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

Nibir Mandal ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN

Abstract

No Abstract.

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Nishant Kumar ¹, Shamik Sarkar ¹, Nibir Mandal ¹

Affiliations
1 Indian Institute of Science Education and Research-Kolkata, FC-6, Salt Lake, Sector III, Kolkata-700 106, IN

Abstract

This paper presents results obtained from numerical model experiments to show different patterns of mantle flow produced by lithospheric movement in subduction zones. Using finite element models, based on Maxwell rheology (relaxation time ∼ 10¹¹S), we performed three types of experiments: Type 1, Type 2 and Type 3. In Type 1 experiments, the lithospheric slab subducts into the mantle by translational movement, maintaining a constant subduction angle. The experimental results show that the flow perturbations occur in the form of vortices in the mantle wedge, irrespective of subduction rate and angle. The mantle wedge vortex is coupled with another vortex below the subducting plate, which tends to be more conspicuous with decreasing subduction rate. Type 2 experiments take into account a flexural deformation of the plate, and reveal its effect on the flow patterns. The flexural motion induces a flow in the form of spiral pattern at the slab edge. Density-controlled lithospheric flexural motion produces a secondary flow convergence zone beneath the overriding plate. In many convergent zones the subducting lithospheric plate undergoes detachment, and moves down into the mantle freely. Type 3 experiments demonstrate flow perturbations resulting from such slab detachments. Using three-dimensional models we analyze lithospheric stresses in convergent zone, and map the belts of horizontal compression and tension as a function of subduction angle.

Keywords

Plate Tectonics, Convergent Zones, Subduction, Maxwell Rheology, Flow Perturbations.

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Santanu Bose ¹, Nibir Mandal ²

Affiliations
1 Experimental Tectonics Laboratory, Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata - 700 019, IN
2 Indian Institute of Science Education and Research, HC 7, Salt Lake City, Kolkata - 700 106, IN

Abstract

This paper investigates the evolution of thrust wedges with concomitant surface erosion, and its bearing on the exhumation processes in orogenic belts. We performed sandbox experiments, simulating syn-orogenic erosion on forelandward sloping surfaces (∼4^o). Experiments show that the erosion process has a significant control on the progression of frontal thrusts. In case of no-erosion condition, wedges with high basal friction develop frontal thrusts with strongly increasing spacing. In contrast, for the same basal friction the thrusts show uniform spacing as the wedge development involves concomitant surface erosion. On the other hand, the erosion promotes reactivation of hinterland thrusts in wedges with low basal friction. We show that erosion-assisted thrust reactivation is the principal mechanism for exhumation of deeper level materials in orogens. Efficiency of this mechanism is largely controlled by basal friction. The exhumation of deeper level materials is limited, and occurs within a narrow, sub-vertical zone in the extreme hinterland when the basal friction is high (μ_b = 0.46). In contrast, the process is quite effective in wedges with low basal friction (μ_b =0.36), resulting in exhumation along gently dipping foreland-vergent thrusts as well as along thrusts, subsequently rotated into steep attitude. The zone of exhumation also shifts in the foreland direction in the course of horizontal movement. Consequently, deeper level materials cover a large area of the elevated part of the wedge.

Keywords

Orogenic Wedge, Thrusting, Sandbox Experiments, Basal Friction, Reactivation.

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