Building the Pamir‐Tibet Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Pamir: 3. Thermobarometry and petrochronology of deep Asian crust

Abstract

Large domes of crystalline, middle to deep crustal rocks of Asian provenance make the Pamir a unique part of the India‐Asia collision. Combined major‐element and trace element thermobarometry, pseudosections, garnet‐zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15–20 km at 37–27 Ma. The Central Pamir was then heated at ~10–20°C/Myr and buried at 1–2 km/Myr to 600–675°C at depths of 25–35 km by 22–19 Ma. The Shakhdara Dome in the South Pamir was heated at ~20°C/Myr and buried at 2–8 km/Myr to reach 750–800°C at depths of ≥50 km by ~20 Ma. All domes were exhumed at >3 km/Myr to 5–10 km depths and ~300°C by 17–15 Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large‐scale, normal‐sense shear zones, and with advection of near‐solidus or suprasolidus temperatures into the upper crust, triggering exhumation‐related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large‐scale thickening and coincident ~20 Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate‐scale events.

The Alichur Dome, South Pamir, Western India–Asia Collisional Zone: Detailing the Neogene Shakhdara–Alichur Syn‐collisional Gneiss‐Dome Complex and Connection to Lithospheric Processes

“…Previous investigations by our group (Hacker et al, ; Rutte, Ratschbacher, Khan, et al, ; Rutte, Ratschbacher, Schneider, et al, ; Schmidt et al, ; Stearns et al, ; Stearns et al, ; Stübner, Ratschbacher, Weise, et al, ) interpreted structural, petrologic, and geothermochronologic data for the Pamir gneiss domes within the framework of published numerical modeling results of channel flow in orogenic plateaus (Rey et al, ) to propose that the domes were exhumed during a Neogene stage of crustal thinning under dominant ~N–S and minor ~E–W extension. Rey et al's () modeling showed that low channel buoyancy, a small melt fraction, and a strong foreland upper crust favor foreland‐directed channel flow extrusion beneath a plateau; in contrast, high channel buoyancy, a large melt fraction, and a weak foreland upper crust favor coupling between extensional gneiss‐dome exhumation within the plateau and shortening in the foreland.…”

“…The Shakhdara–Alichur gneiss‐dome complex encompasses an ~350 km E‐to‐W increase in topographic relief that broadly correlates with an increase in exhumation depths from 10–20 km for the Alichur dome to 30–55 km for the Shakhdara dome (Figures and a) (Hacker et al, ; Stübner, Ratschbacher, Rutte, et al, ). Whereas the top‐S SPSZ bounds both domes to the south, the top‐N ASZ only bounds the Alichur dome to the north.…”

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral, NW Pakistan): Insights from phase equilibria modelling, and garnet–monazite petrochronology

“…(), from 37 Ma in the South Pamir to 29 Ma in the Central Pamir. Early heating and deformation has been attributed to the India–Asia collision leading to the first Indian slab break‐off underneath the South Pamir at 45 Ma leading to regional asthenosphere upwelling and the subsequent underthrusting of the Indian continental margin (Figure d, Guillot et al., ; Hacker et al., ; Mahéo et al., ; Negredo, Replumaz, Villaseñor, & Guillot, ; Smit et al., ). However, Van Hinsbergen et al.…”

“…Final and rapid exhumation/cooling (~50°C/km from 20 to 8 Ma; Faisal et al., ) may have been related to break off of the Indian slab at 25–20 Ma and the subsequent underthrusting of the Indian continental margin (DeCelles, Kapp, Quade, & Gehrels, ; Mahéo et al., ; Replumaz, Negredo, Guillot, & Villaseñor, ; Rolland et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ; Van Hinsbergen et al., ). While in the Miocene the northern part of the South Pamir and the Central Pamir underwent decompression melting during ~N–S extension (Hacker et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ), the rest of the South Pamir and the Karakoram experienced ~N–S crustal thickening and anatexis (e.g. Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ; Hacker et al., ; Stearns et al., ).…”

“…While in the Miocene the northern part of the South Pamir and the Central Pamir underwent decompression melting during ~N–S extension (Hacker et al., ; Rutte, Ratschbacher, Khan, et al., ; Rutte, Ratschbacher, Schneider, et al., ; Stearns et al., ), the rest of the South Pamir and the Karakoram experienced ~N–S crustal thickening and anatexis (e.g. Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ; Hacker et al., ; Stearns et al., ). Related deformation in the Chitral region was essentially accommodated through the reactivation of the Tirich Mir and Reshun transpressive faults (e.g.…”