Assessing Pre-Cenozoic Shortening of the Southern Pamir

Abstract:

An ongoing problem in understanding the evolution of the Himalayan-Tibetan Orogen is determining how much shortening and thickening of the crust occurred before the Cenozoic collision between India and Eurasia. Whereas previously-determined values for the timing and magnitude of internal shortening in the Pamir as a whole suggest >340 km of Cenozoic internal shortening and 150 km within the Southern Pamir alone (Burtman and Molnar, 1993), recent work indicates the magnitude of shortening may be much less. In the Southern Pamir, for example, shortening formerly-interpreted as a Cenozoic nappe structure, which accounted for approximately 100 km of the 150 km, has recently been shown to be a detachment fault bounding a Cenozoic gneiss dome (Stubner et al. 2013). Our study aims to address the timing and magnitude of shortening in the Southern Pamir. The Pamir has had a protracted history of deformation due to the accretion of Gondwana terranes and subsequent tectonism. A post accretion Jurassic carbonate platform within the Central and Southern Pamir provides an ideal strain marker for evaluating the magnitude of post-suturing deformation. Further, syn-tectonic terrigenous deposits associated with deformation of the Jurassic carbonates provide means of addressing the timing of their deformation through detrital zircon maximum depositional ages. Preliminary detrital zircon results from our work and other studies indicate shortening occurred during the Early to Middle Cretaceous, prior to the collision of India and Eurasia, rather than during the Cenozoic as previously interpreted. Further, our geologic field mapping and cross section analysis suggest post-Jurassic internal shortening in the Southern Pamir is ~7 km, much less than the 50 km previously suggested. Our results of this study indicate that internal upper crustal shortening in the Pamir is significantly less than previously proposed and is largely Cretaceous in age rather than Cenozoic.