Interpretations of pre-collisional configurations of Greater India are highly controversial and predict distinct processes during the India-Asia collision. To better determine the possible pre-collisional configuration(s) of Greater India, we conduct a mass-balance analysis combined with previously published geologic, paleomagnetic, and geodynamic evidence. The mass-balance analysis determines the magnitude of northern Greater India (NGI) width needed to provide sufficient crustal accretion to form the Tethyan-Greater Himalaya orogenic wedge in Cenozoic time. Applying endmember crustal thicknesses of 10–40 km to a mass-balance equation yields a broad range of plausible pre-collisional NGI widths of ∼3016±1000 km and ∼956±283 km, respectively, which we further assess considering contrasting models/evidence. The integrated evidence requires a thin NGI continental crust to form 1) continuous Tethyan-Greater Himalayan crustal thickening, 2) a narrow foredeep width of Himalayan foreland basin, 3) continuous Gangdese arc magmatism with oceanic-subduction-style mantle wedge, and 4) low-magnitude exhumation in the North Himalaya and Gangdese arc-forearc from ∼60-30 Ma. Adding the structurally restored ∼740 km wide southern Greater India, the synthesized analyses yield two possible configurations: 1) an ∼1350±440 km wide and ∼23-30 km thick NGI indicating an ∼2080±450 km wide Greater India with ∼500-1000 km wide oceanic basin systems in both Asia and NGI; and 2) a ≥1815±630 km wide and ∼10-23 km thick Zealandia-type NGI indicating a ≥2550±640 km wide pre-collisional Greater India without or with limited ∼500-1000 km Xigaze back-arc oceanic basin. The former is conditionally consistent with the integrated evidence by assuming no Cenozoic oceanic subduction initiation within NGI and predicts multi-stage collision since ∼60 Ma. The latter is consistent with the integrated evidence and predicts an approximate-single-stage collision at ∼60 Ma. Both configurations predict significant post-collisional NGI crustal shortening that may have been accommodated by the Eocene-Oligocene Greater Himalayan structural discontinuities.

One of the most striking geological features of the Pamir is the south-dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati-Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia-sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth-tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U-Pb geochronology combined with rare earth element analysis, and whole-rock major-trace element and Sr-Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (similar to 31-43 kbar/similar to 960-1170 degrees C) are the deepest sourced portions of the lithosphere from similar to 100 to 140 km depth, the protoliths of which represent the mid-lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (similar to 20 kbar/similar to 900 degrees C) represent the Cenozoic lower crustal rocks of Pamir terranes from similar to 70 km depth. These results indicate the south-dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.

An ongoing question in understanding the evolution of the Himalayan-Tibetan orogeny is how much of the observed upper crustal shortening and crustal thickness is related to the Cenozoic collision between India and Asia vs earlier tectonic events along the southern margin of Asia. While the Pamir Mountains located at the western end of the orogen have been proposed to have experienced significant Cenozoic shortening, recent studies have interpreted upper crustal shortening to be primarily mid- to Late Cretaceous. To further understand the timing of upper crustal deformation in the Pamir, we investigated synorogenic clastic deposits within the footwall of the north-dipping Tanymas thrust fault along the suture between the Northern and Central Pamir terranes. Sandstones from these deposits were analyzed by detrital zircon U-Pb, zircon fission track, and muscovite 40Ar/39Ar analyses to assess the age and source of the detritus. Results show the deposits were sourced from the Northern Pamir (hanging wall of the Tanymas thrust) and provide an Early Cretaceous maximum deposition age of ∼130–120 Ma, interpreted to constrain their age and date motion on the Tanymas thrust fault as Early Cretaceous. Our results, integrated with previous studies, show Cretaceous deformation in the Pamir began in the Northern Pamir (∼140–110 Ma) before sweeping into the Southern Pamir in the mid- to Late Cretaceous (∼110–75 Ma). These results are consistent with previous interpretations of an Early Cretaceous phase of shallow- or flat-slab northward subduction followed by slab rollback and southward migration of deformation and magmatism in the mid- Cretaceous.

Different crustal deformation histories between Tibet and the Pamir reflect along-strike variations in geodynamics of the Tethys orogen. To investigate the less well-documented deformation history of the Pamir, which has been a barrier in understanding the nature of these differences, we conducted an integrated study in the Kurgovat-Vanch region, NW Pamir. The lithologies are primarily Ediacaran-to-Carboniferous metasedimentary rocks intruded by Carboniferous plutons, which then experienced Late Triassic to Early Jurassic regional metamorphism. Structural mapping and analyses document a low-angle NW-directed thrust fault, the Poshkharv thrust, separating the overlying upper-greenschist facies Poshkharv complex from the underlying amphibolite facies Kurgovat complex. Regional geologic maps indicate the Poshkharv thrust continues for similar to 300 km across the NW Pamir. Our study also documents another regional thrust fault, the top-to-the-SE Vanch thrust that juxtaposes the Southern Kurgovat complex above the lower-grade Vanch complex in the south. Biotite 40Ar/39Ar thermochronology indicates Early Cretaceous movement on all structures with similar to 135-125 Ma exhumation along the NW-directed Poshkharv thrust and similar to 125-115 Ma exhumation along the SE-directed Vanch thrust. Regional crustal deformation in the Northern Pamir was formed in a Cretaceous retro-arc setting, unrelated to the Cenozoic India-Asia collision. Cretaceous deformation in the NW Pamir was broadly coeval with the NE Pamir, but preceded Cretaceous shortening and coeval arc magmatism in the Southern Pamir. We interpret Early Cretaceous thrusting and crustal thickening followed by southward migration of shortening and magmatic flare-up in the Pamir to have resulted from a transition of Neotethys subduction from northward flat-slab advancing to southward retreating.

While the northward convex Pamir salient has been interpreted to be a Cenozoic feature, resulting from terranes of the Pamir experiencing similar to 300 km of northward translation relative to Tibet and Afghanistan during the India-Asia collision, structural evidence for large magnitudes of translation is generally lacking. We focus on the geology of the Northern Pamir, which forms the outer margin of the salient, to argue that the Pamir salient has not experienced significant northward translation as has been suggested by several recent studies. We then present evidence based on new detrital zircon results that the Pamir salient may be an inherited feature from the Late Paleozoic southern margin of Asia. Kinematic endmember models of northward translation of the Pamir predict either similar to 300 km of arc-parallel extension (radial thrusting model) or truncation of geologic terranes at the margins of the salient by strike-slip faults (transfer faulting model). However, both models are inconsistent with the regional geology and structural evolution, which shows minimal extension along outer arc of the Pamir (similar to 50 km) and no clear truncation or thinning of geologic terranes that make up the Northern Pamir. Regarding the origin of the Pamir salient, a pronounced change in detrital zircon age signature of the Northern Pamir from west to east suggests the regions were not directly connected (i.e. part of a continuous linear belt), and were sourced from distinct cratons. Further, an absence of Triassic arc/forearc flysch deposits and arc plutons along WNW portion of the Northern Pamir suggests the possibility of a transform boundary segment along the Triassic Paleo-Tethys oceanic subduction zone. Based on these observations, we suggest an embayment existed between Tarim and Karakum Cratons after their amalgamation to the southern margin of Asia in the Early Permian. During the Late Triassic, colliding Cimmerian Gondwanan terranes (Central-Southern Pamir) filled in the embayment, resulting in the northward deflection in the trace of the Paleotethyan suture zone and Gondwanan terranes and the current arcuate geometry of the Pamir. Subsequent Cenozoic northward translation of the Pamir is interpreted to be only similar to 50 km, no more than similar to 80-100 km, and may be coupled with northward translation of northwestern Tibet and northeastern Afghanistan. (C) 2020 Elsevier B.V. All rights reserved.

The Pamir gneiss domes represent the most extensive exposure of mid to lower crustal rocks in the Himalayan-Tibetan orogen north of the India-Asia suture zone. Unlike other domes in the Central and Southern Pamir, the Muztaghata dome stands out due to its higher metamorphic grade, more complex structural elements, and variable timing of metamorphism. In order to unravel the P-T-t history of the Muztaghata dome and better constrain the timing of peak metamorphism, we applied petrologic modeling in concert with geochronology to samples from the structure. The Muztaghata gneiss dome is composed of a structurally higher metapelite-dominated terrane in the west and a structurally lower orthogneiss terrane in the east. Our results from the western terrane indicate high-pressure eclogite facies peak conditions of similar to 800 degrees C/22 kbar at similar to 25-20 Ma. Zircon grains from metapelitic samples from the western terrane also yield Early Jurassic metamorphic U-Pb ages with REE signals that indicate coeval garnet growth. Our results from the eastern terrane record high-pressure amphibolite facies peak conditions of similar to 650 degrees C/14 kbar at similar to 24-20 Ma, noticeably lower than the structurally higher western terrane indicating structural juxtaposition during Miocene exhumation. Peak metamorphic conditions from the eastern terrane indicate depths below the current Moho, supporting the interpretation that the Early Miocene Pamir crust was thicker than present. This was followed by rapid exhumation from depths of similar to 75-80 km and partial westward collapse of the Pamir after 20 Ma, possibly driven in part by regional lithospheric delamination.

Bredigite was synthesized by using the Piston-Cylinder in 1.2 GPa and 1 473 K. With external heating device and diamond anvil cell, high temperature and high pressure Raman spectra of bredigite were collected at temperatures 298, 353, 463, 543, 663, 773 and 873 K and with pressure from 1 atm up to 14.36 GPa (room temperature). The SEM image showed that the sample consisted of one crystalline phase with grain size ranging from 10～20 μm. The EPMA data suggest a chemical formula of Ca7.03(2)Mg0.98(2)Si3.94(2)O16 which was identical to the theoretical component of bredigite. The Raman spectroscopic results indicate there were 29 vibration bands of bredigite at high temperature. Some bands were merging, weakening and disappearing increasingly with the temperature, which was obvious in the range of 800~1 200 cm-1. The vibration bands of 909, 927 and 950 cm-1 disappeared at 873, 773 and 873 K, respectively. The results primarily indicated that the structure of bredigite was stable under experimental condition. In addition, isobaric mode-Grüneisen parameters and isothermal mode-Grüneisen parameters were calculated, yielding 1.47(2) and 0.45(3) as their mean values, respectively. Anharmonic coefficients were estimated based on the high temperature and high pressure Raman experiments, showing that the contributions to anharmonic-effect induced with the Si—O vibration modes were smaller than other modes.

Pamir metamorphic crust is dominated by eight metamorphic domes exposed across Pamir from east to west, among which Shakhdara dome is the biggest one. Peak T-P condition of Shakhdara garnet-sillimanite gneiss is T~810°C/P~10kbar represented by its peak assemblage Grt+Ky+Bi+Rt+Pl+Qz, while the peak T-Pcondition undergone in Shakhdara's garnet-clinopyroxene granulite is T~824°C/P~16.3kbar calculated from the peak assemblage Grt+Cpx+Rt+Pl+Qz. As to garnet-amphibolites in Shakhdara, which had experienced relatively severe retrograde, T~683°C~873°C/P~8.6~11.7kbar is revealed as the metamorphism condition of its residual peak assemblage Grt+Pl+Hbl+ilm+Qz. Peak metamorphic age 19~35 Ma of the mafic granulites dated by U-Pb method on zircons may reflect the thickening and heating process of Pamir lower crust from Late Eocene to Early Miocene. The metamorphism of Pamir domes could be compared with that of High Himalaya crystalline complex and we suggest that Cenozoic Pamir intra-continental-subduction alone the formerly formed suture zone after India-Asia Collision could be the trigger of Cenozoic thickening and metamorphism of lower crust in Pamir.