Karakoram Block Research Articles

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 of periodotites, 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 mid-Cretaceous 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. Periodotites 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.

Subduction and collision related magmatism in the Shyok Suture and eastern Karakoram

The Shyok Suture is represented by distinct sets of volcano-plutonic rock assemblages. The high-Mg tholeiitic basalt and calc-alkaline andesites of the Shyok Volcanics have a subduction zone chemical signature. The REE data on tholeiitic basalt suggest a chemical affinity between primitive N-MORB to E-MORB. The calc-alkaline andesites, however, resembles to transitional nature of basalt between E-MORB to OIB. The geochemical data and regional tectonic setting suggest a close similarity between the Shyok Volcanics of northern Ladakh and the Chalt Volcanics of Kohistan.
 The mildly deformed trondhjemite-tonalite-granodiorite of the Tirit Granitoids are composite plutons located south of the Shyok Suture melange. These granitoids are subalkaline, I-type and represented by volcanic arc chemical signatures. The regional tectonic setting, the nature of occurrence and the composition of Tirit Granitoids are similar to the plutonic suites of northern Kohistan (Gindai, Matum Das and Nomal plutons).
 The eastern Karakoram Batholith is dominated by quartz monzonite-tonalite-granodiorite and granite. The subalkaline to calc-alkaline Karakoram Batholith is constituted by both I-and S-type granitoids with volcanic arc and syn-collision chemical signatures. REE data suggest that the I-type granitoids of eastern Karakoram are calc-alkaline magmatism of a subduction zone environment. In contrast, most of the S-type granitoids are crust-derived peraluminous granitoids. New Rb/Sr isotopic whole rock age data indicates that an S-type intrusive phase was active in the eastern Karakoram region during 83±9 Ma. The syn-collision nature of these granitoids are similar to those of north Sost pluton and Karambar pluton of northern Kohistan.
 This indicates that the collision between Kohistan-Ladakh arc and Karakoram block was active during 83±9 Ma.

Discovery of Gondwana plant fossils and palynomorphs of Late Asselian (Early Permian) age in the Karakoram Block

We report the discovery of Early Permian (late Asselian, ∼280–275 Ma) plant fossils and associated palynomorphs from a marine sedimentary sequence of the eastern Karakoram plate, in northern India. These specimens show affinities with those found in marine Lower Gondwana sediments of the Indian subcontinent. This supports the contention that during the Early Permian Period, the Karakoram plate was Peri‐Gondwanan. It is suggested to have had an intermediate position between the Indian plate and the Qiangtang–Lhasa microcontinents, at a latitude of about 35° south.

EARLY ORDOVICIAN AND DEVONIAN CONODONTS FROM THE WESTERN KARAKORAM AND HINDU KUSH, NORTHERNMOST PAKISTAN

Extensive tracts of Devonian and older sedimentary and igneous units occur within the axial region of the western Karakoram Block of northernmost Pakistan over a distance in excess of 200 km between the the headwaters of the Karambar valley in northwestern Gilgit Agency to southwestern Chitral. Conodont data indicate that the oldest sedimentary unit so far discriminated within this belt, the Yarkhun Formation, includes horizons of Ordovician (Arenig) age, consistent with an earlier-presented acritarch-based Arenig age for part of the same unit. Conodont data from the Lun Shales, a stratigraphic potpourri with little-known Silurian and Devonian tracts, demonstrate the presence of Early Devonian (early Emsian) horizons. The Shogrâm Formation, widely distributed through the region, spans an appreciable interval of the Middle and Late Devonian mid-Givetian through until at least early Famennian. A major lacuna in sedimentation may be present, represented by all or most of the earlier half of Frasnian time. A biostratigraphically and possibly biogeographically important new species, Icriodus homeomorphus, is described; it is encountered in horizons of early Famennian age (Late triangularis Zone to ?Early crepida Zone).

Plate tectonic interpretation of the Western Himalaya

The Western Himalaya, which includes the regions of Kumaun, Garhwal, Himachal, Jammu and Kashmir in India displays a complete cross-section of the Himalayan orogenic belt encompassing the principal tectonic zones of Outer Himalaya, Lesser Himalaya, Higher Himalaya, Tethys Himalaya, and Trans Himalaya. The rock formations of the Lesser Himalaya, which are correlatable to those of the northern part of the Indian Peninsular shield, represent deposits on the northern edge of the Indian craton. The Phanerozoic sequence of the Tethys Himalaya zone is interpreted as representing sedimentary deposits of the continental margin. The Central Crystallines of the Higher Himalaya zone, underlying the Phanerozoic sequence of the Tethys Himalaya zone, have been exposed in their present position as a result of uplift and southward movement along the Main Central Thrust. The Trans-Himalaya zone consists of the Indus Suture, Ladakh magmatic arc, Shyok Suture and Karakoram subzones. The Nidar ophiolite and the Shergol and Zildat ophiolitic melanges of the Indus Suture are the tectonised remnants of the Tethys ocean floor. The Ladakh plutonic complex, the Dras volcanics and other related Cretaceous volcanics belong to the plutonic-volcanic series of the Ladakh magmatic arc. The Shyok Suture is interpreted as a back-arc basin, and the Palaeozoic-Mesozoic sequence of the Karakoram subzone represents continental margin deposits of the Karakoram block. The molasse sedimentary deposits of the Outer Himalaya zone are interpreted as representing post-collision sedimentation in a trough that developed in front of the rising Himalaya. The tectonic evolution of Northwestern Himalaya has been involved in the following sequence of events. 1. (a) Subduction of the Indian oceanic plate under the Karakoram (Tibet) block in the Cretaceous, giving rise to the Ladakh magmatic arc and arc-trench gap sedimentation. 2. (b) Collision of the Indian plate with the Ladakh magmatic arc in the middle Eocene forming the Indus palaeosubduction zone. 3. (c) Closure of the Shyok back-arc basin and collision of the Ladakh magmatic arc with the Karakoram block. 4. (d) Suturing of the lithospheric blocks as a result of collision. The penetrative and non-penetrative deformation, regional metamorphism, generation of S-type granites in the Higher Himalaya zone and the formation of nappes and thrusts owe their origin to crustal shortening in the Oligocene-Miocene.