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Interactions between midlatitude westerlies and the Pamir–Tian Shan mountains significantly impact hydroclimate patterns in Central Asia today, and they played an important role in driving Asian aridification during the Cenozoic. We show that distinct west-east hydroclimate differences were established over Central Asia during the late Oligocene (ca. 25 Ma), as recorded by stable oxygen isotopic values of soil carbonates. Our climate simulations show that these differences are present when relief of the Pamir–Tian Shan is higher than 75% of modern elevation (∼3000 m). Integrated with geological evidence, we suggest that a significant portion of the Pamir–Tian Shan orogen had reached elevations of ∼3 km and acted as a moisture barrier for the westerlies since ca. 25 Ma.

National Natural Science Foundation of China [41672158]; Chinese 111 Project Ministry of Education, China - 111 Project [BP2018001]; National Science Foundation (NSF) [EAR1450917]; Fundamental Research Funds for the Central Universities [lzyjbky-2019-99]; second Tibetan Plateau Scientific Expedition and Research Program (STEP) [2019QZKK0602]

Investigation of a >6‐km‐thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid‐Cretaceous succession, an Upper Cretaceous–Lower Eocene succession and an Eocene–Neogene succession. The Lower to mid‐Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul–Mazar subduction–accretion complex in the northern Pamir. This succession is characterized by a convex‐up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine‐grained, marginal marine and sabkha deposits. The succession is characterized by a concave‐up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea‐level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous–early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50–30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10–30 Myr younger than estimates for the initiation of India–Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4‐km‐thick upper Eocene–Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro‐foreland basin associated with crustal thickening of the Pamir during India–Asia collision. Thus, the Tajik Basin provides an example of a long‐lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin‐forming mechanisms are influenced by subduction dynamics, including periods of slab‐shallowing and retreat.

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.

The atmospheric mechanisms responsible for the loess grain‐size variations in central Asia are less clear, and an increasing number of studies have found that the coarse‐grained component is likely supplied by the surface circulation rather than by the westerlies (westerlies circulation). In this study, we attempt to determine the atmospheric dynamics patterns in the Afghan‐Tajik Basin during the past 800 kyr, based on the coarse grain‐size fraction of loess, combined with a study of modern dust transport processes. We suggest that the coarse grain‐size fraction can be used to indicate the intensity of near‐surface winds, while on glacial‐interglacial time scales, the variations of loess grain size in Tajikistan are dominated by Northern Hemisphere ice volume via its effect on the strength of the Siberian High. In addition, we suggest that atmospheric greenhouse gases concentration affected the loess grain size during interglacials via their influence on the temperature of southern high latitudes.

International audience; Surface, seismic, and borehole data characterize the Neogene-Recent Tajik fold-and-thrust belt of the Tajik basin. The basin experienced little sub-detachment basement deformation, acting as a rigid foreland plate during the Pamir orogeny. The Tajik fold-and-thrust belt contains variable thinskinned structural styles, changing along and across strike as a function of the thickness and facies of Upper Jurassic evaporites, which constitute the basal detachment, and the influence of the surrounding thickskinned belts. The southern Tajik fold-and-thrust belt shows regularly spaced, salt-cored, thrusted detachment anticlines that transition northward into imbricated thrust sheets grouped in oppositely verging stacks facing each other across a common footwall syncline. The width of the fold-and-thrust belt decreases northeastward accommodated by the Ilyak fault, a lateral ramp developed over a seismically active dextral basement fault. The southeastern Tajik fold-and-thrust belt contains massive subaerial salt sheets, formed by squeezing of preexisting salt diapirs. The salt-tectonic domain originates from a local depocenter within the Late Jurassic Amu Darya-Tajik evaporitic basin. Serial cross sections, integrating the structural geometries, yielded minimum thinskinned shortening oriented at~90°to the India-Asia convergence direction, increasing from~93 km in the south to~148 km in the center, and dropping tõ 22 km in the northeast; total shortening-including the foreland buttress-is ≥170 km. Most of the shortening in the central-southern Tajik fold-and-thrust belt occurred by hinterland-vergent, high-displacement back thrusts. The Pamir played a dominant role in the transfer of shortening to the sedimentary infill of the Tajik basin with the Tian Shan acting as a semi-passive buttress.

National Science Foundation (NSF) [EAR-1450899]; NSF [EAR-1419748, EAR-0929777, 1338583]; American Philosophical Society; American Association of Petroleum Geologists; Geological Society of America; Exxon Mobil Corporation

New geochronologic, geochemical, and isotopic data for Mesozoic to Cenozoic igneous rocks and detrital minerals from the Pamir Mountains help to distinguish major regional magmatic episodes and constrain the tectonic evolution of the Pamir orogenic system. After final accretion of the Central and South Pamir terranes during the Late Triassic to Early Jurassic, the Pamir was largely amagmatic until the emplacement of the intermediate ( SiO 2 > 60 wt. % ), calc-alkaline, and isotopically evolved (−13 to −5 zircon eHf(t)) South Pamir batholith between 120–100 Ma, which is the most volumetrically significant magmatic complex in the Pamir and includes a high flux magmatic event at ∼105 Ma. The South Pamir batholith is interpreted as the northern (inboard) equivalent of the Cretaceous Karakoram batholith and the along-strike equivalent of an Early Cretaceous magmatic belt in the northern Lhasa terrane in Tibet. The northern Lhasa terrane is characterized by a similar high-flux event at ∼110 Ma. Migration of continental arc magmatism into the South Pamir terrane during the mid-Cretaceous is interpreted to reflect northward directed, low-angle to flat-slab subduction of the Neo-Tethyan oceanic lithosphere. Late Cretaceous magmatism (80–70 Ma) in the Pamir is scarce, but concentrated in the Central and northern South Pamir terranes where it is comparatively more mafic ( SiO 2 60 wt. % ), alkaline, and isotopically juvenile (−2 to +2 zircon eHf(t)) than the South Pamir batholith. Late Cretaceous magmatism in the Pamir is interpreted here to be the result of extension associated with roll-back of the Neotethyan oceanic slab, which is consistent with similarly aged extension-related magmatism in the Karakoram terrane and Kohistan. There is an additional pulse of magmatism in the Pamir at 42–36 Ma that is geographically restricted (∼150 km diameter ellipsoidal area) and referred to as the Vanj magmatic complex. The Vanj complex comprises metaluminous, high-K calc-alkaline to shoshonitic monzonite, syenite, and granite that is adakitic (La/YbN = 13 to 57) with low Mg# (35–41). The Vanj complex displays a range of SiO2 (54–75 wt.%) and isotopic compositions (−7 to −3 eNd(i), 0.706 to 0.710 87Sr/86Sr(i), −3 to +1 zircon eHf(i), 6.0 to 7.6‰ zircon δ 18 OVSMOW), which reflects some juvenile mantle input and subsequent assimilation or mixing with the Central/South Pamir terrane lower crust. The Vanj complex is speculatively interpreted to be the consequence of a mantle drip or small delamination event that was induced by India–Asia collision. The age, geochemistry, outcrop pattern, and tectonic position of the Vanj magmatic complex suggest that it is part of a series of magmatic complexes that extend for >2500 km across the Pamir and northern Qiangtang terrane in Tibet. All of these complexes are located directly south of the Tanymas–Jinsha suture zone, an important lithospheric and rheological boundary that focused mantle lithosphere deformation after India–Asia collision. Miocene magmatism (20–10 Ma) in the Pamir includes: 1) isotopically evolved migmatite and leucogranite related to crustal anataxis and decompression melting within extensional gneiss domes, and; 2) localized intra-continental magmatism in the Dunkeldik/Taxkorgan complex.

The amalgamation of the Central Asian Orogenic Belt in the southwestern Tian Shan in Tajikistan is represented by tectono-magmatic-metamorphic processes that accompanied late Paleozoic ocean closure and collision between the Karakum–Tarim and Kazakh–Kyrgyz terranes. Integrated U-Pb geochronology, thermobarometry, pseudosection modelling, and Hf geochemistry constrain the timing and petro-tectonic nature of these processes. The Gissar batholith and the Garm massif represent an eastward, along-strike increase in paleodepth from upper-batholith (~21–7 km) to arc-root (~36–19 km) levels of the Andean–syn-collisional Gissar arc, which developed from ~323–288 Ma in two stages: (i) Andean, I-type granitoid magmatism from ~323–306 Ma due to northward subduction of the Gissar back-arc ocean basin under the Gissar microcontinent, which was immediately followed by: (ii) syn-collisional, I–S-type granitoid magmatism in the Gissar batholith and the Garm massif from ~304–288 Ma due to northward subduction/underthrusting of Karakum marginal-continental crust under the Gissar microcontinent. A rapid isotopic pull-up from ~288–286 Ma signals the onset of juvenile, alkaline-syenitic, post-collisional magmatism by ~280 Ma, which was driven by delamination of the Gissar arclogite root and consequent convective asthenospheric upwelling. Whereas M–HT/LP prograde metamorphism in the Garm massif (650–750 °C / 6–7 kbar) from ~310–288 Ma was associated with subduction-magma inundation and crustal thickening, HT/LP heating and decompression to peak-metamorphic temperatures (~800–820 °C / 6–4 kbar) at ~288 ± 6 Ma was driven by the transmission of a post-collisional, mantle-derived heat wave through the Garm-massif crust.

International audience; The Tajik basin archives the orogenic evolution of the Pamir hinterland. Stratigraphic‐sedimentologic observations from Cretaceous‐Pliocene strata along its eastern margin describe the depositional environment and basin‐formation stages in reaction to hinterland exhumation and basin inversion. During the Late Cretaceous‐Eocene (preorogenic stage: ~100–34 Ma), a shallow‐marine to terrestrial basin extended throughout Central Asia. An alluvial plain with influx of conglomerate bodies (Baljuvon Formation) indicates a first pulse of hinterland erosion and foreland‐basin formation in the late Oligocene‐early Miocene (synorogenic stage Ia: ~34–23 Ma). Further hinterland exhumation deposited massive alluvial conglomerates (Khingou Formation) in the early‐middle Miocene (synorogenic stage Ib: ~23–15 Ma). Westward thickening growth strata suggest transformation of the Tajik basin into the Tajik fold‐thrust belt in the middle‐late Miocene (synorogenic stage IIa: ~15–5 Ma). Increased water supply led to the formation of fluvial mega‐fans (Tavildara Formation). Latest Miocene‐Pliocene shortening constructed basin morphology that blocked sediment bypass into the central basin from the east (Karanak Formation), triggering drainage‐system reorganization from transverse to longitudinal sediment transport (synorogenic stage IIb: < ~5 Ma). Accelerated shortening (~27–20 Ma) and foreland‐directed collapse (~23–12 Ma) of Pamir‐plateau crust loaded the foreland and induced synorogenic stages Ia and Ib. Coupling of Indian and Asian cratonic lithospheres and onset of northward and westward delamination/rollback of Asian lithosphere (i.e., lithosphere of the Tajik basin) beneath the Pamir at ~12–11 Ma transformed the Tajik basin into the Tajik fold‐thrust belt (synorogenic stage IIa). The timing of the sedimentologically derived basin reconfiguration matches the thermochronologically derived onset of Tajik‐basin inversion at ~12 Ma.

International audience; The Tajik basin and southwestern Tian Shan constitute the northwestern tip of the India-Asia collision zone. Basin inversion formed the thinskinned Tajik fold-thrust belt, outlined by westward convex fold trains, underlain by a décollement in Jurassic evaporites. The belt's leading edge-the Uzbek Gissar-and its transpressional northern lateral margin-the Tajik Gissar-constitute the thickskinned foreland buttresses. Apatite fission-track data indicate~40-to 15-Ma reheating by sediment burial in the Tian Shan. In the Gissar and the Tajik fold-thrust belt, apatite fission-track and (U,Th)/He ages date the major phase of shortening/erosion between~12 and 1 Ma, with exhumation to 2-to 3-km crustal depths within a few Myr after onset of shortening. Shortening spread immediately across the fold-thrust belt, typical for belts floored by a detachment in ductile rocks, and into the foreland buttresses. Reactivation concentrated in the internal (eastern) fold-thrust belt with the thickest evaporates. The youngest ages (~6.6-1.6 Ma) occur along the Vakhsh thrust, the active erosional front of the fold-thrust belt in the northeastern Tajik basin, where it narrows between the converging Tian Shan and Pamir. Our study links major events in the Pamir hinterland with the Tajik basin and Tian Shan foreland. In the late Eocene-early Miocene, the advancing Pamir-plateau crust loaded the foreland, inducing subsidence, reheating, and early shortening. Basin inversion and major shortening/transpression in the foreland buttresses from~12 Ma onward were synchronous with the subcrustal indentation of Indian lithosphere into the Tajik-Tarim basin lithosphere and the onset of its rollback beneath the Pamir.

Neogene, syn‐collisional extensional exhumation of Asian lower–middle crust produced the Shakhdara–Alichur gneiss‐dome complex in the South Pamir. The

Previous research has indicated that variations in moisture conditions in arid central Asia (ACA) were out-of-phase with those of monsoonal Asia during the Holocene. In order to investigate this phenomenon, we compared the pattern of moisture variations in ACA and the region dominated by the East Asia summer monsoon (EASM) during the last four interglacials. The results indicate that moisture variations in ACA lagged those in the EASM region by ~ 6 kyr during MIS 5, by −3 kyr during MIS 7, by 3 kyr during MIS 9, and by 7 kyr during MIS 11. We suggest that this lagged pattern in three out of four interglacials was the result of a zonal climatic teleconnection, westerly wind intensity, and evaporation upstream. Overall, our results shed new light on the climatic variability of central Asia and its origins during the Holocene.

Several hundred thousand year old moraines preserved in the semi-arid environment of High Mountain Asia attest to Middle Pleistocene glaciations, but the regional correlation of glacial stages and the spatial extent of the glacial advances remain poorly constrained. We examined glacial landforms and Quaternary sediments in the Bartang valley, northwestern Pamir, a region with no previous quantitative glacial chronology. Using cosmogenic 10Be exposure ages, we dated glacially polished bedrock, moraines, and mass wasting deposits. Our data show that the northwestern Pamir was heavily glaciated in the Middle Pleistocene (≥220 ky) with large valley glaciers occupying some of the major valleys in the western Pamir. During the penultimate glacial cycle (191–130 ky) these valleys may have been largely ice free. Catastrophic mega debris flows with volumes ≥0.05 km3 occurred after the ice retreat and reflect paraglacial destabilization of glacial sediments. The age of the best-dated mega debris flow (81 ± 4 ky) is similar to moraine ages ∼70–80 ky documented throughout the Pamir, demonstrating that remobilized sediments may provide valuable age constraints on glacial histories. In order to facilitate regional comparison of glacial chronologies, we developed a Gaussian separation algorithm, which determines a moraine age from a distribution of boulder exposure ages based on the assumption that post-depositional processes prevail over inheritance, and that the oldest boulder ages best represent the timing of moraine formation. We compiled moraine boulder exposure ages from the Pamir and adjacent regions and provide a summary of Middle and early Late Pleistocene glacial cycles of western High-Mountain Asia.

Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic−Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7−2.4, 2.2−1.7, 1.1−0.85, and 0.85−0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661‒552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310‒305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305‒270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon e Hf and whole-rock e Nd values of −2.1 to −6.9 and −2.7 to −7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0‒1.2 and ca. 1.1‒2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661‒552 Ma Garm gneiss crystallization ages and the ca. 1.0−2.2 Ga model ages (and the lack of 2.4−3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton‒Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256−238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290‒270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post−Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

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%

We have used geodetic techniques to improve constraints on the crustal motion of the Pamir Plateau. Three campaigns of Global Position System data acquisition between 2011 and 2015 demonstrate that, in association with the India–Asia collision, a complex pattern of crustal motion exists in the Pamir Plateau. In a north–south direction from the Indian Plate to the Hazak Block, the crust has absorbed ~ 35 mm/yr of shortening, of which ~ 35% is distributed around the Hindu Kush region (~ 12 mm/yr), and another ~ 35% is taken up around the Alai Valley (also ~ 12 mm/yr). Global Position System measurements also show ~ 5 mm/yr of shortening between the Pamir Plateau and the Tajik Basin, whereas between the Pamir and the Tarim Basin, an ~ 10 mm/yr extension rate is observed. With respect to the stable Eurasian Plate, the Pamir rotates counterclockwise at a rate of ~ 1.822°Myr− 1, with an Euler pole positioned about the west end of the Tajik Basin (37.03 ± 0.74°N, 65.89 ± 0.12°E). The strain rate field calculated from Global Position System velocities reveals that the crustal motion is consistent with localized deformation around the Hindu Kush and the Alai Valley, the latter representing a zone with strong shallow seismic activity.

The Tajik Basin, located at the west of the Pamir, is characterized by a series of thin-skinned folds and thrusts within its interior. Constraining the onset of deformation of these structures is important both for illustrating basin inversion process and for understanding growth history of the adjacent Pamir. To date, the timing of thrusting and folding within the Tajik Basin has not been well studied. In this study, we determined the timing of fault motion of the Aruntau thrust in the central Tajik Basin by using apatite (U Th)/He thermochronometry. Apatite (U Th)/He age data of sandstone samples collected from the hanging wall of the Aruntau thrust fault, combined with thermal history simulation, constrains the onset of rapid cooling of rock samples at ca. 10–6 Ma. Given the compressional tectonic regime, we propose that the observed rapid cooling was caused by thrust-induced exhumation, and thus, the cooling age of ~10–6 Ma approximately represents the timing of thrusting initiation. Integrated with previous studies, we found that the late Miocene tectonic deformation was not only circumscribed within the Tajik Basin, but also occurred in the areas north and east of the Pamir and within the Pamir. We suggest that the synchronously widespread tectonic deformation in the Pamir-Tian Shan convergence zone reflects the enhanced northward indentation and lateral growth of the Pamir since the late Miocene.

The Tajik Basin is now located to the west of the Pamirs, but it formerly connected with the Tarim Basin, occupied by a large epicontinental sea (proto-Paratethys Sea) once extended from Europe to Central Asia. However, the northward indentation and collision of the Pamirs into the Tian Shan separated the Tajik and Tarim Basins, creating manifold implications and consequences. Consequently, timing of the final seawater retreat from the Tajik Basin is important for understanding the evolution of the proto-Paratethys Sea, the effect of the northward indentation of the Pamirs on the sea regression, and the influence of seawater retreat on the Cenozoic aridification in the Tajik Basin. Here we present a multidisciplinary study of Upper Paleogene strata from the central Tajik Basin. Our results demonstrate that the final seawater retreat from the center of the Tajik Basin occurred at 38.6 Ma, which is earlier than a previous age of 37.4 Ma from the northeastern Tajik Basin. Such a late Eocene sea retreat was quasi-simultaneous with that in the Tarim Basin. This final marine regression was mainly a consequence of the northward indentation and growth of the Pamirs, which was an intracontinental response to the India-Asia collision. Our new detrital zircon ages indicate that the provenance of Upper Eocene-lower Oligocene strata was from the Pamirs rather than Tian Shan, implying that the reactivation of the Tian Shan was later than the early Oligocene. Our paleomagnetic data indicate a 30° counterclockwise rotation of the central Tajik Basin that is much bigger than the previous estimate of 17°, and we propose that such a counterclockwise rotation mostly occurred after the late Miocene. Finally, different from the previous reported aridfication at 39 Ma or 37 Ma, our evidence demonstrates that drought intensified since the start of the Oligocene at 34 Ma, being related to reduced water vapor transport to Central Asia, caused by global cooling-induced sea-level drop, the retreating Paratethys Sea, and weakened sea surface evaporation.

As part of the TIPAGE (TIen shan – PAmir GEodynamic program) project, passive seismological observations were made along an approximately N-S profile crossing the Pamir seismic zone for about one year. From these observations guided waves were recognized. These guided waves occur as a single, continuous, secondary, compressional (P) wave phase behind the first P-wave arrivals. An equivalent phase in the shear (S) wavefield is hardly recognizable. Modelling of the phase shows that an approximately 10 km thick low velocity zone (LVZ) between the Moho and about 160 km depth reproduces the guided waves as a single, continuous phase much better than a 15–20 km thick LVZ. Modelling of the arrival times of the guided waves reveals that a model with a P-wave velocity of 6.3 km/s above about 100 km depth, and a velocity of 7.6 km/s between this depth and the deep cluster of earthquakes at about 150 km depth provides the best fit to the observed travel-time data. One plausible way to explain the low velocity of 6.3 km/s is to invoke the presence of melts in the LVZ. Then, taking a velocity of 6.9 km/s for the lower crust being subducted, about 10–13% melt is required to obtain a velocity of about 6.3 km/s in the LVZ between the Moho and about 100 km depth. This would be in keeping with the estimated burial depths from xenoliths of Gondwana terrane affinity brought to the surface in the southeastern Pamir around 11 million yr. ago. The present-day LVZ is interpreted to comprise continental lower crust. Although guided waves are known to exist associated with subducted oceanic crust or fault zones, this is the first time to the knowledge of the authors that guided waves have been observed resulting from a LVZ associated with subducted continental lower crust.

The 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 (

A regional, balanced cross section is presented for the thin-skinned Tajik fold-andthrust belt, constrained by new structural and stratigraphic data, industrial well-log data, flexural modeling, and existing geologic and geophysical mapping. A sequential restoration of the section was calibrated with 15 new apatite (U-Th)/He ages and 7 new apatite fission-track ages from samples of the major thrust sheets within the Tajik fold-and-thrust belt. Thermokinematic modeling indicates that deformation in the Tajik fold-and-thrust belt began during the Miocene (prior to or ca. 17 Ma) and continues to near present, with long-term shortening rates of ~ 4-6 mm/yr and Pliocene to present rates of ~ 6-8 mm/yr. The Tajik fold-and-thrust belt can be characterized as two distinct, oppositely verging thrust belts. Deformation initiated at opposite margins of the Tajik foreland basin, adjacent the southwest Tian Shan and northwest Pamir Mountains, and propagated toward the center of the basin, eventually incorporating the foreland basin entirely into a composite fold-and-thrust belt. The western Tajik fold-and-thrust belt records at least 35-40 km of total shortening and is part of the greater Tian Shan orogenic system. The eastern Tajik fold-and-thrust belt records ~ 30 km of shortening linked to the Pamir Mountains. The amount of shortening in theTajik fold-and-thrust belt is significantly less than predicted by models of intracontinental subduction, which call for subduction of an ~ 300-km-long slab of continental Tajik-Tarim lithosphere beneath the Pamir. Field observations and structural relationships suggest that the Mesozoic and younger sedimentary rocks of the Tajik Basin were deposited on and across the Northern Pamir terrane and then subsequently uplifted and eroded during orogenic growth, rather than undergoing subduction beneath the Pamir. The Paleozoic-Proterozoic(?) metasedimentary and igneous rocks exposed in the Northern Pamir terrane are equivalent to the middle-lower crust of the Tajik Basin, which has become incorporated into the Pamir orogen. We propose that the southdipping zone of deep seismicity beneath the Pamir, which is the basis for the intracontinental subduction model, is related to gravitational foundering (by delamination or large-scale dripping) of Pamir lower crust and mantle lithosphere. This contrasts with previous models that related the Pamir seismic zone to subduction with or without rollback of Asian lithosphere. Delamination may explain the initiation of extension in the Pamir gneiss domes and does not require a change in plate boundary forces to switch between compressional and extensional regimes. Because the Pamir is the archetype for active subduction of continental lithosphere in the interior of continental plates (intracontinental subduction), the viability of this particular tectonic process may need to be reassessed.

Although the Pamir Plateau is an ideal place to investigate paleo-environmental changes in the westerlies-dominated high Central Asia, there are only few Holocene records from this region. We present a sub-centennially resolved lacustrine record of moisture variations from Sasikul Lake, central Pamir Plateau, based on geochemical, sedimentological, and mineralogical proxies. Our results show that generally dry conditions at Sasikul Lake during the past 2540 years were interrupted by a pronounced wet period between ad 1550 and 1900, corresponding to the ‘Little Ice Age’ (LIA). More negative values of carbonate δ18O, lower total inorganic carbon (TIC) and sand content during the LIA all indicate a relatively wet period with higher lake level. Higher TIC during the ‘Medieval Warm Period’ (MWP; ad 950–1200) reveals a lower lake level relative to the LIA. Low δ18O during this time is probably attributed to changes in the isotopic composition of input water and/or upstream moisture sources. The significant increase in detrital minerals and decrease in carbonate during the LIA provide further evidence for higher allochthonous input during the wet period at Sasikul Lake. The inferred moisture variations are consistent with existing records from regions of the northern Tibetan Plateau and Central Asia that are also influenced by the westerlies, but out-of-phase with those records from the Asian monsoon region, indicating that moisture variations at Sasikul Lake were mainly influenced by the strength and trajectories of the westerlies. The inferred water level at Sasikul Lake decreased significantly during the first half of the 20th century, and then increased in recent decades. This is consistent with the increase in lake area derived from satellite images and the monitoring data of large lake-level changes in Central Asia.

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.

[1] We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation, and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes: one beneath the Pamir and the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (approximately 10 km width), curviplanar arc that strikes east-west and dips south at its eastern end and then progressively turns by 90° to reach a north-south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east-west, yet bends northeast, toward the Pamir, at its eastern end. It may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitates a contortion and oversteepening of the latter.

Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is regarded as a possible place of active continental subduction because of the intermediate-depth seismicity, crustal xenoliths and estimates of crustal shortening versus convergence rates. Here we present for the first time receiver function images from a passive-source seismic array traversing the Tien Shan and the Pamir plateau showing southward subduction of Eurasian continental crust. In the eastern Pamir, we observe a southerly dipping 10–15 km thick low-velocity zone (LVZ) that extends from 50 km depth near the base of the crust to more than 150 km depth with a dip angle increasing to subvertical. While the upper- and mid-crustal material seems to be shortened and incorporated into the Pamir, the lower Eurasian crust detaches and subducts. In its deeper part (>80 km) the LVZ envelopes the intermediate-depth earthquakes. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust.