Geological evolution of the Hindu Kush, NW Frontier Pakistan: active margin to continent-continent collision zone

Abstract

A geological map of the eastern Hindu Kush, northwest of Chitral, Northern Pakistan, is presented. The lithologies are placed into two main categories, divided by the Tirich Mir Fault Zone. To the northwest, the units of the eastern Hindu Kush are dominated by monotonous sequences of graphite-rich pelitic rocks. Southeast of the fault, the phyllites and diamictites are thought to be lateral equivalents of the Northern Sedimentary Belt of the Karakoram. A structural analysis of the area studied identifies a major, early deformation phase which is usually characterized by tight to isoclinal folding with a well developed axialplanar schistosity. This deformation is thought to have been related to the northward-directed subduction and accretion beneath the southern margin of Asia during the Mesozoic, and may have taken place over a considerable period of time. A major phase of crustal melting at c. 24 Ma generated migmatites and biotite + muscovite ± garnet ± tourmaline leucogranites (including dykes and the Gharam Chasma pluton). This age is comparable to that of the Baltoro pluton in the Karakoram to the east, confirming the regional importance of crustal melting along the southern margin of the Asian plate during the earliest Miocene. The crustal melting was associated with thrusting and folding of the earlier schistosity. Subhorizontal stretching lineations indicate a phase of strike-slip deformation that is thought to have been associated with anticlockwise rotation of the regional foliation strike from E to NE and N after the emplacement of the Gharam Chasma pluton at c. 24 Ma. This deformation and rotation was probably a direct result of the northward-moving Indian plate forcing Kohistan to indent into Asia, resulting in a left-lateral transpressional tectonic environment which remains today. The anomalous height of the Tirich Mir massif, relative to other peaks in the Hindu Kush and the nearby Hindu Raj, may be accounted for by the onset of this transpression.Intensely active seismicity to depths of 300 km beneath the Hindu Kush is associated with seismic shear wave velocities that are significantly faster than those beneath Tibet, where earthquake occurrence is restricted to the upper crust, and previous geophysical studies indicate elevated thermal conditions and possible crustal melts. U-Pb ages suggest that post-India-Asia collision crustal melting beneath the Hindu Kush is restricted to c. 24 Ma, whereas in the Karakoram, the record is both more voluminous and more continuous from c. 37 to c. 9 Ma. These observations reflect major differences in the thermal histories of these regions, where the relatively cooler conditions beneath the Hindu Kush are associated with continental subduction-related seismicity.

Rifting, subduction and collisional records from pluton petrogenesis and geochronology in the Hindu Kush, NW Pakistan

“…1). The geology of the area is characterized by multiply deformed, variably metamorphosed, Paleozoic to Mesozoic sedimentary rocks intruded by the elongate Kafiristan, Tirich Mir, BuniZom, Kesu-Kohuzi and Garam Chasma plutons (Calkins et al, 1981;Hildebrand et al, 2000;Fig. 2).…”

“…Existing whole rock Rb-Sr data from the Kafiristan and Tirich Mir plutons outline ages of 483 ± 21 Ma (Debon et al, 1987) and 115 ± 4 Ma (Desio, 1964) respectively. U-Pb Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) analysis on monazite and uraninite grains from a pegmatite dyke in the Tirich Mir fault zone, thought to be related to the main Tirich Mir plutonic body, yielded a discordant, interpreted intrusion age of 114 ± 2 Ma (Hildebrand et al, 2000). More recently, zircon grains analyzed from a different specimen of the Tirich Mir body yielded two concordant analyses interpreted to comprise an age of 121 ± 1 Ma (Heuberger et al, 2007).…”

“…Published ages are typically interpreted based on only a few concordant analyses or extrapolated from arrays of data variably affected by Pb loss (Hildebrand et al, , 2000Heuberger et al, 2007); moreover, there is no previous record of U-Pb geochronology from the Kafiristan pluton. New U-(Th)/ Pb geochronology data were acquired using laser ablation paired with a multicollector inductively coupled plasma-mass spectrometer (LA-MC-ICP-MS) at the University of Santa Barbara, California (for detailed methodology see Cottle et al, 2011Cottle et al, , 2013.…”

“…Their occurrence has been variably attributed to subduction and/or collision-related crustal thickening processes (e.g. Desio, 1964;Debon et al, 1987;Hildebrand, 1998;Hildebrand et al, 2000;Zafar et al, 2000;Zanchi et al, 2000;Hildebrand et al, 2001;Heuberger et al, 2007). This study examines the petrogenesis and tectonic significance of the Kafiristan, Tirich Mir, Buni-Zom and Garam Chasma plutons based on new major, trace and rare earth element (REE) data and integrated U(Th)-Pb (zircon/ monazite) geochronology.…”

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

“…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). The region records a protracted deformational history with the earliest records indicating Late Triassic deformation and metamorphism and recent events culminating in the Early Miocene (Faisal et al, 2014).…”