Higher Himalayan Shear Zone, Sutlej section: structural geology and extrusion mechanism by various combinations of simple shear, pure shear and channel flow in shifting modes

The Higher Himalayan Shear Zone (HHSZ) in the Sutlej section reveals (1) top-to-SW ductile shearing, (2) top-to-NE ductile shearing in the upper- and the lower strands of the South Tibetan Detachment System (STDS_U, STDS_L), and (3) top-to-SW brittle shearing corroborated by trapezoid-shaped minerals in micro-scale. In the proposed extrusion model of the HHSZ, the E₁-phase during 25–19 Ma is marked by simple shearing of the upper sub-channel defined by the upper strand of the Main Central Thrust (MCT_U) and the top of STDS_U as the lower- and the upper boundaries, respectively. Subsequently, the E_2a-pulse during 15–14 Ma was characterized by simple shear, pure shear, and channel flow of the entire HHSZ. Finally, the E_2b-pulse during 14–12 Ma observed simple shearing and channel flow of the lower sub-channel defined by the lower strand of the Main Central Thrust (MCT_L) and the top of the STDS_L as the lower- and the upper boundaries, respectively. The model explains the constraints of thicknesses of the STDS_U and the STDS_L along with spatially variable extrusion rate and the inverted metamorphism of the HHSZ. The model predicts (1) shear strain after ductile extrusion to be maximum at the boundaries of the HHSZ, which crudely matches with the existing data. The other speculations that cannot be checked are (2) uniform shear strain from the MCT_U to the top of the HHSZ in the E₁-phase; (3) fastest rates of extrusion of the lower boundaries of the STDS_U and the STDS_L during the E_2a- and E_2b-pulses, respectively; and (4) variable thickness of the STDS_L and rare absence of the STDS_U. Non-parabolic shear fabrics of the HHSZ possibly indicate heterogeneous strain. The top-to-SW brittle shearing around 12 Ma augmented the ductile extruded rocks to arrive a shallower depth. The brittle–ductile extension leading to boudinage possibly did not enhance the extrusion.     

Locating center of gravity in geological contexts

International Journal of Earth Sciences - This short article at first deduces center of gravity (C.G.) for horizontal single-layered and multi-layered rock bodies. Spatial variation of density is...     

Higher Himalaya in the Bhagirathi section (NW Himalaya, India): its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms

The Higher Himalayan Crystalline (HHC) in the Bhagirathi river section (India) on fieldwork reveals two extensional ductile top-to-N/NE shear sub-zones—the ‘South Tibetan Detachment System’ and the ‘Basal Detachment’—besides a preceding top-to-S/SW ductile shear. A top-to-N/NE brittle shear was identified as backthrusts from the HHC (except its northern portion) that occur repeatedly adjacent to numerous top-to-S/SW brittle shears as fore-thrusts. The northern portion of the HHC—the Gangotri Granite—exhibits infrequent total six extensional and compressional brittle shear senses. The backthrusts could be due to a low friction between the lower boundary of the HHC (i.e. the Main Central Thrust-Zone) and the partially molten hot rock materials of the HHC. Subduction of the Eurasian plate towards S/SW below the Indian plate more extensively in the Garhwal sector could be the second possible reason. Presence of two ductile extensional shear sub-zones may indicate channel flow (or several exhumation mechanisms) of the HHC in a shifting mode (similar to Mukherjee et al. in Int J Earth Sci 101:253–272, 2012). The top-to-S/SW extensional brittle shear exclusively within the upper (northern portion) of the HHC and a top-to-S/SW brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism. Thus, this work provides the field evidences of possibly both channel flow and critical taper conditions from a Higher Himalayan section, besides that by Larson et al. (Geol Soc Am Bull 122:1116–1134, 2010).     

Analyses of fold profiles by changing weight parameters of NURB curves

Analyses of Non-Uniform Rational B-spline (NURB) curve by varying weights at its nodal points and projection ratio produce several kinetically plausible symmetric and asymmetric fold morphologies in 2D promptly and efficiently with varied overall geometries, curvature of limbs, sharpness/bluntness of hinges, extent of hinge zone, tightness/interlimb angles, etc. Some of these folds are new geometries what other approaches, such as those with Bézier curve, did not produce so far. Natural fold profiles can be matched with NURB curves from photographs.     

Airy’s isostatic model: a proposal for a realistic case

This work derives algebraic expressions for Airy-type isostatic equilibrium and disequilibrium of lithospheric blocks within asthenosphere, considering a more realistic case of continuously varying densities of the lithosphere along mutually perpendicular directions. Isostatic (dis)equilibrium must be linked to the porosity and density of the rock matrix, and to pore fluid density since these factors govern the bulk density of wet sediments/porous rocks. Two combinations of exponential and linear increase of density with depth are demonstrated. Isostatic disequilibrium would speed up sinking/uplift of the lithospheric blocks. Given any other (empirical) equations for density variation in three perpendicular directions, isostatic (dis)equilibrium such as Eqs. 14, 17, 18, 28, and 29 can be theorized as well.     

Kinematics of horizontal simple shear zones of concentric arcs (Taylor–Couette flow) with incompressible Newtonian rheology

We present preliminary kinematic analyses of Taylor–Couette flow. We consider deformation of a Newtonian incompressible ductile material inside rotating horizontal listric (concentric circular) boundaries. The velocity profile is curved indicating non-uniform shear strain but leads to the same shear sense. Each material point on progressive shear keeps increasing shear strain linearly with time. A curve of no movement, the ‘neutral curve’, may exist inside the shear zone. Irregular geometries of initially regular markers and their individual non-matching strain paths indicate inhomogeneous deformaion in such Taylor–Couette flow.     

Structures and morphotectonic evolution of the frontal fold–thrust belt,Kameng river section,Arunachal Himalaya,India

The Neogene–Quaternary Siwalik foreland fold and thrust belt is studied for better understanding of tectonics along the Kameng river section of Arunachal Pradesh, India. The Kimi, Dafla, Subansiri, and the Kimin Formation correspond to Lower, Middle and Upper Siwaliks, respectively. The lithology in the foreland basin is dominantly sandstones, siltstones, claystones, carbonaceous shales, and boulder beds in the upper part. The structural style of the sedimentary sequence from the Main Boundary Thrust southward shows first order ramp-flat geometry. The brittle shear transfers slip across glide horizons to shallower depth. Repeated splay generations from a major regional-scale floor transfers slip from one glide horizon to another that shortens and thickens the crust. In the micro-scale, the lithological response in the structural development is well documented as pressure solution seams and other diagenetic deformation signatures. The basement asperity plays a significant role as the moving thrust front produced a major lateral ramp. The differential movement of the mountain front on both sides of the ramp is decipherable. This is especially true at the western part of the SE flowing Kameng river. The tectonic evolution of the area initiated with slip along the MBT \(\sim \)11 Ma ago along with the deposition of the Siwalik sediments. With southward propagation of the mountain front, the foreland basin shifted towards S, produced splay thrusts from the Himalayan Frontal Thrust-1 (HFT-1), which has been uplifting the Kimin and the older terraces.     

Estimating the viscosity and Prandtl number of the Tso Morari crystalline gneiss dome, Indian western Himalaya

The Tso Morari crystalline (TMC) gneiss dome in the Indian Himalaya extruded from a depth of?~120?km through an inclined subduction channel of sub-elliptical cross-section at the leading edge of the Indian plate. The velocity profile of this gneiss dome is derived after (1) presuming its incompressible Newtonian rheology, (2) finding the “best fit” of the outcrop of the gneiss dome to an ellipse, (3) taking into account different lithologies to have existed at the top of the extruding gneiss body, (4) considering the extrusion to have been driven by the buoyant push of the denser mantle beneath the lighter gneiss, and (5) assigning a range of plausible densities for different litho-units. Fitting the known rates of extrusion—from a few centimetres up to about one-hundredth of a millimetre per year—from?~53?Ma onwards of this gneiss dome to its velocity profile constrains its maximum possible viscosity to?~7.5?×?10²² Pa?s. This magnitude is?10²–10⁴ times higher than previous estimates for gneisses and granites. Alternative explanations of our data are the following: (1) There was a fall in extrusion rates of the TMC gneiss from 53?to?<30?Ma because of an increase in the estimated maximum viscosity from 6.2?×?10²⁰ to 7.5?×?10²² Pa?s, possibly indicating a fall in temperature and/or compositional change of the TMC gneiss. (2) Lower the extrusion rates, higher are the estimated viscosities. (3) The TMC gneiss was more viscous probably due to its eclogite content. (4) The estimated maximum viscosity is?~10² times higher than that in collision zones and?10²–10⁴ times than that in the Tibetan lower crust, but broadly conforms to that for the crustal channel, and average lithospheric and asthenospheric values. The high magnitude of maximum possible Prandtl number of?~10²⁸ of the TMC gneiss might be related to isothermal decompression of the gneiss during its extrusion.