EXHUMATION PROCESSES IN HIGH MOUNTAINS: GNEISS DOME FORMATION AND EROSION IN THE PAMIRS

Cenozoic gneiss domes comprise one third of the surface exposure of the Pamir and allow an unparalleled view into deep crustal processes of the India–Asia collision. The largest of these is the ~350 × 90 km Shakhdara–Alichur dome of the southwestern Pamir of Tajikistan and Afghanistan. The dome is bounded by normal-sense shear zones, which exhumed crustal rocks from up to 30-40 km depth. We show that the dome formed between 22 and 2 Ma by N-S extension, coeval and parallel to ongoing N-S convergence between India and Asia. Smaller N-S extensional gneiss domes formed in the Central Pamir (Yazgulom, Sarez, Muskol, Shatput gneiss domes) from 20 to ~12 Ma. We attribute early Miocene extension to gravitational collapse of the crust, which was thickened by crustal stacking in the Eocene-Oligocene. Core-complex formation in the Pamir was compensated by shortening and development of a fold and thrust belt in the Tajik foreland basin from the middle Miocene on.

In the Pamir mountains, a prominent E-W change in topography offers a unique opportunity to study erosion in high mountain environments. The eastern Pamir forms a low-relief, high-elevation plateau, in parts internally drained, whereas the western Pamir is deeply incised by large river systems building up more than 2000 m of local relief. This lateral change in topography correlates with climate variations with arid conditions in the east and higher precipitation delivered from the ‘Westerlies’ in the west and northwest. However, low-temperature thermochronology and cosmogenic nuclide data suggest that long-term erosion rates in the western Pamir are low (<0.1 mm/yr) and that the relief formed by glacial and fluvial incision in the Quaternary and does not reflect long-term high erosion rates. Detrital thermochronologic signals of Pamir drainages with younger ages in western compared to eastern catchments do not necessarily reflect differences in regional erosion but result from the asymmetric distribution of Cenozoic gneiss domes. This study highlights the distinction between tectonic exhumation and erosion in the interpretation of cooling ages.