TRANSITIONS BETWEEN FLEXURAL AND AIRY ISOSTASY AND IMPLICATIONS FOR BASIN DEVELOPMENT IN CONTINENTAL PLATEAUS

Temporal and spatial changes in the mechanism of isostatic compensation are expressed as variable modes of Cenozoic basin evolution in the Altiplano and Tibetan plateaus. On the basis of available geophysical constraints on crustal thickness and density, the present interiors of both plateaus are principally in Airy isostatic equilibrium (local compensation). However, the following evidence for early Cenozoic flexure (regional compensation) suggests a mid- to late-Cenozoic switch to Airy conditions within plateau interiors: (1) Paleogene syndepositional thrusting along basin margins in east-central Tibet (Nangqian-Yushu basins) and south-central Tibet (Lunpola basin system); (2) increased accumulation rates and large-scale tectonic rotation at roughly 40-30 Ma in northeastern Tibet (Xining-Minhe-Longzhong basin complex, Xining-Lanzhou region); and (3) flexural modeling results based on published regional balanced cross sections of the fold-thrust belt of the central Andes (Bolivia). Modern flexure is generally limited to the leading edges of the northward-growing Tibetan plateau and eastward-propagating central Andes. Temporal and spatial changes from flexural to Airy isostatic conditions may be attributed to crustal thickening beyond a threshold value, and accompanying weakening of the crust, during progressive shortening. Following the switch to Airy isostasy, no sediment accommodation is predicted for regions of external drainage. Accordingly, in areas of internal drainage, Neogene fill may simply be the product of sediment ponding (passive infilling) rather than a measure of tectonic subsidence. This pattern may explain the thick Neogene basin fill in areas of presently closed drainage and the paucity of Neogene fill in the externally drained eastern and southeastern Tibetan plateau. Furthermore, systematic identification of flexural to Airy transitions in the stratigraphic record should help place constraints on the timing, rates, and mechanics of deep crustal processes within high-elevation plateaus.