ROLE OF BACKTHRUSTING IN PLATEAU GROWTH

In his 1986 seminal paper published in the Journal of Structural Geology, Ray A. Price proposed the concept of wedge tectonics that emphasizes the importance of backthrusting in development of large-scale orogenic belts. This concept may be applicable to explain the growth processes of major plateaus on Earth, such as Altiplano in South America, Colorado in North America, and Tibet in Asia. One of the common features of these plateaus is that segments of their margins with high structural relief and the steepest topographic gradient exhibit no major foreland-directed thrusts. This relationship is best exemplified by Denver Basin against the eastern edge of the Colorado plateau and Sichuan Basin against the eastern edge of the Tibetan plateau. In both areas the plateaus are bounded by shallow basins (~ 500-1000-m thick) that are thinner than those induced by thrust loading (> 3000-6000 m). The plateau margins also consist of a thick (>10 km), tilted crustal section dipping towards the foreland. The above observations may be explained by the presence of major back-thrust systems that carry a crustal section of low-elevation foreland over high-altitude plateaus, a process similar to the development of the Alberta Syncline in the southern Canadian Rockies. Along the eastern edge of the Colorado plateau, such a structure has been identified as the west-directed Park Range thrust. Although major back-thrust systems have not been identified along the eastern margin of the Tibetan plateau, the inferred structure could be a blind fault-propagation fold that has created large-scale crustal antiforms and causes rapid surface contraction as documented by existing geologic maps and recent geodetic surveys. The most spectacular example of plateau growth by backthrusting occurs in the eastern Kunlun Shan of northern Tibet, where active backthrusting produces Quaternary growth strata and progressive steepening of younger to older terraces along its north flank.