LATE CENOZOIC TRANSPRESSION IN SOUTHWESTERN MONGOLIA AND THE GOBI ALTAI TIEN-SHAN CONNECTION

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental m ountain building at different stages of development. The region is str ucturally dominated by several major E-W left-lateral strike-slip faul t systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be tra ced along the surface for over 350 km. The eastern two-thirds of the f ault system ruptured during a major earthquake (M = 8.3) in 1957, wher eas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the len gth of the fault system. Detailed transects across two of the restrain ing bends indicate that they have asymmetric flower structure cross-se ctional geometries, with thrust faults rooting into oblique-slip and s trike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, wi ll lead to growth and coalescence of the restraining bends into a cont inuous sublinear range, possibly obscuring the original strike-slip fa ult system; this may be a common mountain building process. The largel y unknown Gobi-Tien Shan fault system is a major left-lateral strike-s lip fault system (1200 km + long) that links the southern ranges of th e Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Ti en Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Qua ternary activity. The total displacement is unknown, but small paralle l splays have apparent offsets of 20 + km, suggesting that the main fa ult zone has experienced significantly more displacement. Field invest igations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fa ult system. The spatial relationship between topography and active fau lts in the Barkol Tagh and Bogda Shan strongly suggests that these ran ges are large, coalescing, restraining bends that have accommodated th e fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien S han formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system i s one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topo graphically low, E-W trending corridor between the northern edge of th e Tibetan Plateau and the Hangay Dome, presumably in response to the I ndo-Eurasian collision 2500 km to the south.