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1 Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, 90095-1567, USA
2 Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, People's Republic of China
The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium- to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230–200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (
650 °C, 8 kbar), which structurally overlies a low-grade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125–110 Ma, followed by emplacement over the low-grade metagray-wacke along a north-northeast–directed thrust prior to ca. 100 Ma. Together these results indicate significant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (650–700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7–8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (29 km) and the present dip of the Kongur Shan normal fault (40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases significantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.
Key Words: Pamir • tectonics • metamorphism • age determination • extension • crustal shortening
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