User:CinColon/Tectonics of the Tian Shan

The Tian Shan is a 2500km long, up to 7400m high, range extending through western China, Kazakhstan, and Kyrgyzstan. The central part of the Chinese Tian Shan comprises several intermontane basins separated by up to 4500-5000m high ranges. The Tian Shan contains two late Paleozoic sutures. The older, southern suture marks the collision of a passive margin at the north of the Tarim block and an active continental margin; subduction under the latter was to the north. The younger, northern suture separates a northern Carboniferious island arc from an active continental margin developed over a south-dipping subduction zone. The intracontinental weakness of Asia's interior has caused deformation in the Tian Shan Range after the collision of India into Asia. The predominant mode of deformation in the Tian Shan is thrust faulting. The Chinese Tian Shan displays lateral variations in magnitudes of deformation, estimates range from 2.12 to 21km of crustal shortening.

Regional background
A late Paleozoic continent-continent collision along Tarim's northern margin created an orogenic belt along the southern part of the Tian Shan. The main vergence of this collision was toward the south; the northern edge of the Tarim Block was a north facing passive continental margin prior to collision. The collision exhumed and eroded Devonian sediments in the northwestern part of the Tarim Basin; folds are gentle, long-wavelength structures. During the Carboniferous and Early Permian, clastics and carbonates were deposited unconformably over gently tilted Devonian rocks. Several fragments and island arcs collided with Asia's southern margin in the Mesozoic, creating deformation and uplift in the Tian Shan and Kunlun Shan to the north and south of Tarim and generating pulses of uplift and nonmarine deposition in the Tarim Basin. Mesozoic deformation was minor or absent in most of the basin interior. The collision of India with Asia is the latest in this series of events. Crustal shortening is the predominant style of Cenozoic deformation in the Tian Shan. The Tian Shan propagated outward and rose progressively as a wedged-shaped block. . There are also dextral NW-SE trending strike slip faults, which either merge with or crosscut east-west trending thrust systems. The Talas Fergana Fault is the longest of these structures.

GPS velocity field
Most of the convergence between the Tarim Basin and the Kazakh Platform is absorbed within the Tian Shan, localized zones of shortening at rates of ~2mm/yr to as many as 6mm/yr lie within the Tian Shan. The GPS velocity field reveals that the total amount of convergence in the Tian Shan is not uniformly distributed across the range, with 80-90% of the N-S shortening absorbed along the southern and northern edges, and relatively little deformation accommodated within the interior. Slip rates on NW-SE trending strike-slip faults range from 1 -4 mm/yr. Slip rates on WE-SN trending gently-dipping detachment fault vary from 10-13mm/yr for the southwest Tian Shan to 2-5mm/y for eastern Tian Shan. The elastic strain confined in the upper crustal layer above the detachment ultimately releases through infrequent great earthquakes in the Tian Shan, resulting in considerable folding and faulting at their margins. A recent GPS velocity field study estimates the Tarim Basin is thrust beneath the Tian Shan at ~4-7mm/yr. GPS data places a bound of ~4mm/yr on the rate of crustal shortening across the Chatkal and neighboring ranges on the northwest margin of the Ferghana Valley, and they limit the present day slip rate on the right lateral Talas Ferghana fault to less than ~2mm/yr. GPS data supports geologic evidence that indicates the northern margin of the Pamir overthrusts the Alay Valley and requires a rate of at least 10-15mm/yr.

Kuqa Foreland
The Kuqa basin is bounded by the Tarim basin to the south and the Tian Shan orogenic belt to the north. The Kuqa depression was probably part of the north Tarim continental margin at the Paleozoic time. The paleo-Tian Shan ocean, experienced multi-stage open-close cycles during the Paleozoic, closed at the end of Early Permian and formed the paleo-Tian Shan collisional orogenic belt. The formation and evolution of the Kuqa foreland fold belt is related to the multi-phase reactivated and uplifted Tian Shan orogenic belt during the Mesozoic-Cenozoic. Under the compressive tectonic loading and gravitational loading, the Kuqa depression became an intra-continental foreland basin. The cross-section of the basin is asymmetric and its depositional center lies close to the Tian Shan orogenic belt, the sediment thickness thinned towards the Tarim craton.

Borohoro Range
To the north, the Borohoro Range seperates the Yili basin from the North Tian Shan piedmont in the Junggar basin. The basement of the Borohoro Range is mainly composed of a Devonian to Carboniferous magmatic arc intruded by Devonian granitoids. On the southern and northern sides of the range, the basement is capped by Permian sediments, mainly carbonates. The Borohoro Range is itself separated in two compartments by the small, elongated Hexilagen basin in which Early Jurassic coal series are exposed.

Yili Basin
The Yili basin lies souths of the Borohoro Range and is bordered by sutures and fault zones in the western Chinese Tian Shan belt. Alike the entire Tian Shan belt, the Yili block underwent a polyphase evolution including subduction of oceanic crust and collision with micro-continents and volcanic arcs. The southern boundary of the Yili basin is formed of Proterozoic basement and Early Paleozoic platform sediments, tectonically overlain by oceanic high-pressure metamorphic rocks and ophiolites. It has been involved in a south-dipping subduction associated with the closure of the paleo-Tian Shan Ocean and the subsequent collision with a micro-continent correlated with Central Tian Shan. This tectonic event resulted in top-to-the-north ductile thrusting observed in oceanic high pressure metamorphic rocks and Proterozoic basement as well. During the Late Paleozoic, the northern boundary of the Yili basin was an active continental margin related to the southward subduction of the North Tianshan oceanic basin, this boundary is represented by Late Carboniferous turbidite and ophiolitic melange. The southern and northern boundaries have been both reworked by Permian strike-slip faults.

Issyk-Kul and Fargana Basins
The general strike of the Tian Shan ranges is East-Northeast, but there are significant deviations, especially near the major Talas-Fergana fault. A striking feature of western Tian Shan is the large number of intermontane basins containing Cenozoic rocks, prominent basins include the Fergana basin in the southwest, the Issyk-Kul basin in the east and Naryn basin in the south. On the edges of the Tian Shan, there are foreland basins with Cenozoic sediments several km thick. The Talas-Fergana fault is an active strike slip fault in which Quaternary offset is about 10km and the total offset since the Paleozoic is about 200km. In the Issyk-Kul and Fergana basins, Jurassic sandstones and coal bearing shales of continental origin unconformably overlie Carboniferous (and older) rocks. In Issyk-Kul, the stratagraphic sequence is only a few hundred meters thick and consists mainly of quartzite sandstones. In Fergana, it is less than 1km thick. Metamorphosed ophiolites mark the suture of the Paleozoic Turkestan ocean. This suture runs along the southern edge of the Naryn basin and is offset right laterally by the Talas-Fergana fault. It then runs along the southern edge of the Fergana basin, before veering northwards toward the Aral Sea and the Urals.