STRUCTURAL GEOMETRY AND STRAIN PATTERNS OF THE COLLISION ZONE, ZANSKAR, NW-HIMALAYA

Abstract

The Himalayan Arc has been considered as archetype example of large-scale continental convergence caused by the subduction of the Tethyan Oceanic Crust and subsequent collision of the Indian Plate against the Eurasian Plate along the Indus Suture Zone during the Late Cretaceous-Cenozoic. It is inferred that many tectonic units of the Himalaya has been folded, thrust and uplifted due to collision. The Indus Suture Zone (ISZ), Tethyan Sedimentary Zone (TSZ) and the metamorphic piles of the Higher Himalayan Crystalline (HHC) have suffered maximum crustal shortening and upliftment. Therefore, study of structural geometry and strain patterns of the Indus Suture Zone, Tethyan Sedimentary Zone and the Higher Himalayan Crystalline may elucidate the mechanism of intracontinental crustal shortening. The area under investigation along Suru-Doda Valleys includes upper part of the Higher Himalayan Crystalline and the basal units of the Paleozoic-Mesozoic pile of the Tethyan Sedimentary Zone. The latter is thrust over the remobilised basement of the Higher Himalayan Crystalline along NE-dipping intracontinental Zanskar Shear Zone. The HHC designated here as the Suru Group is comprised of garnetiferous schist, kyanitestaurolite schist/gneiss, kyanite-sillimanite schist/gneiss, sillimanite-muscovite schist / gneiss, sillimanite- K-feldspar schist/gneiss, mylonite gneiss, augen gneiss along with migmatite and intrusive granitic bodies. The Tethyan Sedimentary Zone includes the Haimanta Group (Late Proterozoic-Cambrian), Phe 11 Volcanics (Permian age) and Mesozoic sedimentary succession. The Haimanta Group is further subdivided into the Batal Formation of grey quartzite, phyllite, metasandstone and siltstone and the Karsha Formation consisting of slate, phylite, calc-phylite and marble. The Karsha Formation is thrust over by the amygdaloidal metavolcanics of the Phe Volcanics. In the north of the TSZ, the trench sediments and the ophiolite melange of the Indus Suture Zone (ISZ) override the TSZ sediments along the SW-dipping Dras Thrust. The Higher Himalayan Crystalline (HHC) reveals four major deforraational phases whereas Tethyan Sedimentary Zone has suffered only three episodes of deformation. The earliest HDi phase of deformation in the HHC has caused rarely recognisable tight to isoclinal HFX folds on the lithological layering or metamorphic banding with long-drawn limbs and tightly appressed rounded narrow hinges. Axial plane foliation HS^. parallels the lithological layering or metamorphic banding on limbs of HF* folds but cut across the hinges. Second deforraational phase (HD2) in the Higher Himalaya caused first episode of deformation (TDi) in the Tethyan Sedimentary Zone. As a result of this deforraational phase, reclined plunging HF2 folds in the HHC and TFx folds in the TSZ are developed. These folds persistently plunge towards NE or SW and parallel the prominent stretching/mineral lineations (HL2 in the HHC and TLX in the TSZ) on the NW-SE trending HS2/TS:l axial plane foliations. HS2 foliation in the HHC transposes the HSo. foliation into most pervasive planar structures. Prominently developed TSa. axial Ill plane foliation in the TSZ are parallel to the bedding surfaces. Stretching lineations are contemporaneously developed during HD2/TDr ductile shearing deformation irrespective of the orientation of HS2/TSi foliations. During the HD3/ TD2 deformational phase, isoclinal to close, low to moderately, SE or NW-plunging HF3»/TF2« folds are developed mainly on the earlier planar structures. Foliation parallel to axial surfaces HS3^/TS2j» of these folds cut across the folded HS2/TSi foliations in the hinge zone of HF3ai/TF2« folds and dip gently to moderately towards NE or NNE. Subsequently, gently to moderately E/SE or W/NW plunging HF3b/TF2b folds are developed with characateristic NE dipping HS3b/TS2b crenulations foliation. Extensional crenulation cleavage, foliation boundinge, kink, extension gashes and brittle-ductile shear zone are developed during the last phase of deformation (HD4 in the HHC and TD3 in the TSZ). Superposition of HF2 on HFX folds and HF3fa on HF3- folds have resulted mainly in Type - 3 fold interference patterns. The overprinting of HF3ai folds and HF3b folds on HF2 folds has led to the development of Type -2 fold interference patterns. In the Tethyan Sedimentary Zone, interference patterns are very rarely noticed. Granitic bodies in the HHC and the TSZ have intruded during different periods. Large granitic intrusives at Ringdom in the TSZ and Sankoo in the HHC are probably older than the Himalayan Orogeny whereas leucogranite bodies along the Zanskar Shear Zone around Padam showing syn-Himalayan fabrics are of Himalayan age. IV Migmatisation in the HHC started during HD2 ductile deforra ational phase and continued after the HD3 deforraational phase. Along the Indus Suture Zone, different rocks of the Lamayuru Flysch and Shergol melange have undergone deformation during the emplacement of the ophiolites resulting into highly sheared serpentinites and conglomerate bodies in the Shergol melange. In the conglomerate, E-W trending and southerly dipping space cleavage cointain preferably oriented pebbles of different composition. The Zanskar Shear Zone (ZSZ) represents an important ductile shear zone along the upper margin of the HHC along which the Tethyan Sedimentary Zone is thrust southward on to the basement. Different shear criteria viz., S-C fabrics, asymmetric quartz and feldspar augen, asymmetric pressure shadow around garnet crystals, inclusion trails within snowball garnet, asymmetric lenses of granite, aplite and pegmatite, intrafolial folds, broken and displaced feldspar grains and duplex structures in phyllite etc., indicates top-to-SW sense of movement along the ZSZ. Lately developed extensional crenulation foliation, foliation boudinage, pull-apart structures, brittle-ductile and brittle faults are the best examples for an extension parallel to main foliation along the ZSZ. Synchronously developed extensional crenulation foliation (ecCl and ecc2) trend consistently towards NW-SE but ecCl dips moderate to steeply towards NE whereas ecc2 dips gently towards SW. During D4 deforraational phase, maximum principal stress axis ( ox) of the stress ellipsoid ( ox > o 2 > o 3) plunges steeply towards SW whereas minimum principal stress axis ( o 3) plunges gently towards NE. Regional strain variations in the Higher Himalayan Crystalline and Tethyan Sedimentary Zone have been analysed. Augen in mylonite and granite gneiss of the HHC and amygdules in the Phe Volcanics of the TZS have been used as the strain markers. A comparison of different techniques of strain analysis reveals that the Rf/0 technique is the ideal method and Rs value calculated from this method show close approximation to the Rs value calculated by Fry's centre-to-centre method. Two dimensional strain R£/0 data combined to determine the three dimensional strain ellipsoid, give the following strain patterns during the HD2 deforraational phase in the HHC and TDX deforraational phase in the TSZ. i) Flattening-type deformation has taken place both in the HHC and the TSZ, but strain ellipsoids are much more flattened along the ZSZ. ii) In the Tethyan Sedimentary Zone, strain magnitude ( 6.) gradually increases from Tongde to Ringdom and attains maximum values around Ringdom. iii) Finite strain value ( 6. ) gradually decreases away from the Zanskar Shear Zone within the HHC. iv) Consistent spatial pattern of strain parameters k, K, 6., 3? and r around Ringdom and oblique of main foliation to VI lithological contact with the Karsha Formation possibly indicate the presence of another tectonic boundary at the base of the Phe Volcanics. This is consistent with the observations around Karsha where the Phe Volcanics are thrust southward on the Karsha Formation. v) X-dimension of the strain ellipsoid both in the HHC and the TSZ is consistently oriented towards NE with high pitch on the main foliation and down-the-dip plunge. Along the Indus Suture Zone, pebbles of different lithologies from deforraed conglomerate of the Shergol melange have been analysed for the strain patterns of the subduction zone. Distribution of points in all the strain fields suggests lack of significant deformation of the pebbles. These have undergone rigid-body rotation to develop strong linear fabric on well-developed space cleavage in the conglomerate during the emplacement of ophiolite along the ISZ. Deformation and strain patterns of the HHC and the TSZ have been modelled within the framework of the intracontinental Collision Tectonics having initial ductile shearing at deeper crustal levels. The HHC and the basal part of the TSZ were probably deformed into a broad ductile shear zone with bulk strain field of the flattening-type throughout the progressive deformation. Such shear zone of the overthrust-type had a consistent top-to-southwest sense of ductile shearing with a few zones of high strain like the ZSZ. Whole metamorphic pile was Vll subsequently deformed into orogen-parallel folds with consistent NW/SE trending fabric. Large-scale layer-parallel extension characterised the HHC and signify superposed extension within an orogenic belt. Late stage extensional tectonics manifest rapid uplift of the HHC probably due to ramping along the MCT during the southward propagation of the metamorphic thrust sheet.