GEOMETRY AND STYLE OF PARTITIONED DEFORMATION WITHIN A LATE CENOZOIC TRANSPRESSIONAL ZONE IN THE EASTERN GOBI-ALTAI MOUNTAINS, MONGOLIA

The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with pe aks averaging 2000-3000 m in elevation. The region is seismically acti ve and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengt hs and at stepover zones. This report summarizes structural field inve stigations made in the easternmost Gobi Altai to document the structur al 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 northeast ern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-late ral oblique-slip faults, The northern side of the range is also bounde d by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments, The southern margin of Artsa Bogd ap pears tectonically inactive but contains S-vergent thrust faults and l eft-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion o f a Mesozoic basin. The isolated, high and narrow Tsost Uul range sout h of Artsa Bogd occupies a restraining bend position along the left-la teral Tsost Uul strike-slip fault system. Major faults within the rang e define a half-fewer 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 be nd mountain where the fault steps to the right near its western end. T hroughout the region, Late Cretaceous-Tertiary basalts and Tertiary an d Quaternary sediments are deformed by the major fault systems indicat ing late Cenozoic fault activity. These fault systems and the ranges f ormed along them occur at fairly regular intervals (approximately 20 k m) between the North Gobi Altai fault system and the Gobi Tien Shan fa ult system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of disc rete linear belts of Cenozoic E-W left-lateral transpressional deforma tion south of the Hangay Dome. The regular spacing of the fault system s may suggest more uniform distributed left-lateral how at depth. East ward-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.