Late Quaternary Crustal Shortening And Deformation Mechanism Across The Kuqa Fold-And-Thrust Belt On The South Of The Tianshan Mountains

Influenced by far-field effects of the India-Eurasia collision, the Tianshan Mountains has been reactived and rapidly uplifted since early to middle Miocene time. Recent GPS measurements reveal that the crustal shortening rate across the Tianshan Mountians is about 10-13mm/yr west of ~80°E, decreasing eastward to 2-5 mm/a in the eastern Tianshan Mountains. The rate of shortening in the west accounts for about a quarter of the total India-Eurasia plate convergence rate of 45mm/a. With intensive tectonic deformation, Tianshan is a natural laboratory for research on the mechanism and evolution of intercontinental collision. Instrumentally and historically recorded seismicity implies that active deformation is accommodated mainly by the piedmont zones north and south of the Tianshan Mountains. However, the convergence rates across the Tianshan front are determined on a geologically long time scale, containing much ambiguity due to poor chronological constraints. At the same time, although GPS-derived shortening rates are fairly accurate, they cannot be simply applied to understanding of the orogenetic process for two reasons: Firstly, they represent shortening rates during a very narrow time window and hence could significantly be affected by elastic strain release associated with episodic slip events on major adjacent faults. Secondly, orogenetic processes are essentially inelastic, whereas GPS-derived strain consists of both inelastic and elastic components. Thus, it still needs an accurate, long-term rate of convergence to compare geologic rates with geodetic rates. The piedmont zone south of Tianshan is a typical foreland fold-and-thrust belt, which constitutes the northern-most part of the Tarim foreland basin. This zone has been continuously sinking throughout the Neogene and Quaternary times and records the interaction between the Tianshan Mountains and the Tarim Basin.This study focuses on the Kuqa Basin in the southern piedmont belt of the Tianshan Mountains. A thick pile of Cenozoic sediments underlying the Kuqa basin have been strongly deformed to form a fold-and-thrust belt, which is referred to the Kuqa fold-and-thrust belt in this thesis. It is generally believed, on a broad observational basis, that the thrust front migrates toward the basin with time along a thin-skinned fold-and-thrust belt. The field observations of late Quaternary tectonic landforms indicate that the Yaken, Qiulitage and Kasangtuokai anticlines are all active. There is increasing information on deep structures, especially on blind faults, beneath the Kuqa fold-and-thrust belt with progress of petroleum exploration in the Tarim Basin. It is indicated that the Kuqa fold-and-thrust belt is underlain by a low-angle master detachment thrust, onto which a number of synthetic and antithetic thrust faults merge. Some of these faults are active and accommodate present-day crustal shortening across the Tianshan-Tarim boundary, while others are inactive. It is, however, difficult to discriminate between active and inactive elements only from seismic profiles.This work estimated deep fault activity and determined the distribution of slip rates on the detachment through the fitting of a two-dimensional dislocation fault model to late Quaternary surface deformation data obtained by geomorphological mapping and cosmogenic 10 Be dating. The conclusions of this thesis are summarized below.(1) In the Kuqa fold-and-thrust belt, surface deformation was quantified by dating tectonically deformed geomorphologic surfaces. Under an assumption of the zero surface erosion rate, the depth-profile analyses of in situ cosmogenic 10 Be concentrations in geomorphologic surface sediments indicate that the minimum age of the abandonment of all dated geomorphologic surfaces can be divided into 3 stages: 125.4~140ka, 43.9~79.9ka and 18.9~26.2ka. Comparison with the late Quaternary temperature curve suggests that abandonment ages of the geomorphologic are not completely correlated with episodic climate change. Meanwhile the importance of regional tectonics cannot be ignored, since which may play a decisive role in the preservation of geomorphologic surfaces.(2) The step-by-step forward modeling reveals that, in a first-order approximation, the observed deformation in the foreland sedimentary wedge as a whole is due to up-dip decrease of the slip rate along the detachment, facilitating the reactivation of older faults as out-of-sequence faults. The Yaken anticline, where the strongest convergence occurred, was formed solely as a terminal effect of the low-angle detachment faulting and its amplitude is proportional to the slip rate near the leading edge. The slip-rate changes on the down-dip portion of the detachment that hardly affect the amplitude and pattern of deformation over the Yaken anticline. The adjacent Qiulitage anticline was formed as a joint effect of a ramp in the detachment and up-dip reduction of the slip rate. The flat-ramp-flat structure in the northern part of the detachment, which slides beneath the Tianshan Mountain, caused the uplift and slight tilt of the Kasangtuokai anticline. The modeling curve also illustrates that there is almost no uplift further north, next to the Tianshan front and at inner Tianshan. However, this point needs more geological evidence to constrain.(3) The forward modeling also provides a constraint on the rate of crustal shortening across the Tianshan Mountain front in the late Quaternary. It is estimated to be 11.54~12.96mm/a and 10.92~12.11mm/a by matching the surface deformation of terrace T2 and T1,3 of the Qiulitage anticline. It is noted, however, that these values are the maximum limits of crustal shortening rates, because the surface-exposure ages of the terraces used in this study are the minimum values on an assumption of a zero surface-erosion rate.It is now clear that the late Quaternary surface deformation of this area is principally caused by slip on the blind detachment fault. However, previous studies reported a number of active faults, which offset late Quaternary geomorphic surfaces in the northern and southern limbs of the Qiulitage and Kasangtuokai anticlines, and are characterized by discontinuous and short traces. In contrast, this work suggests that these surface faults are secondary discontinuities of the master detachment.