6 km s−1 and exhibit the highest values of 3.55–3.6 km s−1 where the P velocity is lowest. Thus, σ for the main layer of the upper crust is 0.26 beneath the profile except beneath the south central part of the profile where it decreases to 0.22. The low value of 0.22 for σ under the central Pamir, the along-strike equivalent of the Qiangtang terrane in Tibet, is similar to that within the corresponding layer beneath the northern Lhasa and southern Qiangtang terranes in central Tibet and is indicative of felsic rocks rich in quartz in the α state. The lower crust below 27 km b.s.l. has P velocities ranging from 6.1 km s−1 at the top to 7.1 km s−1 at the base. Further, σ for this layer is 0.27–0.28 towards the northern end of the profile but is low at about 0.24 beneath the central and southern parts of the profile, which is similar to the situation found in the northeast Tibetan plateau. The low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite-facies and possibly also eclogite-facies metapelites in the lower part. Within the uppermost mantle, the average P velocity is about 8.10–8.15 km s−1 and σ is about 0.26. Assuming an isotropic situation, then a relatively cool (700–800°C) uppermost mantle beneath the profile is indicated. This would in turn indicate an intact mantle lid beneath the profile. An upper mantle reflector dipping from 104 km b.s.l., 120 km from the southern end of the profile to 86 km b.s.l., 155 km from the northern end of the profile has also been identified. The preferred model presented here for the crustal and lithospheric mantle structure beneath the Pamir calls for nearly horizontal underthrusting of relatively cool Indian mantle lithosphere, the leading edge of which is outlined by the Pamir seismic zone. This cool Indian mantle lithosphere is overlain by significantly shortening, warm Asian crust. The Moho trough that is a feature seen beneath some other orogenic belts, for example the Alps and the Urals, beneath the northern Pamir may mark the southern tip of the actively underthrusting Tien Shan crust along the Main Pamir thrust.

Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China)

The Pamir salient is the western expression of mountain growth related to Indo-Eurasian convergence. Though a rough framework has emerged describing the tectonic evolution of the Pamir, detailed knowledge of the spatial and temporal evolution of Cenozoic deformation is necessary to determine how strain progressed through the orogenic belt. Here we present new stratigraphic, zircon provenance, and stable isotope data from Jurassic to Miocene strata along the Pamir’s northeastern margin near the town of Oytag (Wuyitake) in the Tarim Basin (west China). Prominent ∼40 Ma peaks in Oligocene to early Miocene detrital zircon grains record the erosion of an Eocene belt of shoshonitic rocks in the central to southeastern Pamir. This is roughly coincident with an ∼4‰ shift in the oxygen isotopic composition (δ 18O) of carbonates during the Eocene and/or Oligocene (from an average of −8.7‰ to −12.6‰), suggesting a reorganization of atmospheric circulation during that time. This could have been caused by uplift of Tarim Basin-bounding ranges and/or retreat of the Paratethys Sea. A subsequent change from Eocene to Jurassic aged detrital zircon grains in the early to middle Miocene indicates provenance shifted from source rocks in the central and/or SE Pamir to the hanging wall of the Main Pamir Thrust (MPT), coincident with prograding facies at that time. This suggests deformation progressed outward toward the northeast margin of the Pamir plateau in the early to middle Miocene. Our results corroborate outward advancement of Himalayan deformation, affecting all margins of the Tarim Basin by the middle Miocene.

Late Miocene–Pliocene deceleration of dextral slip between Pamir and Tarim: Implications for Pamir orogenesis

The timing of the late Cenozoic collision between the Pamir salient and the Tien Shan as well as changes in the relative motion between the Pamir and Tarim are poorly constrained. The northern margin of the Pamir salient indented northward by ~ 300 km during the late Cenozoic, accommodated by south-dipping intracontinental subduction along the Main Pamir Thrust (MPT) coupled to strike-slip faults on the eastern flank of the orogen and both strike-slip and thrust faults on the western margin. The Kashgar–Yecheng transfer system (KYTS) is the main dextral slip shear zone separating Tarim from the Eastern Pamir, with an estimated cumulative offset of ~ 280 km at an average late Cenozoic dextral slip rate of 11–15 mm/a (Cowgill, 2010). In order to better constrain the slip history of the KYTS, we collected thermochronologic samples along the eastward-flowing, deeply incised, antecedent Tashkorgan–Yarkand River, which crosses the fault system on the eastern flank of the orogen. We present 29 new biotite 40Ar/ 39Ar ages, apatite and zircon (U–Th–Sm)/He ages, and apatite fission track (AFT) analysis, combined with published muscovite and biotite 40Ar/ 39Ar and AFT data, to create a unique thermochronologic dataset in this poorly studied and remote region. We constrain the timing of four major N-trending faults; the latter three are strands of the KYTS. The westernmost, the Kuke fault, experienced significant dip-slip, west-side-up displacement between > 12 and 6 Ma. To the east, within the KYTS, our new thermochronologic data and geomorphic observations suggest that the Kumtag and Kusilaf dextral slip faults have been inactive since at least 3–5 Ma. Long-term incision rates across the Aertashi dextral slip fault, the easternmost strand of the KYTS, are compatible with slow horizontal slip rates of 1.7–5.3 mm/a over the past 3 to 5 Ma. In summary, these data show that the slip rate of the KYTS decreased substantially during the late Miocene or Pliocene. Furthermore, Miocene–present regional kinematic reconstructions suggest that this deceleration reflects the substantial increase of northward motion of Tarim rather than a significant decrease of the northward velocity of the Pamir.

Exhumational history of the north central Pamir

[1] The Pamir plateau forms a prominent tectonic salient along the western end of the Tibet-Tarim margin. Despite its tectonic significance, relatively little is known about the timing of major Cenozoic tectonic events in the Pamir. Here we present new apatite and zircon (U/Th)-He ages, bulk rock geochemistry, and Al-in-hornblende barometry results from the Karakul graben, a prominent north-south oriented rift basin located ∼50 km south of the Main Pamir Thrust. Although cooling ages do not record the onset of extension, graben-bounding normal faults provide exposures of otherwise slowly eroding rocks which record two Cenozoic thermal events. Existing geochronology and new results suggest that granitic rocks in the Karakul region were shallowly emplaced, cooled very quickly through ∼300°C, and have experienced less than 10 km of exhumation since the late Triassic. A long period of relatively slow exhumation throughout much of the late Mesozoic and Cenozoic was punctuated by two periods of accelerated exhumation during the middle Eocene (∼50–40 Ma) and early Miocene (∼25–16 Ma). We interpret the first period of accelerated exhumation as a result of tectonic uplift and subsequent erosion due to the northward propagation of the India-Asia collision. We attribute the second period of rapid exhumation to a renewed phase of tectonism and plateau uplift in the Pamir, perhaps related to a break off event along the down-going Indian plate at ∼25 Ma or to the onset of slip along the nascent Karakoram fault.

Paleomagnetic constraints on neotectonic deformation in the Kashi depression of the western Tarim Basin, NW China

A paleomagnetic study is reported of Eocene to Pliocene formations from the Kashi depression, which aims to constrain the pattern of neotectonic deformation within the western sector of the Tarim Basin in northwest China. With the exception of Pliocene specimens from one locality (East Kulukeqiati) which show large within site-mean variations in declination, most sites from five sampled formations yield well-grouped characteristic remanent magnetizations and positive fold tests and are of probable post-depositional detrital origin. First-order consistency of paleomagnetic results from a range of rock ages and localities demonstrates that only small inter-locational vertical-axis rotation has occurred here and indicates that the Kashi depression is decoupled from the remainder of Tarim to the east and has behaved as a quasi-rigid block which has rotated by 20–30° counterclockwise relative to Eurasia and North China since the late Pliocene. The crustal-scale Talas-Ferghana Fault cuts the Tian Shan and meets the Kashi depression in the region immediately to the northwest of the study region and we find no paleomagnetic evidence for differential rotations to suggest that this fault zone extends southwards across the Kashi depression to link with the North Pamir Thrust Fault (NPTF). Instead, we argue that the southern extension of this zone is a transform-orogen junction with southward motion of the eastern wall accommodated by southward thrusting at the margins of the south Tian Shan and the Tarim Basin. We propose that dextral transpression around the margins of the crustal block incorporating the Kashi depression was responsible for the contrasting amounts of thrusting on the NPTF in the southwest and the South Tian Shan Thrust Fault in the north. Extensive evidence for neotectonism in the bordering zones of this block, as well as some paleomagnetic evidence from low unblocking temperature components, indicates that the deformation produced by block rotation is ongoing.