Upper crustal deformation in many active orogens appears to be concentrated along first-order fault systems that are thousands of kilometers long and hundreds of kilometers wide parallel and perpendicular to strike, respectively. But what controls the formation and evolution of these intracontinental “microplate” boundaries? To address this question, we are investigating the evolution of the SW termination of the active, left-slip Altyn Tagh fault, the largest intracontinental strike-slip system in Asia. Although the fault presently terminates into E-W striking, N-directed thrust belts at both its NE and SW ends, this structural geometry cannot account for 475 ± 70 km of displacement recently documented along the central section of the fault. To address this problem, we have conducted 1:100,000-scale structural mapping in westernmost Tibet, at the SW terminus of the Altyn Tagh fault, to test the idea that early to middle Cenozoic slip along the Altyn Tagh fault was absorbed at its SW end by a S-directed thrust belt prior to formation of the presently active N-directed thrust system. Our mapping indicates that western Tibet has been shortened by a regionally extensive system of Late Cretaceous or younger, N-dipping, S-directed thrusts. This S-directed thrust belt shows systematically decreasing structural levels from north to south. For example, in the north, the steeply north-dipping Tianshuihai thrust places a sequence of greenschist facies Proterozoic metasediments over Carboniferous strata in its footwall. About 45 km to the south, these Carboniferous strata are in turn thrust southwards over Cretaceous strata that are deformed by S-directed thrusts and associated folds within the Loqzung range. The Aksai Chin anticline is the southernmost structure we have mapped within the S-directed thrust belt. This E-W trending fold has a N-S interlimb width of 12 km and deforms both Carboniferous rocks in its core and unconformably overlying Cretaceous strata on its limbs. Cretaceous beds dip 25-45° N on the N limb but are vertical to 10° overturned on the south limb. Shortening due to folding alone is at least 35% but could be as high as 65% within the Aksai Chin anticline, and ongoing work aims to address total shortening across the thrust belt. The S-directed thrust belt coincides with a late Paleozoic to early Mesozoic subduction-accretion complex, while the zone of most intense shortening within the belt coincides with a Mesozoic suture along the N edge of the Qiangtang block. Thus, heterogeneous crustal strength inherited from Paleozoic and Mesozoic assembly of Asia has likely controlled the location and geometry of first-order fault systems produced during Cenozoic orogeny.

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