Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Yogesh Kumar ¹, R. C. Patel ¹

Affiliations
1 Department of Earth Sciences, Kurukshetra University, Kurukshetra - 136 119, IN

Abstract

The Chiplakot Crystalline Belt (CCB) in Kumaon, NW-Himalaya is made up of biotite schist biotite augen gneiss and psammite. These rocks are mylonitised and deformed in a NC-dipping broad ductile shear zone (across the whole CCB) of the overthrust-type formed due to intracontinental crustal shortening during Cenozoic within the Indian Plate.

Two distinguishable structural regimes characterize the CCB viz, the pre-Himalayan deformation and Himalayan deformation. Pre Himalayan deformation is associated with at least one distinct episode of deformation (D₁) while Himalayan deformation is characterized by three distinct episodes of deformation (D₂, D₃ and D₁). Out of these three deformations, D₂ is the most prominent deformation and wide spread throughout the CCB. It is distinguished by a prominent S₂ foliation axial planar to the rec1ined F₂ folds and NE-plunging L₂ stretching/Mineral lineation. The early tight to isoclinal F₁ folds on lithological layering have been stretched and rotated towards NC-plunging F₂ folds during the SW-directed D₂ ductile shear deformation S-C fabric, asymmetric augen, and sheared lenses etc are ubiquitously developed during D₂ throughout the CCB. During progressive translation towards shallower structural level and southward migration of the thrust sheets, the rocks in the shear zone underwent D₃ and at last, D₁ deformation in compressional regime. D₃ episode is characterized by N-NW plunging F₃ folds and D₄ by kink bands, etc.

In contrast to main crustal shortening fabrics, fabrics related to extensional tectonics within the CCB have also been identified, which was even initiated during late D₂ and continued post dating D₃. Different shear indicators such as extensional crenulation cleavage (ecc) foliation boudinage, extensional shear bands, brittle normal faults and quartz veins with fringe folds are developed throughout the CCB.

Microstructural studies of the constituent minerals of the shear zone demonstrate the deformation mechanisms and degree of shear strain vary from the basal and upper thrusts towards the central part of the CCB. Mylonitic locks in the middle of the CCB contain less than 10% of relic fragments and the deformation mechanism is characterized by dynamic recrystallization of feldspar and quartz, indicating deformation at amphibolite grade. Preferred orientation of long axes of quartz grains indicates relatively high shear strain in this zone. Towards the basal and upper parts at the mylonite zone, the percentage of relic fragments becomes more than 10%. In these zones, feldspars are fractured quartz is dynamically recrystallised though to lesser degree than in the middle of the zone and long axes of most of quartz grains are sub-parallel to C-plane. These evidences, combined with occurrence of secondary chlorite, biotite in these rocks suggests that mylonitization occurred at greenschist grade and deformation in medium shear strain in comparison to middle zone. At the base and upper parts of the mylonite zone feldspar are fractured and altered to Sericite and quartz shows limited plastic deformation in the form of undulose extinction and subgrain formation. Quartz grains have no preferred orientation indicating low shear strain in this zone.

Keywords

Structural Geology, Extensional Tectonics, Microstructural Studies, Deformation Mechanism, Chiplakot Crystalline Belt (CCB), Kumaon, NW-Himalaya.

Full Text

     

R. C. Patel ¹, Vikas Adlakha ¹, Paramjeet Singh ¹, Yogesh Kumar ², Nand Lal ¹

Affiliations
1 Department of Geophysics, Kurukshetra University, Kurukshetra - 136 119, IN
2 RITES, New Delhi, IN

Abstract

The crystallines in the Kumaon Himalaya, India are studied along Goriganga, Darma and Kaliganga valleys and found to be composed of two high-grade metamorphic gneiss sheets i.e. the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Crystalline (LHC) zones. These were tectonically extruded as a consequence of the southward directed propagation of crustal deformation in the Indian plate margin. The HHC and its cover rocks i.e. the Tethyan Sedimentary Zone (TSZ) are exposed through tectonic zones within the hinterland of Kumaon Himalaya. The HHC records history of at least one episode of pre-Himalayan deformation (D₁), three episodes of Himalayan deformation (D₂, D₃, D₄). The rocks of the HHC in Kumaon Himalaya are thoroughly transposed by D₂ deformation into NW-SE trending S_m (S₁+S₂). The extent of transposition and a well-developed NE-plunging L₂ lineation indicate intense strain during D₂ throughout the studied portion of the HHC. Ductile flow continued, resulting in rotation of F₁ and F₂ folds due NE-direction and NW-SE plunging F₃ folds within the HHC. The over thickened crystalline was finally, superimposed by late-to-post collisional brittle-ductile deformation (D₄) and exposed the rocks to rapid erosion.

Apatite Fission Track (AFT) and Zircon Fission Track (ZFT) studies from the Kumaon Himalaya suggest reactivation of the Main Central/Munsiari Thrust (MCT/MT) and Vaikrita Thrust (VT), rapid exhumation and a system that has been in topographic and exhumation steady-state since at least ₄ Ma.

Keywords

Higher Himalayan Crystallines, Deformation, Exhumation, Kumaon Himalaya.

Full Text