Basin Analysis in the Qaidam Basin, NW China, and Implications to the Cenozoic Tectonics of Northern Tibetan Plateau

In either of two end-member models of Tibetan tectonics (tectonic extrusion vs. continuum shortening), the Northern Tibetan Plateau (NTP) would be the youngest portion as the strain related to the Indo-Eurasia collision was transferred towards north. Numerous studies support the Miocene-Pliocene intensive tectonic activities in NTP. However, a growing body of evidence from thermochronologic, structural, and isotopic studies supports the Eocene deformation and initial topographic growth in NTP. Therefore, understanding the tectonic history of NTP is critical to understanding the formation and evolution of Tibetan Plateau and the style of intracontinental deformation during continent-continent collision.

The Qaidam basin is a huge intermontane basin (>120,000km2) on NTP; the thick Cenozoic strata (>14km) is ideal for recovering Cenozoic tectonic history of NTP. Here we present results from a regional, holistic basin analysis in Qaidam which was conducted on ten sections, ranging from the Eocene to Quaternary and with a thickness of ~36km. Sections are correlated within a newly-constructed comprehensive chronological framework. Collectively, they document a punctuated tectonic history.
(1) Eocene conglomeratic unit from the North Qaidam records the Eocene faulting on basin-bounding thrust faults. Supporting evidence includes the very coarse lithology and paleocurrent directions that are strongly transverse to adjacent mountain ranges, suggesting the proximate sedimentation by high-gradient depositional systems.
(2) Oligocene-early Miocene strata are dominated by fine-grained fluvio-lacustrine sediments, suggesting the relatively quiescent sedimentation corresponding to high rate (>20-30mm/yr) and large amount (>310km) slip motion along the sinistral strike-slip Altyn Tagh fault that bounds the NTP to the northwest. This relative quiescence in sedimentation is supported by slow cooling/exhumation of bedrocks.
(3) Mid-Miocene to Quaternary strata are dominated by coarse clastic sediments and preserve a coarsening-upward sequence. With strong unimodal paleocurrents, they suggest the proximate sedimentation by high-gradient depositional systems, coeval with the intensive crustal shortening and rapid cooling/exhumation of bedrocks. Combined with geologic, sedimentary, thermochronologic, and isotopic studies, we relate the early Eocene deformation to the initiation of Indo-Eurasia collision and suggest that high topography was built since the middle Miocene.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California