The giant Shakhdara migmatitic gneiss dome, Pamir, India‐Asia collision zone: 1. Geometry and kinematics

Abstract

Cenozoic gneiss domes comprise one third of the surface exposure of the Pamir and provide a window into the deep crustal processes of the India‐Asia collision. The largest of these are the doubly vergent, composite Shakhdara‐Alichur domes of the southwestern Pamir, Tajikistan, and Afghanistan; they are separated by a low‐strain horst. Top‐to‐SSE, noncoaxial pervasive flow over the up to 4 km thick South Pamir shear zone exhumed crust from 30–40 km depth in the ~250 × 80 km Shakhdara dome; the top‐to‐NNE Alichur shear zone exposed upper crustal rocks in the ~125 × 25 km Alichur dome. The Gunt shear zone bounds the Shakhdara dome in the north and records alternations of normal shear and dextral transpression; it contributed little to bulk exhumation. Footwall exhumation along two low‐angle, normal‐sense detachments resulted in up to 90 km syn‐orogenic ~N‐S extension. Extension in the southwestern Pamir opposes shortening in a fold‐thrust belt north of the domes and in particular in the Tajik depression, where an evaporitic décollement facilitated upper crustal shortening. Gravitational collapse of the Pamir‐plateau margin drove core‐complex formation in the southwestern Pamir and shortening of the weak foreland adjacent to the plateau. Overall, this geometry defines a “vertical extrusion” scenario, comprising frontal and basal underthrusting and thickening, and hanging gravitationally driven normal shear. In contrast to the Himalayan vertical extrusion scenario, erosion in the Pamir was minor, preserving most of the extruded deep crust, including the top of the South Pamir shear zone at peak elevations throughout the dome.

Crustal versus mantle core complexes

“…However, this would apply only to core complexes whose width of exhumed metamorphic rocks is in the order of 100km like the Southern Rhodope core complex in northern Greece (Brun and Sokoutis, 2007) or the Shakhdara migmatitic gneiss dome in Pamir (Stübner et al, 2013) …”

Neogene Source-to-Sink Relations between the Pamir and Tarim Basin: Insights from Stratigraphy, Detrital Zircon Geochronology, and Whole-Rock Geochemistry

“…Many of the intrusive and volcanic rocks in the central and southeast Pamir have late Eoceneearly Oligocene ages (Budanov et al 1999;Ratschbacher personal communication), which are likely the sources of ∼40 Ma igneous-origin zircons in modern river sediments in the western Pamir (Lukens et al 2012) and Cenozoic deposits at Oytag in the western Tarim Basin (Bershaw et al 2012;Sun and Jiang 2013). Migmatization and leucogranite crystallization that occurred at ∼22-19 Ma (Stü bner et al 2013a(Stü bner et al , 2013b, synchronous with high-temperature metamorphism in the central and south Pamir terranes (Stearns et al 2013), indicate considerable crustal thickening beneath the Pamir during subduction of the Indian plate (Schmidt et al 2011;Stearns et al 2013). In the eastern Pamir, the Tashkurgan alkaline complex emplaced at ∼11 Ma (Robinson et al 2007;Ke et al 2008;Jiang et al 2012), coeval with the eruption of Tajikistan volcanic rocks to the southwest (Ducea et al 2003;Hacker et al 2005).…”

“…Note present-day northward movement velocities of 23 ‫ע‬ 2 mm/yr and 20 ‫ע‬ 2 mm/yr for the north Pamir and Tarim Basin, respectively, relative to the Eurasia determined by GPS (Zubovich et al 2010 and Liu 2006;Bosboom et al 2011;Bershaw et al 2012;Sun and Jiang 2013), the Pamir has been an area of significant research over the past few decades. Some researchers have focused on tectonic deformation and rock exhumation within the Pamir hinterland (e.g., Arnaud et al 1993;Strecker et al 1995;Sobel and Dumitru 1997;Robinson et al 2004Robinson et al , 2007Amidon and Hynek 2010;Cowgill 2010;Sobel et al 2011Sobel et al , 2013Lukens et al 2012;Cao et al 2013aCao et al , 2013bStü bner et al 2013aStü bner et al , 2013b. However, relatively less attention has been given to understand the linkage between mountain building, sediment routing, and basin growth throughout the Cenozoic (Brookfield 2008;Bershaw et al 2012;Sun and Jiang 2013;Zheng et al 2000Wei et al 2013).…”

Pamir Plateau formation and crustal thickening before the India-Asia collision inferred from dating and petrology of the 110–92 Ma Southern Pamir volcanic sequence

“…large crustal domes, which formed along the pre-existing structures related to the Mesozoic terrane accretion (Angiolini et al, 2013;Pashkov and Budanov, 1990;Schwab et al, 2004;Stubner et al, 2013a;Stubner et al, 2013b).…”

“…Metamorphic rocks with an Oligocene-Miocene high-grade metamorphic peak, which may represent the crystalline basement of the Southeastern Pamir, characterize the Southwestern Pamir (Schmidt et al, 2011;Stubner et al, 2013a;Stubner et al, 2013b).…”