Mantle exhumation along the Tirich Mir Fault Zone, NW Pakistan: pre-mid-Cretaceous accretion of the Karakoram terrane to the Asian margin

Abstract

The left-lateral strike-slip Tirich Mir Fault, Chitral, NW Pakistan, is associated with a belt ofperidotites, metagabbros and gneisses named the Tirich Boundary Zone (TBZ), separating the Late Palaeozoic-Mesozoic units of the East Hindu Kush from the Palaeozoic successions of the Karakoram block. These rocks were metamorphosed up to upper amphibolite facies conditions, followed by a greenschist facies overprinting, and then thrust on to very low grade metasediments; they were finally intruded at shallow levels by the midCretaceous Tirich Mir pluton. Ultramafic rocks along the fault zone include well-preserved spinel lherzolites and harzburgites (Tirich Gol, Barum valley, Arkari Gol), whereas schistose serpentinites occur in the Rich Gol. Whole-rock analyses and mineral chemistry of olivine, clinopyroxene, orthopyroxene and spinel from these peridotites show a depleted signature. Microstructural and petrological features suggest a mantle origin for these ultramafic bodies, which equilibrated at temperatures ranging from 1000-1100°C. Peridotites are faulted against partially metamorphosed igneous bodies including hornblende-gabbros, hornblende cumulates and quartz-diorites. Metamorphic rocks of the TBZ, which lay south of the ultramafic-mafic complex, include quartzites, amphibolites, garnet-sillimanite (_+ kyanite + K-feldspar)-biotite gneisses and mica schists, locally displaying migmatitic textures.A sub-continental character of the peridotites indicated by low temperatures of equilibration and by the presence of a deep crustal sequence. These characters along with the absence of an ophiolitic sequence may suggest that the TBZ represents a fragmented crust-mantle boundary developed along a zone of attenuated continental crust. The TBZ is interpreted as a sheared lithospheric section of a Jurassic-Early Cretaceous orogenic complex, formed as a consequence of the accretion of the Karakoram terrane to the southern side of the Pamir belts, which were progressively accreted to the Asian margin.

Mesozoic–Tertiary exhumation history of the Altai Mountains, northern Xinjiang, China: New constraints from apatite fission track data

“…Each collision event would have caused compression across Asia spanning several to tens of millions of years. In terms of the Cretaceous cooling seen in the AFT results exhumation and erosion linked to collision of the Lhasa block with Asia in the Lower Cretaceous (Zanchi et al, 2000) is most the likely cause. In the Junggar Basin on the southern margin of the Altai (Fig.…”

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

“…While ophiolitic rocks have been documented along the Rushan-Pshart Suture (Shvolman, 1980) indicating an oceanic basin between the Central and the Southern Pamir, the Wakhan-TBZ is defined by a narrow belt of mantle peridotites, metagabbros and gneisses (Zanchi et al, 2000). The absence of an ophiolitic sequence sensu stricto along the Wakhan-TBZ led Zanchi et al (2000) to the interpretation that it originated from a lower crust-upper mantle sequence, hence suggesting a subcontinental oceanic basin. The closure time of the Rushan-Pshart oceanic basin is still unclear.…”

“…The closure of Wakhan basin is less understood and thought to have occurred slightly later after the closure of Rushan Ocean (Angiolini et al, 2013;Robinson, 2015). As suggested by Zanchi et al (2000), welding of the Southern Pamir and the Western…”

“…It is generally interpreted that the Cimmerian superterrane detached from the Gondwanaland during the early Permian and drifted northward, finally accreting to the Eurasian margin by the late Triassic-early Jurassic (Angiolini et al, 2013;Angiolini et al, 2014;Gaetani, 1997;Zanchi and Gaetani, 2011;Zanchi et al, 2000). On its way to the north, the Cimmerian terrane was allegedly dissected into three blocks now represented by the Central et al, 2015) or even later in the Eocene (40 Ma; Bouilhol et al, 2013).…”

Microscale strain partitioning? Differential quartz crystallographic fabric development in Phyllite, Hindu Kush, Northwestern Pakistan

“…1). The geology of the area is dominated by Paleozoic protoliths, mainly low-grade metasedimentary rocks that locally reach sillimanite grade (Gaetani et al, 1996;Zanchi et al, 2000;Hildebrand et al, 2001;Zanchi and Gaetani, 2011;Faisal et al, 2014). These metasedimentary rocks are intruded by a series of plutonic bodies that range in age from Paleozoic (Kafiristan -483 ± 21 Ma; Debon et al, 1987), through Mesozoic (Tirich Mir -114 to 121 Ma, Desio, 1964;Hildebrand et al, 2000;Heuberger et al, 2007), to Cenozoic (Garam Chasma -24 Ma;Hildebrand et al, 1998).…”