North Pamir Research Articles

Geochronology, Geochemistry, and Geodynamic Implications of Permo-Triassic Back-Arc Basin Successions in the North Pamir, Central Asia

Abstract The Permo-Triassic period marks the time interval between Hercynian (Variscan) orogenic events in the Tien Shan and the North Pamir, and the Cimmerian accretion of the Gondwana-derived Central and South Pamir to the southern margin of the Paleo-Asian continent. A well-preserved Permo-Triassic volcano-sedimentary sequence from the Chinese North Pamir yields important information on the geodynamic evolution of Asia’s pre-Cimmerian southern margin. The oldest volcanic rocks from that section are dated to the late Guadalupian epoch by a rhyolite and a dacitic dike that gave zircon U-Pb ages of ~260 Ma. Permian volcanism was largely pyroclastic and mafic to intermediate. Upsection, a massive ignimbritic crystal tuff in the Chinese Qimgan valley was dated to 244.1±1.1 Ma, a similar unit in the nearby Gez valley to 245±11 Ma, and an associated rhyolite to 233.4±1.1 Ma. Deposition of the locally ~200 m thick crystal tuff unit follows an unconformity and marks the onset of intense, mainly mafic to intermediate, calc-alkaline magmatic activity. Triassic volcanic activity in the North Pamir was coeval with the major phase of Cimmerian intrusive activity in the Karakul-Mazar arc-accretionary complex to the south, caused by northward subduction of the Paleo-Tethys. It also coincided with the emplacement of basanitic and carbonatitic dikes and a thermal event in the South Tien Shan, to the north of our study area. Evidence for arc-related magmatic activity in a back-arc position provides strong arguments for back-arc extension or transtension and basin formation. This puts the Qimgan succession in line with a more than 1000 km long realm of extensional Triassic back-arc basins known from the North Pamir in the Kyrgyz Altyn Darya valley (Myntekin formation), the North Pamir of Tajikistan and Afghanistan, and the Afghan Hindukush (Doab formation) and further west from the Paropamisus and Kopet Dag (Aghdarband, NE Iran).

How do pre-existing weak zones and rheological layering of the continental lithosphere influence the development and evolution of intra-continental subduction?

• The depth of a preexisting weak zone is one of the key factors in initiating intra-continental subduction. • Crustal internal friction angle plays an important role in altering the deformation type of a continental collision system. • The reactivation of the suture may be an auxiliary condition for the evolution of the south-dipping continental subduction underneath the North Pamir. Intra-continental subduction is of special importance for studying the formation of intra-continental orogens, crust-mantle structural evolution, and the far-field effects of continental collision, whose mechanism is still a matter of discussion. In this work, we investigated the role of pre-existing weak zones and the continental lithospheric rheological layering in the formation and evolution of the intra-continental subduction based on a 2D finite element numerical technique. The model results indicate that the deeper the intra-continental weak zone is and the faster the convergence velocity is, the more likely it is to develop into a new intra-continental subduction. Altering the rheological strength of the overriding plate may not have a substantial impact on the intra-continental subduction mode when the depth of the pre-existing weak zone is larger than half of the lithospheric thickness. In contrast, the lithospheric rheological strength is closely related to the continental collision system’s deformation style: Models with a weaker overriding plate are inclined to delaminate continuously under collision, whereas a strong overriding plate results in the subducting plate’s roll-back. The reactivation of the suture that runs deep into the lithosphere as a result of the Indian-Asian continental collision could be one of the crucial factors controlling the formation of the south-dipping subduction under the North Pamir.

Calcite U–Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia

Abstract. The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U–Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.

Middle to late Miocene growth of the North Pamir

AbstractHow and when the Pamir formed remains an open question. This study explores Pamir tectonics recorded in a sedimentary section in the eastern Tajik Basin. A prominent lithofacies change that has been recognised regionally is assigned to the middle Miocene (13.5 Ma based on preferred magnetostratigraphic correlation). Closely following this change, detrital zircon U‐Pb age spectra and mudstone bulk‐rock εNd values exhibit a sediment source change from the Central to the North Pamir estimated ca. 12 Ma. At the same time, the stable oxygen and carbon isotopic values of carbonate cements show negative and positive shifts, respectively. Combined with previous studies in both the Tajik and Tarim basins, these results suggest that the North Pamir experienced a middle–late Miocene phase of deformation and surface uplift. This supports models proposing middle–late Miocene Pamir tectonism, and climate models implying that coeval Pamir orogenesis deflected Westerly moisture and affected Asian environments.

Mesozoic–Cenozoic Tectonic Evolution and Uplift in Pamir: Application of Fission Track Thermochronology

AbstractThe Pamir Plateau can be divided into three secondary tectonic units from north to south: the North, the Middle and the South Pamir Blocks. The North Pamir Block belonged to the southern margin of Tarim‐Karakum, thermochronological study of the Pamir structural intersection indicates that accretion of the Middle Pamir Block to the Eurasian Continental Margin and its subduction and collision with the North Pamir Block occurred in the Middle–Late Jurassic. Due to the Neo‐Tethys closure in the Early Cretaceous, the South Pamir Block began to collide with the accretion (the Middle Pamir Block) of the Eurasian Continental Margin. Affected by the collision and continuous convergence between the Indian Plate and the Eurasian Plate since the Cenozoic, Pamir is in a multi‐stage differential uplift process. During 56.1–48.5 Ma, North Pamir took the lead in uplifting, that is, the first rapid uplift in the Pamir region began there. The continuous compression and contraction of the Indian and Eurasian plates during 22.0–15.1 Ma forced the Pamir tectonic syntaxis to begin its overall uplift, i.e. Pamir began to enter the second rapid uplift stage in the Early Oligocene, which lasted until the Middle Miocene. During 14.6–8.5Ma, South Pamir was in a rapid uplift stage, while North Pamir was in a relatively stable state, showing asymmetry of tectonic deformation in the Pamir region in space. Since 6.5 Ma, Pamir began to rapidly uplift again.

The Carboniferous Arc of the North Pamir

Abstract In this study, we investigate the age and geochemical variability of volcanic arc rocks found in the Chinese, Kyrgyz, and Tajik North Pamir in Central Asia. New geochemical and geochronological data together with compiled data from the literature give a holistic view of an early to mid-Carboniferous intraoceanic arc preserved in the northeastern Pamir. This North Pamir volcanic arc complex involves continental slivers in its western reaches and transforms into a Cordilleran-style collision zone with arc-magmatic rocks. These are hosted in part by Devonian to Carboniferous oceanic crust and the metamorphic Kurguvad basement block of Ediacaran age (maximum deposition age) in Tajikistan. We discuss whether a sliver of Carboniferous subduction-related basalts and intruded tonalites close to the Chinese town of Mazar was part of the same arc. LA-ICP-MS U-Pb dating of zircons, together with whole rock geochemistry derived from tonalitic to granodioritic intrusions, reveals a major Visean to Bashkirian intrusive phase between 340 and 320 Ma ago. This 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 the Tarim cratonic crust challenge the idea of a continuous cratonic Tarim-Tajik continent beneath the Pamir.

Nonextensive Analysis of Aftershocks Following Moderate Earthquakes in Tien Shan and North Pamir

Aftershock sequences have been identified for 60 earthquakes (K ≥ 11) from the catalog of the Institute of Seismology, National Academy of Sciences, Kyrgyz Republic (to be referred to as KIS in what follows). The catalog includes 56 259 events that have occurred in Tien Shan and North Pamir during the period from 1980 to 2001. We have obtained some statistical characteristics of the catalog and determined its completely reported part. A map of seismic activity has been made. We determined some parameters of main events and aftershock sequences. The lowest number of aftershocks was 3 events, the largest number was 2370, the shortest duration of the aftershock process was 2 days and the longest 2046 days. The earthquake catalog and the aftershock sequences were treated from the standpoint of nonequilibrium thermodynamics using the Tsallis statistic (the parameters q and a). We sought to obtain a description of earthquakes over energy using a modified stick-slip earthquake source model and the principle of maximum entropy. This approach was used to obtain a quantitative estimate of the nonextensive Tsallis parameter q for seismicity, as well as to estimate the variation of that parameter just before a large earthquake and during aftershocks. It was shown that seismicity is a system with memory and long-range spatial correlations. The Tsallis parameter q for aftershock sequences exceeds the value of q based on the entire catalog, thus indicating an unstable state of the earthquake-generating system during aftershocks. One observed a sharp increase in the Tsallis parameter q in an area that includes the precursory area before the main event and a sharp drop at once after it, gradually returning to the average level observed before the main event.

Interaction of the Indian and Asian Plates Under the Pamir and Hindu‐Kush Regions: Insights From 3‐D Shear Wave Velocity and Anisotropic Structures

AbstractThe Pamir‐Hindu‐Kush region is widely cited as a best place to study opposing continental subduction on Earth. Yet, subducting slab morphology of the Indian and Asian plates under this region remains elusive. Here we report new shear wave velocity and radial anisotropy models from Rayleigh and Love wave tomography to constrain the geometry of these two slabs. Together with previous tomographic studies, we show that the Indian slab exhibits along‐strike variation with subhorizontally underthrusting into the Pamir and high‐angle subduction under the Hindu‐Kush. We speculate that the Indian slab was probably detached and sank into the mantle transition zone under the Pamir, while break‐off process is ongoing under the Hindu‐Kush. The Asian lithospheric mantle and lower crust are inferred to be subducting southward into the North Pamir. Two opposing subducting slabs could have interacted under the Pamir, resulting in thickening crust, horizontally stretched midcrustal flow, and horizontal mantle flow. These dynamic processes could explain surface geologic observations and potentially constrain the causes of intermediate‐depth seismicity.

Convergence of the Pamir and the South Tian Shan in the late Cenozoic: Insights from provenance analysis in the Wuheshalu section at the convergence area

AbstractIn response to collision and convergence between India and Asia during the Cenozoic, convergence took place between the Pamir and South Tian Shan. Here we present new detrital zircon U-Pb ages coupled with conglomerate clast counting and sedimentary data from the late Cenozoic Wuheshalu section in the convergence zone, to shed light on the convergence process of the Pamir and South Tian Shan. Large Triassic zircon U-Pb age populations in all seven samples suggest that Triassic igneous rocks from the North Pamir were the major source area for the late Cenozoic Wuheshalu section. In the Miocene, large populations of the North Pamir component supports rapid exhumation in the North Pamir and suggest that topography already existed there since the early Miocene. Exhumation of the South Tian Shan was relatively less important in the Miocene and its detritus could only reach a limited area in the foreland area. Gradually increasing sediment loading and convergence of the Pamir and South Tian Shan caused rapid subsidence in the convergence area. Since ca. 6–5.3 Ma, the combination of a major North Pamir component and a minor South Tian Shan component at the Wuheshalu section is consistent with active deformation of the South Tian Shan and the North Pamir. During deposition of the upper Atushi Formation, a larger proportion of North Pamir–derived sediments was deposited in the Wuheshalu section, maybe because faulting and northward propagation of the North Pamir caused northward displacement of the depocenter to north of the Wuheshalu section.

Miocene exhumation of northeast Pamir: Deformation and geo/thermochronological evidence from western Muztaghata shear zone and Kuke ductile shear zone

The West Muztaghata Shear Zone (WMSZ) and Kuke ductile shear zone (KS) are located at the western and eastern margins, respectively, of the Muztaghata gneiss dome (MD) in the northeast Pamir. We report new geological data, including quartz fabric analyses, zircon U-Pb ages, and mica 40Ar-39Ar ages, to constrain the geometry, kinematics and evolution time of the WMSZ and KS. The presence of garnet and kyanite in the mylonites from the WMSZ indicate that the shear zone roots into the lower crust. The deformation characteristics of mylonites of the WMSZ show top-to-W or SW shear sense, and formed at mid-high temperatures. Zircons from high-grade mylonites and synkinematic leucogranites record a consistent U-Pb ages of ∼20 Ma, interpreted to reflect high-grade metamorphism and partial melting in the northeast Pamir in the early Miocene prior to ductile shearing along the WMSZ. The adjacent undeformed 12 Ma Kuzigan pluton, indicates deformation along the WMSZ ceased by then. 40Ar-39Ar dating results for mica show that the WMSZ underwent continued exhumation to the middle-upper crust until ∼12–8 Ma. To the east, the KS has E-dipping foliations with a top-to-E normal shear sense and preserves low to medium deformation temperature fabrics. Previous data and new biotite dating from the KS yield 40Ar-39Ar ages of ∼12–8 Ma, indicating broadly synchronous activity along the KS in the middle Miocene. Integrated with previous studies, our results suggest that continuous India-Asia convergence led to thickening of the Pamir crust, which propagated from south to north, initiating in the South Pamir in the late Eocene and advanced through the North Pamir around 20 Ma. This was immediately followed by rapid exhumation during east-west stretching and dome formation, broadly coeval but slightly outlasting exhumation of the other Central Pamir gneiss domes.

Mid-Miocene (post 12 Ma) Displacement along the Central Karakoram Fault Zone in the Nubra Valley, Ladakh, India from Spot LA-ICPMS U/Pb Zircon Ages of Granites

ABSTRACT The Karakoram fault zone is a prominent right lateral fault that connects the frontal thrust of the North Pamir with the Indus suture zone near Mount Kailas. Its nature and age of initiation is controversial. In the Nubra valley, Ladakh, India, a Karakoram range granite is thrust over Cretaceous magmatic arc rocks and this thrust is cut by a western strand of the Karakoram fault zone. Three different lithologies from this granite gave weighted mean zircon U/Pb ages of 12.92±0.77 Ma, 12.41±0.43 Ma, and 11.72±0.31 Ma. The ages indicate a relatively short intrusive history of about 1 Ma for the phases: the geochemistry is practically identical to the Pangong leucogranites in the same tectonic block. The Karakoram fault zone in this area is thus less than ~12 Ma old which supports a post middle Miocene (Serravallian) age of Karakoram fault initiation in this area.

Indentation of the Pamirs with respect to the northern margin of Tibet: Constraints from the Tarim basin sedimentary record

AbstractThe Pamirs represent the indented westward continuation of the northern margin of the Tibetan Plateau, dividing the Tarim and Tajik basins. Their evolution may be a key factor influencing aridification of the Asian interior, yet the tectonics of the Pamir Salient are poorly understood. We present a provenance study of the Aertashi section, a Paleogene to late Neogene clastic succession deposited in the Tarim basin to the north of the NW margin of Tibet (the West Kunlun) and to the east of the Pamirs. Our detrital zircon U‐Pb ages coupled with zircon fission track, bulk rock Sm‐Nd, and petrography data document changes in contributing source terranes during the Oligocene to Miocene, which can be correlated to regional tectonics. We propose a model for the evolution of the Pamir and West Kunlun (WKL), in which the WKL formed topography since at least ~200 Ma. By ~25 Ma, movement along the Pamir‐bounding faults such as the Kashgar‐Yecheng Transfer System had commenced, marking the onset of Pamir indentation into the Tarim‐Tajik basin. This is coincident with basinward expansion of the northern WKL margin, which changed the palaeodrainage pattern within the Kunlun, progressively cutting off the more southerly WKL sources from the Tarim basin. An abrupt change in the provenance and facies of sediments at Aertashi has a maximum age of 14 Ma; this change records when the Pamir indenter had propagated sufficiently far north that the North Pamir was now located proximal to the Aertashi region.

Brachiopods from the Cisuralian–Guadalupian of Darvaz, Tajikistan and implications for Permian stratigraphic correlations

In this paper, we describe the upper Cisuralian Safetdara and Gundara formations of the Darvaz mountains, North Pamir, which were part of the Kunlun Arc, developed along the active Eurasian margin. The Safetdara Formation comprises massive limestones (mainly cyanobacterial, Tubiphytes and Archaeolithoporella boundstones) alternating with well-bedded bioclastic and oncoidal limestones and an interval of recessive shales. The formation crops out above the Chelamchi Formation consisting of turbiditic siltstones and sandstones with bioclastic silty limestones yielding massive limestone olistoliths. The Gundara Formation consists of fine sandstones at the base, followed by well-bedded marly bioclastic, oncoidal and microbial limestones, bearing a rich silicified brachiopod fauna in life-position. Two new taxa have been identified in this association: the cemented coralliform Gundaria insolita n. gen. n. sp. and the pedicle attached Hemileurus politus n. sp. The inferred environmental setting is that of shoal deposits of warm, shallow, high energy, clear marine waters for the Safetdara Formation. The agglutinated microbial reefs to cluster reefs of the Gundara Formation were probably growing in a muddier, quieter and probably slightly deeper setting.The foraminifers of the Brevaxina Zone suggest a Bolorian age for the top of the Chelamchi Formation, the Safetdara Formation and the base of the Gundara Formation. Kungurian conodonts have been found in the lower part of the Safetdara Formation. The biostratigraphic data from the sedimentary succession of North Pamir, integrated with those already obtained from Southeast Pamir, allow to refine the correlations between the Tethyan regional scale and the International Time Scale. In particular, it seems now clear that the Bolorian and the lower part of the Kubergandian correlate to the Kungurian.

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.

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 right-slip faulting along the eastern margin of the Pamir salient, northwestern China

The Pamir salient defines the western end of the Himalayan-Tibetan orogen and has overthrust the Tarim-Tajik basin to the north by ∼300 km along a late Cenozoic, south-dipping intracontinental subduction zone. Field mapping, structural measurements, and analysis of mesoscale structures along a 32-km-long reach of the Yarkand River document the tectonic evolution of the east flank of this salient, between the North Pamir to the west and the Western Kunlun Shan to the east. The study area is cut by a set of four, north-northwest–striking, steeply dipping brittle faults. Microstructures and asymmetric outcrop- to map-scale folds indicate right slip along these faults. Between these structures, fault-bounded panels of Phanerozoic strata are deformed by en echelon folds with axes that trend more westerly than the adjacent faults, consistent with dextral transpression. The fault system described here extends for ∼350 km along the eastern flank of the Pamir salient. Transpressional right slip along this set of faults, here called the Kashgar-Yecheng transfer system, appears to have accommodated late Cenozoic separation of the North Pamir from the Western Kunlun Shan during south-directed intracontinental subduction beneath the leading edge of the Pamir salient. Correlation of major faults suggests total slip along the Kashgar-Yecheng transfer system is likely on the order of ∼280 km. This offset estimate implies long-term slip rates of 7–15 mm/a along the Kashgar-Yecheng transfer system when combined with previous sedimentologic, stratigraphic, and thermochronologic data that indicate deformation along the east flank of the Pamir started between the late Eocene and early Miocene. These results imply that the first-order structures on the western and eastern flanks of the Pamir are asymmetric: previous work has shown that deformation in the west was accommodated by anticlockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin. This rotation is generally interpreted to reflect northwest-directed radial thrusting, in contrast to the transpressional right-slip transfer faulting on the east side reported here.

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.

EARLIEST ARTINSKIAN (EARLY PERMIAN) FUSULINIDS REWORKED IN THE TRIASSIC LERCARA FORMATION (NW SICILY)

The Permian limestone boulders reworked in the Triassic Lercara Formation (NW Sicily) contain carbonate microfossils consisting of algae, smaller foraminifers and fusulinids. The following fusulinids are described: Quasificsulina ultima Kanmera, Robustoschwagerina cf, R, schellwieni (Hanzawa), Chalaroschwagerina (?) globosa (Schellwien). The group of Chalaroschwagerina (?) vulgaris (Schellwien) is emended. Based on this fusulinid assemblage, the limestone boulders of Lercara are age-dated as early Yakhtashian; i.e,, earliest Artinskian (circa -280 to -275 Ma). Therefore at least two distinct olistostromes are present in Sicily: the post-Yakhtashian olistostrome studied here, and the second one post-Dorashamian in age. Similar assemblages do not occur in adjacent European areas (Camic Alps, Montenegro or Greece), but they have been recorded in Darvas (North Pamir) and Japan, revealing constant paleoecological conditions throughout the Paleotethys.

Distribution of main ore deposits and strong earthquakes in regard to the lithospheric structure of Central Asia

The territory of Central Asia which includes the Hercynian folded structures of the central and western parts of the Tien Shan, the Touran plate, the Pamirs, and the Kopetdagh alpides, contains various ore deposits including giant units. It is also a high seismic region where strong and destructive earthquakes occur rather often. Earthquake foci (hypocenters) in the Tien Shan region mainly appear in the upper crust (up to depths of 25-30 km). In the Pamirs, these events can occur both in the upper crust and in an upper part of the upper mantle (up to depths of 220-240 km). The lower crust in Central Asia (foot depths 30-80 km) appears to be aseismic in almost all areas. The earthquake epicenters trace large sublatitudinal steeply dipping faults that divide tectonic elements and blocks and displace the Moho discontinuity for 5 to 10 km (North Ferghana, South Ferghana, Atbashi-Inylchek, South Tien Shan, North Pamir, and other faults). Large geotectonic units of the Tien Shan and the Pamirs are of well-defined metallogenic specialization: caledonides of the North Tien Shan - ferrous/polymetallic/rare-earths; hercynides of the Middle Tien Shan - copper/ferrous/polymetallic/gold with fluorite and uranium; hercynides of the South Tien Shan -gold/rare metals (W, Mo, Sn, Hg, Sb) etc. The ore deposits are arranged as a belt zonation (the South Tien Shan) or as a node distribution (the Middle Tien Shan). Antagonism between large endogenic ore deposits and epicentral zones of strong earthquakes is being confirmed by evidence from other countries. Within the lithosphere, one can find a certain connection between the distribution of ore deposits and high velocity bodies. The absence of strong crustal earthquake foci in zones of lower seismic velocities, high conductivity, and higher plasticity (Vp/Vs > 1.75) has been established. In the present report, the authors offer an interpretation of the causes of these associations.

A preliminary study of regional seismic discrimination in central Asia with emphasis on western China

AbstractIn support of a Comprehensive Test Ban Treaty, we have started to evaluate regional seismic-event discrimination capabilities for central Asia, emphasizing western China. We have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband, digital station WMQ in western China and over 120 earthquakes and five nuclear explosions at station AAK in Kyrgyzstan. The explosions are from the Kazakh Test Site (KTS) and Lop Nor, China. The earthquakes are mostly from northwest China. Event magnitudes (measured as either mb or ML) range between 2.5 and 6.5 and maximum event-station distance is about 1700 km. We formed phase, spectral, cross-spectral, short-period/long-period, and long-period ratios to test many possible event discriminants. All ratios were corrected for distance effects before forming ratio-versus-magnitude discrimination plots.The most consistent discriminants for separating earthquakes from explosions are P/S phase ratios for frequencies above 3 to 4 Hz. P-wave (f > 1 Hz) to Rayleigh-wave (0.03 Hz < f < 0.1 Hz) ratios are also effective, but surface-waves are difficult to measure for magnitudes below about 4.5. For the P/S ratios, separation between earthquakes and explosions increases with frequency, and for the P/R ratios, separation increases with increasing P frequency. The P (3 to 6 Hz)/S (0.75 to 1.5 Hz) cross-spectral ratios also separate earthquakes and explosions. These cross-spectral ratios may prove quite useful because, due to weak high-frequency Lg signals from explosions, the high-frequency P/Lg ratio sometimes cannot be evaluated, but the P (3 to 6 Hz)/Lg (0.75 to 1.5 Hz) can be evaluated successfully.P spectral ratios (3 to 6 Hz/0.75 to 1.5 Hz) also separate earthquakes and explosions but not quite as consistently as the P/S ratios. However, P spectral ratios may prove useful in cases where path effects block or highly attenuate Sn and Lg waves, rendering the P/S ratios unavailable. In contrast to Nevada Test Site (NTS) events, the Lg (3 to 6 Hz/0.75 to 1.5 Hz) spectral ratio does not separate earthquakes and nuclear explosions in central Asia. This observation may be a source effect, as Asian explosions are thought to be detonated in highly lithified rocks below the water table, while most of the smaller (mb < 4.8) NTS explosions have been detonated in poorly lithified rocks above the water table.For station AAK, the short-period spectral and cross-spectral ratios identify a few earthquakes (not recorded at WMQ) from the north Pamir region that plot with the Lop Nor explosions. The waveforms from these earthquakes lack surface waves and a distinct Lg phase. These events, which we interpret as having occurred in a subducted slab, do separate from the explosions on high-frequency (f > 4 Hz) P/S discrimination plots. For station WMQ, long-period R/L (Rayleigh-wave/Love-wave) ratios separate KTS explosions and Asian earthquakes. However, R/L ratios fail to identify the large Lop Nor explosions recorded at station AAK because the Love waves are as large as the Rayleigh waves, suggesting that strong tectonic release occurs with these explosions. Because we have examined only one path at a single station for these explosions, source-radiation pattern may be influencing this observation.