Thickening and growth of lower crust during continental collision: constraints from geochronology of the Pamir
Abstract
Continental collision is an integral part of the evolution of Earth, and collisional orogens — ancient and active — have been investigated profitably from many perspectives to understand the processes and rates of this evolution. Most studies of active continental collision have focused on the Tibetan Plateau. Just west of the Tibetan Plateau is the Pamir Plateau. Both Plateaux are thought largely to have formed during the collision between India and Asia, having absorbed ≥2000 km of N-S convergence. Both contain a similar set of terranes that rifted from Gondwana and were accreted to the southern margin of Eurasia during the Mesozoic. There are two end-member models for crustal thickening and growth of the Pamir plateau during collision: 1) a model based on folding and imbrication of the crust and subduction of mantle lithosphere, and 2) a model that calls upon large-scale flow of the lower crust in response to gravitational potential energy gradients. In contrast to Tibet, the Pamir have numerous exposures of middle-lower crustal rocks exhumed during the Cenozoic. Therefore, we can test these models in the Pamir within the framework of three hypotheses: The Pamir lower crust was 1) thickened and exhumed by local intracontinental shortening during the Cenozoic, 2) thickened by Cenozoic long-distance flow and exhumed by local intracontinental shortening during the Cenozoic, or 3) thickened and exhumed mostly before the India-Asia collision. Deciphering the P-T-t-D evolution of the middle-lower crustal rocks of the Pamir is essential in testing these hypotheses. Quantitative thermobarometry was carried out on rocks collected from four lower crustal domes across the Pamir to determine their pressure-temperature history, and therefore exhumation depth. The sampled domes (Kurgovat, Yazgulem, Shakhdara, and Muskol-Sares) are dispersed throughout the plateau. Many of the rocks are metapelites, with typical peak metamorphic assemblages of garnet + kyanite ± staurolite + biotite + An20 plagioclase ± K-white mica. Thermobarometry using the garnet-Al2SiO5-quartz- plagioclase, garnet-biotite-muscovite-plagioclase, and garnet-biotite Fe-Mg exchange reactions combined with Perple_X pseudosection calculations yields pressures of ∼6 kbar and temperatures of ∼600 °C for the Kurgovat dome, Yazgulem dome and Muskol-Sares domes. The Shakhdara dome gives higher peak metamorphic conditions at ∼700 °C and ∼10 kbar. Preliminary LA-ICP-MS ages of rutile, interpreted to belong to the peak metamorphic assemblage, yield Cenozoic ages. It is therefore unlikely that thickening happened prior to the India-Asia collision (third hypothesis). Currently we focus our geochronology on the spatial distribution of the metamorphic ages to further constrain the metamorphic evolution of the Pamir.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V41D2304V
- Keywords:
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- 1115 GEOCHRONOLOGY / Radioisotope geochronology;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures;
- 3660 MINERALOGY AND PETROLOGY / Metamorphic petrology;
- 8108 TECTONOPHYSICS / Continental tectonics: compressional