CRUSTAL ARCHITECTURE OF THE MONGOLIAN HIGH ALTAI

We report preliminary results from 6 weeks of detailed field work carried out during summer, 2000 in northwestern Mongolia investigating the crustal architecture of the Altai at latitude 48°N. The region can be divided into 5 separate Cenozoic structural domains each dominated by a major dextral strike-slip fault system. Gentle bends along these faults are marked by transpressional uplifts including asymmetric thrust ridges, restraining bends containing flower structure cross-strike fault geometries, and triangular thrust-bounded massifs. These transpressional uplifts (Tsambagarav Massif, Altan Huhey Uul, Sair Uul, Asgat Nuruu, Omno Hayrhan Uul, Mengildyk Nuruu) comprise the highest mountains in the Altai and are structural and metamorphic culminations exposing polydeformed greenschist-amphibolite grade basement recording at least 2 phases of Paleozoic ductile deformation overprinted by Cenozoic brittle structures. Cenozoic thrust faults with the greatest amounts of displacement bound the west and southwest sides of ranges throughout the region and consistently verge WSW. Many of these faults are considered active because they deform Quaternary alluvium and form prominent scarps. We have joined our transect to previous work carried out in the Chinese Altai to create the first modern crustal section across the entire northwestern Altai. Samples were collected for fission-track dating, Ar-Ar dating of mylonites and metamorphic fabrics, and for geochemical analysis to further refine the timing of deformation and tectonic significance of basement rocks. Our preliminary results indicate the importance of restraining bends in nucleating Cenozoic topographic uplift and acting as nodes from which transpressional mountain ranges have grown along-strike and perpendicular to fault strike. On a more regional scale, the Siberian and Hangay cratons have played a major role in localizing Late Cenozoic deformation along their boundaries. The geometric relationship between the craton boundaries and the dominantly northeast SHmax (derived from India1s continued northeastward indentation) has dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai and Sayan-Baikal regions.