TECTONIC HISTORY OF THE EAST GOBI BASIN

The East Gobi Basin (EGB) of southeast Mongolia has a multiphase subsidence and deformation history. This history includes four distinct compressional, extensional, and/or strike-slip tectonic regimes, resulting from amalgamation and deformation of the Asian continent. Basement rocks of the EGB are generally upper Paleozoic flysch and volcanic arc sequences formed in a remnant ocean basin. Following amalgamation of North China and Mongolia, a Permian to Jurassic, north-south compressional regime caused regional flexural subsidence in foreland basins (Phase 1). This was followed in the Late Jurassic-Early Cretaceous by regional northwest-southeast extension (Phase 2). At this time, the basin was partitioned into both high-strain and low-strain provinces with contemporaneous formation of a metamorphic core complex to the southwest and high-angle fault bound grabens to the northeast. Synrift sequences were inverted in the mid-Cretaceous (Phase 3), in part by reactivation of normal faults. A quiescent or ‘sag' phase 4 followed inversion, represented by a cover of relatively flat-lying Upper Cretaceous sedimentary rocks deposited across the EGB. In places, this sequence continues into the Paleogene.

A principle controlling factor for each phase of basin formation was movement along the East Gobi fault zone (EGFZ). In addition to well-documented extensional and inversion structures, the EGFZ experienced at least two episodes of left-lateral slip. Ductile shear is evident in the latest Triassic (during phase 1), and brittle strike-slip faults also cut Upper Cretaceous strata (phase 4), indicating post-Mesozoic (as young as Quaternary) movement as well. These deformation events may have driven local subsidence along the fault zone, but ultimately sinistral slip is responsible for offset and dismembering of the EGB into distinct subbasins with differing subsidence histories. Exact timing and slip magnitude on the EGFZ are currently being evaluated.

Multiple authors have proposed a connection between early Cenozoic movement of the EGFZ and the Altyn Tagh Fault in China. With future work, understanding the EGFZ will not only aid in reconstruction of the tectonics and subsidence history of the EGB, but may aid in the overall tectonic reconstruction of Asia.