Abstract
AbstractNeogene, syn‐collisional extensional exhumation of Asian lower–middle crust produced the Shakhdara–Alichur gneiss‐dome complex in the South Pamir. The <1 km‐thick, mylonitic–brittle, top‐NNE, normal‐sense Alichur shear zone (ASZ) bounds the 125 × 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km‐thick, mylonitic–brittle, top‐SSE South Pamir normal‐sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22–17 Ma Alichur‐dome‐injection‐complex leucogranites transition from foliation‐parallel, centimeter‐ to meter‐thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization‐temperature constraints suggest Alichur‐dome exhumation from 10–15 km depth. Thermochronologic dates bracket footwall cooling between ~410–130 °C from ~16–4 Ma; tectonic cooling/exhumation rates (~42 °C/Myr, ~1.1 km/Myr) contrast with erosion‐dominated rates in the hanging wall (~2 °C/Myr, <0.1 km/Myr). Dome‐scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E–W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between ~20–4 Ma to: (i) Mostly northward, foreland‐directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non‐underthrust Central Pamir.
Highlights
We show that footwall exhumation along the top‐NNE, normal‐sense, mylonitic
We show that the longer‐lasting, faster, and higher‐magnitude exhumation of the kinematically linked Shakhdara dome along the South Pamir shear zone (SPSZ) from 21–2 Ma reflects more significant tectonic exhumation than that of the Alichur dome along the Alichur shear zone (ASZ)
We showed that activities of the mostly dextral shear Gunt shear zone (GSZ) and the mostly normal‐shear ASZ and SPSZ temporally overlap
Summary
Gneiss domes in both Cordilleran‐style and collisional orogens record an early phase of burial during plate convergence, followed by exhumation during either lithosphere‐scale extension (for Cordilleran‐style domes) or continued plate convergence (for syn‐collisional domes). Extensional shear zones that bound and exhume syn‐collisional gneiss domes are incongruent with the regional framework of ongoing plate convergence and represent components of a complex fault network that includes shortening and/or relay structures that extend laterally and/or vertically through the crust. Incongruent syn‐collisional extensional gneiss‐dome exhumation involves extension that is either perpendicular (e.g., Laskowski et al, 2017; Murphy et al., 2002; Ratschbacher et al, 1989) or parallel (e.g., de Sigoyer et al, 2004; Horton et al, 2015) to the plate‐
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