Indian Continental Crust Research Articles

Continental subductions beneath Tibet and the Himalayan orogeny: a review

ABSTRACTArgand (1924) and later Holmes (1965) proposed that the Himalayan orogeny and Tibet uplift were the result of large underthrusting of the Indian continental crust beneath Tibet. Since then, plate motion studies have yielded new constraints on the relative displacements of India and Asia and a large number of geological and geophysical investigations have helped to determine the structure of the Himalayas and Tibet, the best natural laboratory for understanding mountain belts. The aim of this paper is to review briefly the various models of Himalayan mountain building in light of recent geological and geophysical data and to estimate the role of continental subductions.

Structural elements of the southern Tethyan Himalaya crust from wide‐angle seismic data

A deep seismic common midpoint (CMP) profile, shot across the southern margin of the Tethyan Himalaya as the first stage of Project International Deep Profiling of Tibet and the Himalaya (Indepth), imaged the top (Main Himalayan Thrust or MHT) and bottom (Moho) of the Indian continental crust underthrusting the Himalaya. We used portable seismographs to record the CMP‐profile shots at a wide range of offsets, up to 155 km. Our short‐offset data corroborate the CMP‐profile data, while our large‐offset data are dominated by a band of reflectivity. We interpret the bright onset of this reflective band as the basal detachment of the South Tibetan Detachment system and a phase at the end of the reflective band as the MHT. We used the CMP‐profile first‐break data to model the uppermost 2 km in detail. The depth of young extensional basins of the Yadong rift system along the CMP profile was constrained to a maximum of 2 km, yielding a throw of 4.6 km across the eastern flank of the rift and a rough estimate of approximately 1.5% east‐west extension by normal faulting in the Tethyan Himalaya. The wide‐angle data were used to construct a crustal seismic‐velocity model down to the MHT. The South Tibetan Detachment System (STDS) basal detachment reflector is observed dipping 12.5°NNE from a depth of about 6 km beneath the surface under the south end of the CMP profile to a depth of 22 km, then flattening to a dip of only 2.5°NNE. Thus our observations suggest that the STDS basal detachment is a deep‐rooted basement fault. For the MHT we observe a dip of 7.5°NNE from 20 km depth below sea level at the crest of the High Himalaya crystalline sheet to 36 km below sea level (40 km beneath the surface) at a distance of about 70 km south of the Indus‐Yarlung Suture (IYS). From geometrical arguments we suggest that Indian crust underthrusts the surface expression of the IYS within the crust but does not form the lower crust of central and northern Tibet.

Differential Unroofing and Inherited Radiogenic Strontium in the Western Himalaya

In the western Himalaya, the Nanga ParbatHaramosh massif (NPHM; Indian continental crust) has been differentially exhumed relative to the adjacent Kohistan island arc terrane (Zeitler 1985). This study aims to constrain the detailed cooling history of the region by using both Rb-Sr and laser probe 4~ dating of muscovite and biotite from closely-spaced samples. Such a study should also help to evaluate the resolution of mineral ages in determining cooling rates in regions of rapid exhumation. Samples dated include granite sheets and their metamorphic country rocks, and come from the NPHM, the Kohistan arc, and from the intervening steeplydipping, high-grade shear zone.

Deep seismic reflection evidence for continental underthrusting beneath southern Tibet

THE Himalaya and adjacent Tibetan plateau, constituting Earth's largest region of elevated topography and anomalously thick crust, formed as a consequence of Cenozoic collision between India and Asia—itself considered the archetypal continent–continent collision1–3. Here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling. Our ∼100-km-long profile, acquired in the Tethyan Himalaya, shows a mid-crustal reflection that prob-ably marks the active thrust fault along which the Indian plate is underthrusting southern Tibet; upper-crustal reflections with geo-metries suggestive of large-scale structural imbrication of the upper crust; and Moho reflections from the base of the double-normal-thickness crust underlying the region. These results lend substantial support to the view that crustal thickening beneath southernmost Tibet was accomplished by wholesale underthrusting of Indian continental crust beneath the structurally imbricated upper crust comprising the Tethyan Himalaya.

Geology of the northern part of the Nanga Parbat massif, northern Pakistan, and its implications for Himalayan tectonics

The northern outcrop termination of Indian continental crust lies at the Nanga Parbat massif, from where this contribution presents new field data. The tectonic contact with the structurally overlying Kohistan island arc is concordant and ductile, is associated with interleaving of Nanga Parbat and Kohistan lithologies, and may be correlated with the Main Mantle thrust found elsewhere in the NW Himalayas. This ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall, the northern outcrop termination of the massif is controlled by erosion through a gently, northward-plunging antiformal structure. This folds both the Main Mantle thrust and the underlying, concordant 'Layered Unit' of the Nanga Parbat basement. Thus there is no indication that the massif acted as a promontory to the Indian continent during collision nor that it is a structure entirely bound by neotectonic faults. Ductile shear fabrics associated with the 'Main Mantle thrust' are cross-cut by leucogranite sheets and pegmatites. These may represent the stockwork to a significant crustal-melt granite body described here in the northern massif. This Jutial granite shows many geochemical characteristics in common with similar bodies in the High Himalayas which are consistent with anatexis of the buried Indian continental basement rocks. However, the granite is enriched in heat producing elements (particularly Th: 22 ppm) and shows extremely high 87 Sr/ 86 Sr ratios (>0.88). The high concentrations of radiogenic Sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the Nanga Parbat massif, suggesting that these may represent a stockwork to a largely buried body of which the Jutial granite is a small exposure. The enrichment in heat-producing elements within the granite may reflect similarly high heat production in the source Indian continental crust requiring in turn a fundamental re-examination of the thermal evolution of this crust during Himalayan metamorphism and exhumation.

Rheology of Indian continental crust and upper mantle

A rheological model of the Indian shield has been constructed using the thermal structure derived from available surface heat flow and heat generation data and the flow properties of characteristic minerals and rocks like quartz, diabase and olivine which respectively represent the upper crust, lower crust and upper mantle. Lateral variations in the thicknesses of the brittle and ductile crust and of the brittle upper mantle have thus been obtained for different tectonic environments. Implications of these results to interpretation of the seismic structure of the Indian shield have been pointed out.

Radioactivity, Heat Flow, and Rifting of the Indian Continental Crust

The relationship between heat flow (Q) and heat production (A) in India is quite different for the northern and southern parts of the peninsula. In the north, locations in a variety of tectonic settings (including Gondwana rifts) show a linear relationship between Q and A with a common reduced heat flow (Q*) about $40 mWm^{-2}$. In southern India, the heat production of surface rocks is greater than is expected for the measured heat flow. Relative to other continental shields, g* is high in northern India and about average in the south. Apparently the Indian crust evolved with a content of radioactive elements higher than in most shields, and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle. This depletion may coincide with a regional granulite event about 2,500 m.y. ago. The high heat flow and heat production in northern India may explain several unique characteristics of the Indian peninsula. One feature is the continued reactivation of rif...

The tectonic history of Kohistan and its implications for Himalayan structure

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.