Neotectonics of the Min Shan, China: Implications for mechanisms driving Quaternary deformation along the eastern margin of the Tibetan Plateau

The Min Shan region, located along the eastern margin of the Tibetan Platea u north of the Sichuan Basin, provides an important natural laboratory in w hich to study the rates and patterns of deformation and their relationship to mountain building at the margin of the plateau. The topographic margin o f the plateau is coincident with a north-trending mountain range, the Min S han, that stands nearly 2 km above the mean elevation of the plateau (simil ar to 3500 m in this region). We exploit the preservation of a series of va riably deformed Quaternary sediments along the western flank of the range t o investigate the Pleistocene-Holocene deformation field within the Min Sha n region. Mapping and field observations of remnant alluvial fans of late P leistocene age indicate that deformation within the Min Shan involved subst antial (similar to 10 degrees), rapid, down-to-the-northwest tilting. The g eometry of the deposits and the partial preservation of an erosion surface beneath the basin suggest that much of the modern relief of the Min Shan re lative to the Tibetan Plateau is a consequence of this late Pleistocene til ting. Rates of tilting inferred from luminescence dating of interbedded loe ss have been remarkably rapid (similar to 10(-8) rad/yr), Similarly rapid r ates of Holocene differential rock uplift are inferred from tilted lacustri ne sediments in the southwestern part of the range. The range is bounded on the west by the Min Jiang fault zone, an east-vergent reverse fault, Howev er, Holocene alluvial terraces in headwaters of the Min River are preserved across the fault in several places, indicating that displacement rates on the Min Jiang fault are <1 mm/yr. Active faulting only occurs along the eas tern foot of the range (Huya fault) for a short distance (similar to 60 km) , despite 3 km of relief on the eastern range front. The relationship betwe en these structures and the tilting observed in the Min Jiang basin is enig matic; the faults do not appear to exert a strong control on the rates and pattern of deformation within the basin, A simple flexural model demonstrat es that rates of tilting on the western flank of the Min Shan are too high to be simply attributed to an isostatic response to surficial loading and u nloading of the lithosphere, Present-day horizontal shortening across the M in Shan is geodetically determined to be less than 2-3 mm/yr suggesting tha t only a small part of the observed tilting can be attributed to horizontal shortening. Thus, tilting and concomitant differential rock uplift in the Min Shan appear to require an additional driving component. We suggest that Quaternary deformation along the western Min Shan may reflect the surface response to thickening of a weak lower crust at the margin of the Tibetan P lateau.