Late Cenozoic tectonic evolution of the southern Chinese Tian Shan

Structural, sedimentological, magnetostratigraphic, and 40Ar/39Ar thermochronological investigations were conducted in the southern Chinese Tian Shan. On the basis of our own mapping and earlier investigations in the area, the Late Cenozoic southern Tian Shan thrust belt may be divided into four segments based on their style of deformation. From west to east, they are (1) Kashi-Aksu imbricate thrust system, (2) the Baicheng-Kuche fold and thrust system, (3) the Korla right-slip transfer system, and (4) the Lop-Nor thrust system. The westernmost Kashi-Aksu system is characterized by the occurrence of evenly spaced (12-15 km) imbricate thrusts. The Baicheng-Kuche and Korla systems are expressed by a major north dipping thrust (the Kuche thrust) that changes its strike eastward to become a NW striking oblique thrust ramp (the Korla transfer zone). The Lop Nor system in the easternmost part of the southern Chinese Tian Shan consists of widely spaced thrusts, all involved with basement rocks. Geologic mapping and cross-section construction suggest that at least 20-40 km of crustal shortening with a horizontal shortening strain of 20-30% has occurred in the southern Chinese Tian Shan during the late Cenozoic. These estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections. The timing of initial thrusting is best constrained in the Kuche basin where crustal shortening may have occurred at 21-24 Ma, the time of a major facies transition between lacustrine and braided-fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy. This estimate represents only a minimum age, as development of thrusts in the southern Chinese Tian Shan may have propagated southward toward the foreland. Thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting. If our estimate of timing for the thrust initiation (21-24 Ma) is correct, using the estimated magnitude of shortening (20-40 km) and shortening strain (20-30%), the averaged rates of late Cenozoic horizontal slip and shortening strain are 1-1.9 mm yr(-1) and 2.9-4.5x10(-16) s(-1), respectively. Our reconnaissance 40Ar/39Ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the Chinese Tian Shan suggests that the magnitude of Cenozoic denudation is no more than 10 km, most likely less than 5 km. We demonstrate via a simple Airy-isostasy model that when the thermal effect on changes in surface elevation is negligible, determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the Tian Shan. Using this simple model, the loosely constrained magnitude of crustal-shortening strain and denudation in the southern Chinese Tian Shan implies that it may have been elevated 1.0-2.0 km since the onset of Cenozoic thrusting.