Biraja Das | Banaras Hindu University, Varanasi (original) (raw)
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Papers by Biraja Das
Journal of Asian Earth Sciences, 2019
Geometry and kinematics of a hitherto ignored crustal scale tectonic discontinuity from the westc... more Geometry and kinematics of a hitherto ignored crustal scale tectonic discontinuity from the westcentral sector of the Himalaya, viz. the North Almora Fault (NAF) is discussed in this paper. Choice of the tectonic model for response of the Himalayan-South Tibetan ensemble to the Tertiary Himalayan continental collision is largely dictated inter alia by the geometry and kinematics of the large orogen parallel strike slip faults in the region. Based on field, laboratory and satellite imagery data, it is concluded that the North Almora Fault/ Thrust is not one but two closely parallel tectonic discontinuities viz. (i) the North Almora Fault, a steep south dipping high angle brittle fault with a dominant east slip and subordinate dip slip component and (ii) the North Almora Thrust, a low angle south dipping thrust marking the northern boundary of the large Almora-Jajarkot Nappe. The >500km long dextral strike slip North Almora Fault mimicking the ∼800 km long Karakoram Fault is responsible for significant strain partitioning in the west-central Lesser Himalaya. The North Almora Fault assumes special salience for the Himalayan tectonics as it partitions sizable strains in addition to the Karakoram Fault in the region and lends support to the oblique convergence model that best describes the Himalayan extensional and strike slip deformation.
Acta Geologica Sinica - English Edition, 2019
The chemical and petrological correlation of metamorphic nappes and klippes overlying the Protero... more The chemical and petrological correlation of metamorphic nappes and klippes overlying the Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses, not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the Ramgarh granitic gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic gneisses is likely to have been controlled by feldspar crystallization in restites. During the anatexis at > 776°C temperature and >6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The Rb/Sr ratio ∼0.54 and Nb ∼10 ppm is consistent with the granodioritic composition. The negative Sr anomaly in the underlying Ramgarh granitic gneisses indicates a distinct mantle derived source/plagioclase fractionation with a notable correspondence to oth...
Journal of Earth System Science, Feb 15, 2023
Journal of Asian Earth Sciences, 2019
Geometry and kinematics of a hitherto ignored crustal scale tectonic discontinuity from the westc... more Geometry and kinematics of a hitherto ignored crustal scale tectonic discontinuity from the westcentral sector of the Himalaya, viz. the North Almora Fault (NAF) is discussed in this paper. Choice of the tectonic model for response of the Himalayan-South Tibetan ensemble to the Tertiary Himalayan continental collision is largely dictated inter alia by the geometry and kinematics of the large orogen parallel strike slip faults in the region. Based on field, laboratory and satellite imagery data, it is concluded that the North Almora Fault/ Thrust is not one but two closely parallel tectonic discontinuities viz. (i) the North Almora Fault, a steep south dipping high angle brittle fault with a dominant east slip and subordinate dip slip component and (ii) the North Almora Thrust, a low angle south dipping thrust marking the northern boundary of the large Almora-Jajarkot Nappe. The >500km long dextral strike slip North Almora Fault mimicking the ∼800 km long Karakoram Fault is responsible for significant strain partitioning in the west-central Lesser Himalaya. The North Almora Fault assumes special salience for the Himalayan tectonics as it partitions sizable strains in addition to the Karakoram Fault in the region and lends support to the oblique convergence model that best describes the Himalayan extensional and strike slip deformation.
Acta Geologica Sinica - English Edition, 2019
The chemical and petrological correlation of metamorphic nappes and klippes overlying the Protero... more The chemical and petrological correlation of metamorphic nappes and klippes overlying the Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses, not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the Ramgarh granitic gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic gneisses is likely to have been controlled by feldspar crystallization in restites. During the anatexis at > 776°C temperature and >6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The Rb/Sr ratio ∼0.54 and Nb ∼10 ppm is consistent with the granodioritic composition. The negative Sr anomaly in the underlying Ramgarh granitic gneisses indicates a distinct mantle derived source/plagioclase fractionation with a notable correspondence to oth...
Journal of Earth System Science, Feb 15, 2023