End-member models of the Cenozoic collision of India with Asia (continuum crustal thickening vs. extrusion of crustal blocks) make specific predictions about the rates, kinematics, and spatial extent of structures in and around the Tibetan Plateau. The Altyn Tagh fault, an active greater than 1600 kilometer long left-lateral structure that marks the northern margin of the Tibetan Plateau, plays key, yet different roles in these models. Continuum thickening models predict slow rates of slip on the ATF and require that all displacement on the ATF (approximately 375 kilometers), is accommodated by crustal shortening in the Qilian Shan and Qaidam basin. Extrusion models predict high rates of slip on the ATF and require it to continue beyond the plateau or transfer its slip to other structures.
Recent studies of Cenozoic sedimentary basins along the central-eastern ATF have demonstrated two phases of motion: first, Late Oligocene-Early Miocene large magnitude (approximately 310 kilometers) and high rates of slip on the central-eastern segment of the fault and second, limited mid-Miocene-Recent slip ( approximately 65 kilometers) and lower rates . These results suggest that the Altyn Tagh fault accommodated plate-like lateral extrusion in the Oligocene-Early Miocene, and that the mode of deformation changed to distributed shortening and thickening in the mid-Miocene. However, the inferred earlier phase of Oligocene-Early Miocene lateral extrusion requires that slip be transferred from the ATF beyond the NE Tibetan Plateau, but studies of known candidate structures have failed to identify any with the appropriate slip magnitude or timing. The hypothesis being tested is that Oligocene-Early Miocene eastward lateral extrusion of Tibet along the ATF was accommodated by left-lateral faults in the Alxa region, northeast of Tibet, which subsequently became inactive in the mid-Miocene.
Candidate structures have been mapped in the Alxa region and are left-lateral faults that cut Cretaceous strata. Fieldwork in the Alxa region includes geological mapping, kinematic analysis of these major faults, and basin analysis to determining the distribution and structural geometry of the fault systems. Combining the techniques above with provenance analysis, documentation of cross-cutting and overlapping relationships with Cretaceous-Cenozoic sedimentary deposits, and paleomagnetic determinations of declination anomalies in Cretaceous-Cenozoic rocks provides detailed slip histories and differential vertical axis rotations for each of these structures.
The final results of this research will advance understanding of how the Himalayan-Tibetan mountain belt evolved and how collision between India and Eurasia was accommodated. As a prominent natural laboratory, this region is critical for studying the first order processes that control how continents deform, and thus how mountain belts evolve through time. These processes of continental deformation and mountain uplift are in turn important, because they influence regional and global climate, distribution of natural resources, and influence a wide range of other Earth System processes. In addition to the scientific objectives of this research, the project is supporting graduate and undergraduate student training at Louisiana State University, the University of Indiana, and the University of California, Santa Cruz and will provide international research experiences for these students. The research also involves collaboration between U.S. researchers and their counterparts in China. Funding for the Indiana University part of the collaboration is provided by the NSF Tectonics Progran and the NSF Office of International Science and Engineering.
