EXHUMATION DEPTHS OF THE LOWER CRUSTAL DOMES OF THE PAMIR

Our understanding of the processes involved in building orogenic plateaux relies heavily on what can be inferred from exposures of the upper crust. Lower crustal exposures, however, can shed light on the processes occurring deep within 70+ km-thick plateaux and are therefore useful in determining the mechanics of plateau formation. Both the Tibetan and Pamir plateaux are archetypal continental collisional orogens and are both made of microcontinents and island arcs accreted to the southern margin of Asia during the Paleozoic to Mesozoic. The Tibetan plateau, however, lacks critical exposures of lower crust needed to interpret deformation that is occurring at depth within the crust. In contrast, the Pamir plateau has undergone greater Cenozoic exhumation, exposing deeper crustal rocks that record the history of plateau formation that is missing in Tibet. The Pamir are therefore an ideal location in which to study the processes that build orogenic plateaux.

Quantitative thermobarometry was carried out on rocks collected from three lower crustal domes across the Pamir to determine the pressure–temperature history, and therefore exhumation depth, of the lower crust. The sampled domes (Kurgovat, Yazgulem, and Shakdhara) are dispersed north to south across the western half of the plateau, and may therefore have formed diachronously as the Pamir grew northward. Many of the rocks are metapelites, permitting the use of well-understood thermobarometers such as GASP, GHPQ, GBMP and GARB. The typical peak pressure assemblage, garnet + kyanite + biotite + An20 plagioclase ± K-white mica, replaced staurolite, and is itself overgrown by sillimanite and more anorthitic plagioclase. Garnet cores are homogeneous and rims are partially resorbed with long-wavelength rimward increases in Mn. Preliminary data indicate peak metamorphic pressures of 9-11 kbar at temperatures of 700-800°C; exhumation to pressures of 4-6 kbar occurred at temperatures of 500-700°C. These data imply exhumation depths of 32-40 km for the lower crustal domes of the Pamir.