No abstract

Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over ~30 km of crustal section during the India‐Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7–10 km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90 km. Two thrust sheets, comprising Cambro‐Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite‐facies metamorphic rocks within the domes—equivalent to lower grade rocks outside the domes—form fold nappes with dome‐scale wavelengths. E‐W stretching occurred contemporaneously with top‐to‐ ~ N imbrication and folding. At ~22–12 Ma, bivergent (top‐to‐N and top‐to‐S), normal‐sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at ~12 Ma with opposite‐sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant ~N‐S shortening interacted with ~E‐W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir‐Tibetan Plateau and short‐lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India‐Asia collision, and the Miocene‐Recent dextral shear and ~E‐W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.

Geothermochronologic data outline the temperature‐deformation‐time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top‐to‐ ~N thrust‐sheet and fold‐nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome‐bounding normal‐sense shear zones. Synkinematic minerals date normal sense shear‐zone deformation at ~22–17 Ma. Age‐versus‐elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir‐wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India‐Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep‐seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

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.