Xuhua Shi

Millennial slip rates along the eastern Kunlun fault: Implications for the dynamics of intracontinental deformation in Asia

The role of major strike-slip faults in the Indo-Asian collision zone is central to our understanding of the ways in which continental crust and lithosphere deform in response to continental collision. We investigated how slip varies along the eastern segments of the Kunlun fault in northeastern Tibet. Millennial slip rates were determined based on landforms that are offset by the fault and that were dated using a combination of 14 C and cosmogenic radionuclide exposure dating techniques. We developed estimates for slip rates at four new locations along the fault in addition to four previously published sites. All of these sites are located along the eastern 300 km of the fault system, and our results reveal a systematic eastward decrease in slip rate along this portion of the fault since the late Pleistocene. This displacement gradient is consistent with the termination of the Kunlun fault near ∼102°E. Coincident variations in fault slip rates and geometry reflect complex kinematics along the fault zone. Although other faults exist in the region, our observations suggest that none of these accomplishes transfer of slip from the primary Kunlun fault system. Instead, we interpret that either the eastern Kunlun fault is relatively young and propagating eastward, or that left-lateral slip is absorbed by interaction of the fault zone with regional rotation of the eastern fault tip. Both of these scenarios contrast with previous interpretations and indicate that the Kunlun fault does not accommodate the eastward extrusion of the central Tibetan Plateau lithosphere.

Holocene slip rate along the Gyaring Co Fault, central Tibet

Although geodetic measurements of interseismic deformation in interior Tibet suggest slow strain accumulation, active slip along the right-lateral Gyaring Co Fault is suggested to be between 8 and 21 mm/yr. Reliable geologic constraints on the slip rate along this fault are sparse. Here we document 12 ± 2 m of right-lateral displacement of lacustrine shorelines across the Gyaring Co Fault. Optically stimulated luminescence ages of the shorelines are tightly clustered between 4.1 and 4.4 ka. These data require an average slip rate of 2.2–3.2 mm/yr along the central Gyaring Co Fault during the latter half of the Holocene. Consideration of seismic cycle effects allows the possibility of slightly higher average slip rates, up to 2.2–4.5 mm/yr. Overall, our results suggest that the slip rate along the Gyaring Co Fault is similar to other strike-slip faults in interior Tibet, supporting the notion that active deformation in this region is distributed among numerous, slowly moving faults.

Young, active conjugate strike–slip deformation in West Sichuan: evidence for the stress–strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe–Xiaojiang fault in the northeast and the right-lateral Red River–Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike–slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE–SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW–SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike–slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of ∼15–35 km. Using 5.7–3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike–slip faulting, we have determined an average slip rate of ∼2.6–9.4 mm/year. These two sets of strike–slip faults intersect at an obtuse angle that ranges from 100° to 140° facing east and west; the fault sets define a conjugate strike–slip pattern that expresses internal E–W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20°. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E–W shortening and N–S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.

Evaluating the size and extent of paleolakes in central Tibet during the late Pleistocene

Subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleo-lake in central Tibet, the East Qiangtang Lake (EQL), during the last interglacial period (Marine Isotope Stage [MIS] 5e). Reconstructions based on relict lake deposits suggest that the EQL would have been ~400 m deep and over ~66,000 km2. Models of flexural rebound driven by lake recession predict that shorelines near the EQL center, at the present-day location of Siling Co, would have rebounded 60-90 m above their initial elevation. New 36Cl chronology of the highest relict shorelines around Siling Co indicates that they reflect lake levels between 110-190 ka. These shorelines, however, are presently >300 m below their predicted elevations, implying a substantially smaller water load. Our results reveal that the expansion of Tibetan lakes during MIS 5e was relatively limited. Instead, individual lakes were supplied by river networks, much as they are today.