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

Abstract

[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 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.

Rates of river incision across the main tectonic units of the Pamir identified using optically stimulated luminescence dating of fluvial terraces

“…Figs. 1, 2, own field observations and Stübner et al 2013). Away from the Pamir boundaries, seismic records indicate an intra-plateau neotectonic activity that relates to the east-west extension of the Karakul rift system, and scattered seismic activity in the Southern Pamir (e.g., Fan et al, 1994;Strecker et al, 1995;Sippl et al, 2013).…”

“…Dome exhumation in the eastern Pamir has remained constant since ∼7 Myr (Robinson et al, 2010). Thermochronology indicates that exhumation of the Shakhdara and Yazgulom domes reached a peak at ∼15 Myr and continued until at least ∼2 Myr (Stübner et al, 2013). Neotectonic activity is focused at the active frontal range of the Pamir that bends almost 180 • from northern Afghanistan into western China (Bershaw et al, 2012).…”

“…The domes cover up to 30 % of the Pamir and expose crystalline basement and metamorphic rocks (Vlasov et al, 1991;Brunel et al, 1994;Schwab et al, 2004;Robinson Schmidt et al, 2011;Stübner et al, 2013) The Kurgovat Dome of the Northern Pamir consists of high-grade metamorphosed Triassic rocks. The domes in the Central Pamir (Yazgulom, Sarez, Muskol and Shatput domes; Fig.…”

“…The domes in the Central Pamir (Yazgulom, Sarez, Muskol and Shatput domes; Fig. 1) and the Southern Pamir (Shakhdara and Alichur domes) have exhumed high-grade metamorphic rocks of Oligocene-Miocene age (Schmidt et al, 2011;Stübner et al, 2013). The Pamir gneiss domes relate to orogen-wide exhumation between ∼21 Myr and 13 Myr (Lukens et al, 2012).…”

“…The topography of the Pamir is dominated by a series of Oligo-Miocene domes (Hubbard et al, 1999;Ducea et al, 2003;Schwab et al, 2004;Amidon and Hynek, 2010). The timing and mechanism of dome exhumation and the distribution of neotectonic activity are a focus of research in the Pamir (e.g., Schmidt et al, 2011;Schneider et al, 2013;Sippl et al, 2013;Stübner et al, 2013). The Pamir lies in a climatic transition zone between the Westerlies and the Indian Summer Monsoon (ISM).…”