Late Cenozoic Crustal Reactivation of the North Tibetan Foreland, Western Hexi Corridor, Beishan, and Gobi Corridor: Implications for Intraplate Fault Networks, Mountain Building Processes and Earthquake Hazards in Slowly Deforming Regions of Central Asia

Abstract

The active deformation field between northern Tibet and central Mongolia is dominated by diffuse sinistral transpressional reactivation of the southern Altaids Phanerozoic terrane collage.   The angular relationship between NE-directed SHmax and pre-existing basement trends is the dominant control on Quaternary fault kinematics.  Along Tibet’s northern margin, the Altyn Tagh system is widening northwards by transpressional duplexing.  The Nanjieshan and Sanweishan comprise sinistral oblique-slip thrust ridges within a regional asymmetric flower structure centered on the Altyn Tagh Fault.  In the southern Beishan, interconnected lensoidal domains of transpressional and transtensional faulting are subtly indicated by Quaternary fault scarps, low-relief rejuvenated landscapes and alluvial sedimentation.  The SE Beishan and western Hexi Corridor region contain numerous Late Quaternary fault systems including the Heishan-Jinta'Nanshan sinistral strike-slip corridor and the Helishan-Longshoushan fault array that connects eastwards with the transtensional grabens of the Yabrai and Langshan in the eastern Alxa Block.  Further north, the Paleozoic terrane collage of the Gobi Corridor was repeatedly reactivated during the Permo-Triassic, Jurassic, Cretaceous and Neogene.  Late Cenozoic reactivation was likely facilitated by thermal weakening of the crust due to Jurassic-Miocene volcanism, and diffuse Cretaceous rifting and crustal thinning.  Although terrane boundaries and other faults are reactivated in many areas, thrust and oblique-slip reactivation of WNW striking shallowly dipping sedimentary bedding and metamorphic fabrics is equally important.  Conversely, modern E-W trending strike-slip faults in the Gobi Altai typically crosscut older basement trends. In the Altai and Gobi Altai, the Late Cenozoic fault array has created a transpressional  basin and range physiographic province.  Coalescence of separate ranges into topographically continuous mountain belts in the Altai, Gobi Altai and easternmost Tien Shan is an important mechanism of transpressional mountain building not predicted by classical plate tectonic models.  Throughout the vast deforming region north of Tibet, tectonic loading is shared amongst a diffuse fault network challenging assumptions about earthquake recurrence intervals and seismic hazard forecasting.