ARIDITY AND THE EVOLUTION OF CLOSED BASINS IN ASIA

Western China and adjacent areas of central Asia are characterized by low relief, internally drained sedimentary basins that are divided by actively uplifting mountain ranges. Based on analysis of the GTOPO30 DEM, 95% of the area of these closed geomorphic basins received less than 40 cm/yr average annual precipitation during the 1951-1980, and 58% received less than 20 cm/yr precipitation. Closed basins that presently contain lakes are only slightly less arid: 92% and 60% of the associated lake area occurs in areas with less than 60 and 40 cm/yr average precipitation, respectively. These data do not take into account local orographic effects or longer-term fluctuations in precipitation, but it is nonetheless clear that the existence of large tectonically-closed basins requires aridity, and that large lakes generally occur near the edge of the desert. Many of the closed geomorphic basins in central Asia overlie much longer-lived nomarine sedimentary basins. The margins of these basins typically show evidence for repeated and extensive contractional deformation, yet their interiors have remained surprisingly stable. The basin fills commonly span the entire Mesozoic and Cenozoic, and reach up to 10-15 km in thickness. Carroll et al. (2004) proposed the term “walled basins”, based on analogy to the modern configuration of orogenic walls surrounding closed geomorphic basins. Walled basins represent a major tectonic element of Asia, and appear to play an important role in post-amalgamation stabilization of the continent. However, an apparent paradox exists with respect to one of the most prominent phases of walled-basin deposition, which took place during the Early to Middle Jurassic. The presence of thick Jurassic fluvial and lacustrine strata within most of the major basins of China implies the need for aridity, but these same intervals also contain well-developed coals, suggesting humid conditions. Further work is needed to resolve these issues.