Conference Presentations by PRITAM GHOSH
As a dominant thrust sheet of the Himalayan fold thrust belt (FTB), the Main Central thrust (MCT)... more As a dominant thrust sheet of the Himalayan fold thrust belt (FTB), the Main Central thrust (MCT) records a long deformation history along it. In order to examine how the deformation conditions varied spatially and temporally as the fault progressed through time we have initiated a study to examine the fault zones from three exposed structural positions, along its transport direction, in the eastern Sikkim Himalayan FTB. In this region, the MCT system is composed of a structurally higher MCT1 and a lower MCT2 sheet, and is folded by the growth of the Lesser Himalayan duplex into a pair of antiform and synform. The three studied MCT2 zones are from the northern limb of the antiform (south of Mangan), northern limb of the synform (Algarah), and the forelimb of the synform near the foreland at Gorubathan.
We evaluate the operative deformation mechanisms along two dominant thrust faults, from similar s... more We evaluate the operative deformation mechanisms along two dominant thrust faults, from similar structural positions, in the core of the Sikkim Himalayan fold thrust belt (FTB): the MCT and the Pelling thrust (PT). The hanging wall rocks are quartzo-feldspathic gneiss for both MCT and PT that define a strong mylonitic foliation within the fault zone. The amphibolite facies MCT zone is ~1170m thick, while greenschist facies PT zone is ~938m thick. The mean MCT mylonitic foliation is 41̊, 057 with a mean pucker axis 38°, 115° and a mineral lineation of 15̊, 030̊. The PT mylonitic foliation is 32̊,020° , pucker axis is 22̊, 013̊, and a streching mineral lineation of 25̊, 016̊.
Grain-size reduction is higher in PT ~(82%) than in MCT ~(57%). In both the fault zones, ~60%-70% quartz is dominantly deformed by dislocation creep. However, percentages of quartz deformed by microfracturing is higher at MCT (20%-30%) than in PT(15%-20%). Similarly, microfracturing is more prominent for feldspar at MCT ~(50%-75%) than at PT ~(40%-55%). Pressure solution is notably higher in PT ~(17%-20%) than in MCT ~(1%-3%). Asymetric porphyroclasts of feldspar suggest consistent top-to-south shearing within both the fault zones; this observation agrees with regional transport direction.
Plastic strain of deformed quartz grains record stronger flattening in MCT(k=0.03) than in PT(k=0.20). Rigid grain vorticity analysis indicates ~58% - 72% pure shear and ~28% - 42% simple shear in MCT and ~43% - 70% pure shear and ~30% - 58% simple shear in PT.
M.Sc. Thesis by PRITAM GHOSH
Sandbox analogue models were used to study the interaction of folding and faulting in orogenic we... more Sandbox analogue models were used to study the interaction of folding and faulting in orogenic wedges. Orogenic wedges grow largely through sequential thrusting and sub-sequent motion away from the plate boundary.
An orogenic wedge, as modeled here, dominantly consists of sediments accreted against the folding, fore-thrust and back-thrust, which belong to the accretionary prism. Non-cohesive dry sand was used in experiments as a deforming material, which is conventionally used for brittle crustal deformation for the proper scaling of the model to the nature. In a series of experiments with red sand and mica, orogenic wedge showed four different modes of deformation, depending on the different experimental set-up and condition.
The four types of experiment show that, (a) the formation of folding or faulting at the initial stage of deformation in orogenic wedges, extremely dominated by the experimental environment and the rheological behavior of materials. (b) In all types of experiment, thrusts were initiated (from folding) in between the area of hinge zone and inflection points of fore-limbs of nearly upright non-plunging folds. (c) Experimental results shows that wedge height and taper angle depend on the initial thickness of the bed. (d) Thrust spacing increases with increase in initial bed thickness. (e) Formation of folds and thrusts, and their propagation appear to depend mostly on how stresses work and propagate through the model and how material strength varies with in the model. The main stresses at play are born at the moving piston (horizontal tectonic force), from thickening and isostatic compensation in the model (gravitational forces), and from elastic resistance to buckling.
Papers by PRITAM GHOSH
Examining microstructural and kinematic evolution of dominant thrust faults from hinterland of th... more Examining microstructural and kinematic evolution of dominant thrust faults from hinterland of the Sikkim Himalayan FTB: Insights into orogenic wedge deformation PRITAM GHOSH*, KATHAKALI BHATTACHARYYA Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata (IISER K), Mohanpur-741246, West Bengal *Email: [email protected] We evaluate the structural evolution of two dominant thrust faults, with mineralogically similar protoliths, from similar structural positions in the hinterland in a deforming orogenic wedge. We focus our study on the hinterland-most exposures of Main Central thrust (MCT) at Mayangchu and the Pelling thrust (PT) at Mangan in the Sikkim Himalayan fold thrust belt (FTB) (Fig. 1). Both the exposures lie on the northern limb of the folded MCT-PT antiform in the hinterland-most part of the wedge (Fig. 1). Both the MCT and the PT are folded in an E-W trending antiform-synform pair, by the growth of the underlying Lesser Himalayan duplex (Bhattacharyya and Mitra, 2009, Mitra and Bhattacharyya, 2011). We quantify the various operative deformation mechanisms and strain recorded in these two fault zones and evaluate a comparative study on their structural evolution. The coarse-grained, quartzo-feldspathic gneissic protoliths transform into quartz-mica mylonite zone defining both the fault zones. The thicknesses of the amphibolite facies MCT mylonite zone and greenschist facies PT zone are somewhat similar in the hinterland at~1170m.and at ~938m., respectively. However, in the foreland most part, the same fault zones become narrower with the PT 158m thick (Bhattacharyya and Ghosh, 2014) suggesting that the deformation conditions spatially varied along the fault zones (Bhattacharyya and Mitra, 2014). The mylonitic foliation is dominantly defined by preferential orientations of recrystallized quartz along with muscovite, biotite and chlorite. The mean MCT mylonitic foliation at Mayangchu is 41°, 057° with a mean stretching lineation defined by stretched biotite and sillimanite at 15°, 030°; the latter also defines the regional transport direction. The easterly plunging pucker axis lineation (18°, 078°) indicates east-west trending fault bend fold and N-S plunging pucker axis lineations (28°, 203°) are associated with doubly plunging structure of the underlying Lesser Himalayan duplex (Bhattacharyya and Mitra, 2009, Mitra and Bhattacharyya, 2011). The average PT mylonitic foliation in Mangan is 32°,020° , pucker axis lineation is 22°, 013°, and a mean stretching mineral lineation defined by stretched biotite and chlorite is 25°, 016°(Fig. 1). The pyroxene-amphibole bearing biotite rich quartz-feldspathic gneissic protolith has undergone ~ 70% grain size reduction in quartz and ~55% reduction in feldspar within the MCT zone. Similarly biotite rich coarse grained quartzo feldspathic protolith records ~85% grain size reduction in quartz and ~70% in feldspar within the PT mylonite zone. Quartz has undergone grain size reduction dominantly by dislocation creep and feldspar by microfracturing. Interestingly, microfracturing is more dominant in MCT zone than in PT for both quartz and feldspar grains. Additionally, pressure solution is significantly higher in PT ~ (17%-20%) than in MCT ~ (1%-3%). Therefore, there is a spatial variation in deformation mechanisms in the MCT and PT zones in the study areas. Shear-sense analysis on asymmetric feldspar porphyroclasts (~70%-85% porphyroclasts) record top-to-the south shearing from both the zones which agrees with the regional transport direction. Based on recrystallized quartz grain sizes (Stipp et. al., 2002) ), the temperatures at MCT and PT are estimated at 435° - 510°C and 405° - 425°C, respectively. To quantify the strain partitioning between deformed quartz and feldspar grains, we estimated plastic strain from relict quartz and feldspar grains (Bhattacharyya et. al., 2015). Based on strain analysis, both the MCT and PT zones record strong flattening strain. Penetrative strain from deformed quartz grains and relict feldspar grains record stronger flattening in MCT zone than in PT fault zone. We propose that pressure solution and microfracturing have accommodated a significant proportion of strain in the fault rocks, thereby under-representation of the finite plastic strain from the strain markers We have initiated vorticity analyses on the fault rocks using both porphyroclasts (Passchier, 1987; Wallis et al, 1993) and subgrain ( Das et al., 2014) methods. Based on porphyroclast analysis (Wk), the MCT and PT record ~28% - ~42% simple shear and ~33% - ~57% simple shear, respectively. Subgrain method (Wm) indicates ~42% - ~65% simple shear in MCT and ~46% - ~73% simple shear in PT. Therefore, at a first order, both the fault zones record non-steady deformation. References: Bhattacharyya, K., Dwivedi, H.V., Das, J.P., Damania, S., 2015. Structural Geometry, microstructural and strain analyses of L-tectonites from…
Journal of Structural Geology, 2020
Abstract We compare and contrast the structural evolution of two successive internal thrusts, the... more Abstract We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex; the PT is its roof thrust. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hangingwall MCT sheet that records one additional cleavage possibly indicating time-transgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The Rs-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. Rs-θ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher Rxz, and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated general-shear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT.
Geological Society of America Abstracts with Programs, 2018
Journal Of Structural Geology, 2020
We compare and contrast the structural evolution of two successive internal thrusts, the Main Cen... more We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex; the PT is its roof thrust. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hangingwall MCT sheet that records one additional cleavage possibly indicating timetransgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The R s-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. R s-θ ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher R xz , and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated generalshear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT.
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB... more Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (Wk) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (~56-96%) that progressively became simple-shear dominated (~29-54%), as recorded by the oblique fabric and sub-grains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen. The L-tectonites record ~30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB... more Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (W k) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (~56e96%) that progressively became simple-shear dominated (~29e54%), as recorded by the oblique fabric and sub-grains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen. The L-tectonites record ~30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.
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Conference Presentations by PRITAM GHOSH
Grain-size reduction is higher in PT ~(82%) than in MCT ~(57%). In both the fault zones, ~60%-70% quartz is dominantly deformed by dislocation creep. However, percentages of quartz deformed by microfracturing is higher at MCT (20%-30%) than in PT(15%-20%). Similarly, microfracturing is more prominent for feldspar at MCT ~(50%-75%) than at PT ~(40%-55%). Pressure solution is notably higher in PT ~(17%-20%) than in MCT ~(1%-3%). Asymetric porphyroclasts of feldspar suggest consistent top-to-south shearing within both the fault zones; this observation agrees with regional transport direction.
Plastic strain of deformed quartz grains record stronger flattening in MCT(k=0.03) than in PT(k=0.20). Rigid grain vorticity analysis indicates ~58% - 72% pure shear and ~28% - 42% simple shear in MCT and ~43% - 70% pure shear and ~30% - 58% simple shear in PT.
M.Sc. Thesis by PRITAM GHOSH
An orogenic wedge, as modeled here, dominantly consists of sediments accreted against the folding, fore-thrust and back-thrust, which belong to the accretionary prism. Non-cohesive dry sand was used in experiments as a deforming material, which is conventionally used for brittle crustal deformation for the proper scaling of the model to the nature. In a series of experiments with red sand and mica, orogenic wedge showed four different modes of deformation, depending on the different experimental set-up and condition.
The four types of experiment show that, (a) the formation of folding or faulting at the initial stage of deformation in orogenic wedges, extremely dominated by the experimental environment and the rheological behavior of materials. (b) In all types of experiment, thrusts were initiated (from folding) in between the area of hinge zone and inflection points of fore-limbs of nearly upright non-plunging folds. (c) Experimental results shows that wedge height and taper angle depend on the initial thickness of the bed. (d) Thrust spacing increases with increase in initial bed thickness. (e) Formation of folds and thrusts, and their propagation appear to depend mostly on how stresses work and propagate through the model and how material strength varies with in the model. The main stresses at play are born at the moving piston (horizontal tectonic force), from thickening and isostatic compensation in the model (gravitational forces), and from elastic resistance to buckling.
Papers by PRITAM GHOSH
Grain-size reduction is higher in PT ~(82%) than in MCT ~(57%). In both the fault zones, ~60%-70% quartz is dominantly deformed by dislocation creep. However, percentages of quartz deformed by microfracturing is higher at MCT (20%-30%) than in PT(15%-20%). Similarly, microfracturing is more prominent for feldspar at MCT ~(50%-75%) than at PT ~(40%-55%). Pressure solution is notably higher in PT ~(17%-20%) than in MCT ~(1%-3%). Asymetric porphyroclasts of feldspar suggest consistent top-to-south shearing within both the fault zones; this observation agrees with regional transport direction.
Plastic strain of deformed quartz grains record stronger flattening in MCT(k=0.03) than in PT(k=0.20). Rigid grain vorticity analysis indicates ~58% - 72% pure shear and ~28% - 42% simple shear in MCT and ~43% - 70% pure shear and ~30% - 58% simple shear in PT.
An orogenic wedge, as modeled here, dominantly consists of sediments accreted against the folding, fore-thrust and back-thrust, which belong to the accretionary prism. Non-cohesive dry sand was used in experiments as a deforming material, which is conventionally used for brittle crustal deformation for the proper scaling of the model to the nature. In a series of experiments with red sand and mica, orogenic wedge showed four different modes of deformation, depending on the different experimental set-up and condition.
The four types of experiment show that, (a) the formation of folding or faulting at the initial stage of deformation in orogenic wedges, extremely dominated by the experimental environment and the rheological behavior of materials. (b) In all types of experiment, thrusts were initiated (from folding) in between the area of hinge zone and inflection points of fore-limbs of nearly upright non-plunging folds. (c) Experimental results shows that wedge height and taper angle depend on the initial thickness of the bed. (d) Thrust spacing increases with increase in initial bed thickness. (e) Formation of folds and thrusts, and their propagation appear to depend mostly on how stresses work and propagate through the model and how material strength varies with in the model. The main stresses at play are born at the moving piston (horizontal tectonic force), from thickening and isostatic compensation in the model (gravitational forces), and from elastic resistance to buckling.