Geometry and style of partitioned deformation within a late Cenozoic transpressional zone in the eastern Gobi Altai Mountains, Mongolia

Abstract

The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with peaks averaging 2000–3000 m in elevation. The region is seismically active and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengths and at stepover zones. This report summarizes structural field investigations made in the easternmost Gobi Altai to document the structural geometry and style of late Cenozoic transpressional deformation in the region in order to better understand processes of intracontinental mountain building and the distant intracontinental strain response to the Indo-Eurasian collision. The Artsa Bogd range marks the northeastern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-lateral oblique-slip faults. The northern side of the range is also bounded by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments. The southern margin of Artsa Bogd appears tectonically inactive but contains S-vergent thrust faults and left-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion of a Mesozoic basin. The isolated, high and narrow Tsost Uul range south of Artsa Bogd occupies a restraining bend position along the left-lateral Tsost Uul strike-slip fault system. Major faults within the range define a half-flower structure cross-sectional geometry. To the south of the Tsost Uul range, the Gobi Bulag left-lateral strike-slip fault system is marked by small push-up ridges and one major restraining bend mountain where the fault steps to the right near its western end. Throughout the region, Late Cretaceous-Tertiary basalts and Tertiary and Quaternary sediments are deformed by the major fault systems indicating late Cenozoic fault activity. These fault systems and the ranges formed along them occur at fairly regular intervals (approximately 20 km) between the North Gobi Altai fault system and the Gobi Tien Shan fault system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of discrete linear belts of Cenozoic E-W left-lateral transpressional deformation south of the Hangay Dome. The regular spacing of the fault systems may suggest more uniform distributed left-lateral flow at depth. Eastward-directed lower crustal and lithospheric mantle flow is suggested by existing seismic anisotropy data for the eastern Gobi Altai and is believed to be the driving force for the upper crustal deformation.