Collaborative Research: Extrusion and Rotation During Intracontinental Deformation: The Role of the Kunlun Fault in the Indo-Asia Collision

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Despite recent advances in understanding of the mechanical and thermal response of continental lithosphere to collisional orogenesis, important controversies remain. One of these centers on the role of large strike-slip faults during intracontinental deformation, and whether these structures 1) control the lateral 'escape' of quasi-rigid blocks in response to continental convergence (e.g., Tapponnier et al., 1982), or 2) reflect the passive localization of strain in a pervasively deforming and shearing crust (e.g., England and Molnar, 1990). The models make very different predictions regarding the variation of displacement along strike-slip faults, the relationship of fault displacement to deformation of the surrounding crustal blocks, and the nature of accommodation of slip at the terminations of the faults. In eastern Tibet, continuing debate over the nature of active deformation reflects, to a large degree, the limited number of rigorous geologic tests of these predictions. The Kunlun fault is a first-order structural feature in the central and eastern Tibetan Plateau, where it presents a key opportunity to test among competing hypotheses for the role of strike-slip faults in the active deformation of eastern Tibet. Although Holocene slip rates appear to be uniform at ~11mm/yr along the central portion of the fault (Van der Woerd et al., 2000), several observations suggest that significant left-lateral shear along the eastern Kunlun fault does not reach the margin of the Tibetan Plateau: 1) the active trace of the fault on remote sensing (e.g., Tapponnier and Molnar, 1977) cannot be distinguished east of ~102E; 2) field observations (Kirby) confirm that scarps associated with the Kunlun fault are not present east of this region; and 3) geodetic surveys indicate that, at present, little resolvable left-lateral shear passes through the eastern margin of the plateau (Chen et al., 2000). Determining what happens to left-lateral shear along the easternmost portion of the Kunlun fault is critical if we are to understand its kinematic and dynamic role in deformation of eastern Tibet and more generally the role of strike-slip faults during intracontinental deformation. The PI's propose to test several hypotheses regarding the mechanisms of transfer and/or accommodation of displacement at the apparent termination of an intracontinental strike-slip fault: ce Hypothesis 1: Displacement is transferred to kinematically linked, strike-slip faults that: a. transmit displacement across and beyond the plateau margin, or b. transmit displacement to shortening structures at the plateau margin. ce Hypothesis 2: Displacement is absorbed by distributed shortening within the plateau resulting in crustal thickening. ce Hypothesis 3: Displacement represents passive rotation of faults in response to a diffuse, clockwise regional shear. Testing these hypotheses will focus on the following tasks: ce Determining Late Pleistocene-Holocene slip rates along the easternmost segment of the Kunlun fault, with special attention to potential variations along strike. ce Establishing the geometry, kinematics, and rates of displacement on candidate accommodation structures (both within the plateau and at its margin). ce Assessing the magnitude and distribution of differential rock uplift and river incision in the Anyemaqen Shan (the prime candidate for shortening within the plateau) This study promises to bring a detailed chronologic perspective to bear on the nature of accommodation of strain at the terminations of large, intracontinental strike-slip faults. The PI's will document the presence or absence of displacement gradients present near the ends of such structures. The study will define the relationship of fault displacement to regional deformation patterns and will determine some of the mechanisms by which displacement is transferred to other structures. Finally, it will determine to what degree fault displacements are linked to deformation of the bounding blocks. The combined results will yield critical new insights into the problem of extrusion versus rotation during continental deformation.