Cenozoic deformation of the Qaidam basin, western China

Abstract

This thesis investigates the timing and structural styles of Cenozoic deformation of the Qaidam basin in western China. Chapter 2 documents the existence of a new class of contractional fault-related fold termed basement-involved structural wedges in the northwest Qaidam basin, and discusses how we can quantitatively describe and model them. We develop a series of forward models that illustrate the geometry and kinematic evolution of these wedge structures. Our results provide a better understanding of how to identify basement-involved wedge system and model their geometry and kinematic development in ways that can help us better understand regional tectonics. Chapter 3 explores the application of map-based restoration method to define the deformation of the Qaidam basin. We develop a new workflow to improve the capabilities of these methods to distinguish the tectonic component of deformation from other processes such as compaction and basin subsidence due to sedimentary loading. This improved methodology is used to document the style and magnitudes of deformation over late Cenozoic time in the Qaidam basin, and can be applied to other basins that are affected by large basin subsidence. Chapter 4 investigates the deformation of the Qaidam basin based on regional seismic cross-sections. Our interpretation results indicate that the basin is controlled by both northeast and southwest dipping basement involved thrust faults that typically form fault-propagation folds and imbricated structural wedges. Despite the presence of both northeast and southwest dipping thrust faults throughout the basin, our analysis invoking wedge structures concludes that the dips of the basal (basement-involved) thrust ramps are systematic, rooting beneath both the Qiman Tagh and Qilian Ranges. Activity of these mountain ranges is driven by the Altyn Tagh and perhaps Kunlun faults. We use syntectonic growth strata to document the timing and magnitudes of Cenozoic deformation throughout the basin, and show that deformation initiated at 37.8 Ma, concurrent with or shortly after the Himalayan collision. After a subsequent period of tectonic quiescence, the main phase of basin deformation initiated at 22 Ma. The overall tectonic shortening began in the northwest and propagated to the southeast over time, causing migration of the basin depocenter. The second phase of shortening corresponds with the inception of the Altyn Tagh fault, which accommodates eastward extrusion of tectonic blocks from the Tibetan Plateau. Thus, deformation in the Qaidam basin records the transition from north-south convergent to escape tectonics in the Tibetan Plateau.