Southern Tianshan Research Articles

Estimation of the shortening rate since late Pleistocene in the Aksu area on the southern flank of the Tianshan, China

This research focused on the Aksu area in the central part of the southern Tianshan. Along the 60 km wide Aksu fold-and-thrust belt, active thrusts reach the surface and offset the youngest sediments. Our research was based on the geomorphologic study that examined the advance and retreat of glaciers cut by thrusts in the Tomur area in the north of Aksu. Our fieldwork revealed that two fault scarps were clearest across three different moraines that represent the maximum of advance of glaciers during three glacial periods along the Tailan River in the Tomur area. The measured heights of the fault scarps that cut the moraines, together with the moraines-inferred age, imply a shortening rate of 1.85 mm/a on the Aksu area since late Pleistocene. This rate, similar to that of the Korla area on its east side and of the Kaping area on its west side, but lower than that of the Kashgar area farther west and of the Manas area in the northern margin of the belt, implies that the distribution of shortening across the Tianshan changed markedly along the mountain.

Research on present crustal deformation in the southern Tianshan (Jiashi) region by GPS geodesy

The pattern and range of present crustal movement as well as characteristic of deformation-strain play a key role in cognizing seismogenic mechanism in the Jiashi region, Xinjiang, northwestern China. Using GPS geodesy in 1994 and 1998, here we give geodetic evidence of rapid convergence of about 19 mm/a across the western Tianshan, which is about 50% greater than the seismic moment solution (13 mm/a) by assembling major earthquake in the 20th century. The discrepancy of deformation rate between geodetic observation and seismic energy releasing indicates the possibilities that, there exists a lot of aseismic strain and strain during seismic interval in the western Tianshan, or seismic fault slip in this century could not be enough to compensate the present deformation in the whole area. Whichever it maybe, from the view of accumulation of stress-strain, it suggests that there will be potential strong earthquakes in the western segment of Tianshan and the northeastern corner of Pamir in a long period.

Crustal Structure and Tectonic Environment for Strong Earthquakes in the Tianshan Earthquake Belt

AbstractThe deep structure of the Tianshan earthquake belt is investigated, based on the seismic tomography of the northwestern China. The result indicates: a ductile detachment layer with low velocities exists in the middle crust of the Tianshan earthquake belt; the depths of faults in the southern Tianshan are beyond theMoho, whereas those in the northern Tianshan only reach middle crust; the depth of the Moho beneath the Tianshan is more than 50 km, moreover, the crust‐mantle boundary is composed of a wide velocity transition belt. Strong earthquakes are mainly concentrated in the middle‐lower crust where a clear velocity variation is observed near the range‐basin boundaries. A low velocity zone is found between the Pamir, southern Tianshan and Tarim, extending in the NNE direction. The earthquakes in Wuqia and Jiashi are separately located in the west and east sides of this low velocity zone. It is inferred that the relative motion between the Pamir, southern Tianshan and Tarim is a major reason to cause deformation within this low velocity zone and fracturing near its boundaries. Besides, the intrusion of hot mantle materials had played an important dynamic role in tectonic activities for this region.

Zircon U-Pb dating of Yushugou terrain of high-pressure granulite facies in southern Tianshan Mountain and its geological significance

The Yushugou terrain of high-pressure granulite fades in southern Tianshan Mountain is composed mainly of an ophiolite suite. Most selected zircons are round or elliptical in shape, and some are of tetragonal prism with round edges. The granulometric analyses showthat they are well sorted in sedimentation. ZrO2/HfO2 ratios in zircons range from 45 to 57. These characters, together with the petrologic and geochemical characters of plagioclase-garnet-orthopyroxenite bearing zircons, indicate that the protolith of plagioclase-garnet-orthopyroxenite may be derived mainly from volcanic base surge sedimentary debris in oceanic islands and from clays formed by seafloor weathering. Zircons are simply of pyroclastic debris. The ophiolite formation age of (440±18) Ma and the first-stage metamorphic age (amphibolite or granulite facies) of (364±5) Ma were obtained with a method of multiple grains in different groups and a method of concordia plot. These ages provide important information on the temporal and spatial occurrence of southern Paleozoic Tianshan Ocean, the subduction rate of the oceanic crust and the formation mechanism of ophiolite of granulite facies.

Eclogite Occurrences in the Southern Tianshan High-Pressure Belt, Xinjiang, Western China

Eclogite pods and layers of eclogites in mafic blueschists are the most prominent witnesses of eclogite facies metamorphism in the southern Tianshan high-pressure belt of Western China. The P-T evolution was deciphered in order to understand the geotectonic framework of these rocks. Geothermobarometric evaluation for matrix assemblages reveals peak metamorphic condition between 14 and 21 kbar at 500 to 600°C. Prograde growth zoning and mineral inclusions in garnet are evidence for a clockwise P-T path. The presence of intimate eclogite/metabasic blueschist interlayering and the high P-T estimates suggest that the eclogite-facies rocks from the southern Tianshan high-pressure belt formed in a tectonic regime similar to the Franciscan type. Therefore the southern Tianshan high-pressure belt is most important in order to resolve the Paleozoic tectonic evolution of central Asia.

Variscan terrane of deep-crustal granulite facies in Yushugou area, southern Tianshan

The Yushugou terrane of deepcrustal granulite facies in southern Tianshan consists of two parts, granulite and metaperidotite. The whole terrane is a metamorphism of (high-pressure) grunulite facies, and typical mineral associations are: Gt-Cpx-P1-Tit-Ilm (±Qz) (silica-saturated and oversaturated mafic rocks), Gt-Ky (pseudomorph)-P1-Ru-Ilm± Qz (metapelitic rocks) and Spi-Opx-Cpx-01 (meta-ultramafic rocks). The peak-stage P-T conditions are 795— 964°, 0.97—1.42 GPa, which are obtained with mineral chemistry, assemblage analyses and P-T estimation. The Sm-Nd isochron age of peak-stage metamorphic minerals is (315 ± 3.62) Ma. All of these indicate that the terrane is a deep-crustal body, which subducts to the depth of 40—50 km in the middle late-Paleozoic, undergoing metamorphism of (high-pressure) granulite facies, and exhumed again to the surface by tectonic uplifting.

Closing history of the southern Tianshan oceanic basin, western China: an oblique collisional orogeny

The Tianshan (Tien Shan) Range is an important Paleozoic collisional orogenic belt and the key to understand the central Asia tectonic evolution. This paper integrates our research results with the existing Chinese and international literature on sedimentology, geochemistry, isotopic geochronology, paleontonology and paleomagnetism of the Tianshan and Tarim regions to propose that the oblique collision may have played an important role in the late Paleozoic closing of the southern Tianshan oceanic basin. As a result of the Sinian (latest Proterozoic, younger than 800 Ma) continental extension and rifting process, the Tarim and Yili blocks separated from their parent continent in the Late Cambrian–Ordovician. The southern Tianshan oceanic crusts between the two blocks subducted northward beneath the southern margin of the Yili block in the Silurian. During the Devonian–Early Carboniferous, the Tarim block rapidly drifted to the north and rotated about 46° clockwise. This process induced the collision of the Yili micro-continent with the eastern segment (present geographical position) of the Tarim continent in the Late Devonian, and the southern Tianshan oceanic crust evolved to be a west-facing remnant oceanic basin. During the Late Carboniferous–Early Permian, the Tarim block, located within an almost constant latitude range, rotated about 26° clockwise with respect to the Yili micro-continent, which ultimately closed the remnant oceanic basin in a `scissors-like' manner from east to west and completed the Tarim–Yili collision. Subsequent A-type subduction of the Tarim continental crust and lithosphere-scale sinistral shearing generated a magmatic arc on the southern margin of the Tarim–Yili suture zone. The Late Permian–Early Triassic clastics deposited in a peripheral foreland basin developed above the arc.

3-D velocity images of the crust and upper mantle of the tianshan area

3-D velocity images of the crust and upper mantle in the Tianshan are established by means of seismic tomograghy. From the results, some understanding can be achieved as follows: (1) The northern and central parts of the Tianshan are the uplifted areas with high velocities. The low velocity areas in front of the northern and southern flanks of the Tianshan are formed by the southern margin of the Junggar Basin, Turpan Basin, Kuqa depression, Kalpin fault block and Kaxgar depression. The Ili Basin and the western Kunlun appear as intracrustal stable high velocity blocks, while the Bachu uplift extends down to the crustal bottom. Along the main peaks of the Tianshan and Mt. Kongur of the western Kunlun, the Moho depresses on a great scale to form five large low velocity areas. (2) In the northern Tianshan and western Junggar Basin, the lithosphere is thicker and features a high velocity probably with an asthenolith layer existing on the top of the upper mantle. In the southern Tianshan, the feature of the lithosphere is not very clear, and the existence of a comic low velocity block between 120–280 km depth in the Kaxgar area is presumably related with the upwelling of astenolith from the upper mantle. (3) Some relation does exist between velocity structures and seismic activities, especially those gradient belts between the low velocity zone in the upper crust and the low velocity zone in the lower crust may be the tectonic positions for the preparation of moderate-strong earthquakes. (4) Seismically active areas on the northern and southern flanks of the Tianshan are locations with most inhomogeneous crustal media and well-developed deep faults. Fractures or interlayer-gliding are very likely to occur under the action of tectonic forces and thus to induce earthquakes in these areas.

Paleozoic Plate-Tectonic Evolution of the Tarim and Western Tianshan Regions, Western China

The plate-tectonic evolution of the Tarim basin and nearby western Tianshan region during Paleozoic time is reconstructed in an effort to further constrain the tectonic evolution of Central Asia, providing insights into the formation and distribution of oil and gas resources. The Tarim plate developed from continental rifting that progressed during early Paleozoic time into a passive continental margin. The Yili terrane (central Tianshan) broke away from the present eastern part of Tarim and became a microcontinent located somewhere between the Junggar ocean and the southern Tianshan ocean. The southern Tianshan ocean, between the Tarim craton and the Yili terrane, was subducting beneath the Yili terrane from Silurian to Devonian time. During the Late Devonian-Early Carboniferous, the Tarim plate collided with the Yili terrane by sinistral accretional docking that resulted in a late Paleozoic deformational episode. Intracontinental shortening (A-type subduction) continued through the Permian with the crea...