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

Abstract

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 ~18–15 Ma to ~2 Ma at ~10 mm/yr slip and with vertical exhumation rates of 1–3 mm/yr; it resulted in up to 90 km ~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°C/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°C/km since the end of doming.