The giant Shakhdara migmatitic gneiss dome, Pamir, India-Asia collision zone: 2. Timing of dome formation

摘要

[1] Cenozoic gneiss domes-exposing middle-lower crustal rocks-cover ～30% of the surface exposure of the Pamir, western India-Asia collision zone; they allow an unparalleled view into the deep crust of the Asian plate. We use titanite, monazite, and zircon U/Th-Pb, mica Rb-Sr and ~(40)Ar/~(39)Ar, zircon and apatite fission track, and zircon (U-Th)/He ages to constrain the exhumation history of the ～350 × 90 km Shakhdara-Alichur dome, southwestern Pamir. Doming started at 21-20 Ma along the Gunt top-to-N normal-shear zone of the northern Shakhdara dome. The bulk of the exhumation occurred by ～NNW-ward extrusion of the footwall of the crustal-scale South Pamir normal-shear zone along the southern Shakhdara dome boundary. Footwall extrusion was active from～18-15Mato～2Maat~10mm/yr slip and with vertical exhumation rates of 1-3 mm/yr; it resulted in up to 90km～N-S extension, coeval with～N-S convergence between India and Asia. Erosion rates were 0.3-0.5 mm/yr within the domes and 0.1-0.3 mm/yr in the horst separating the Shakhdara and Alichur domes and in the southeastern Pamir plateau; rates were highest along the dome axis in the southern part of the Shakhdara dome. Incision along the major drainages was up to 1.0 mm/yr. Thermal modeling suggests geothermal gradients as high as 60℃/km along the trace of the South Pamir shear zone and their strong N-S variation across the dome; the gradients relaxed to ≤40-45℃/km since the end of doming.