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Numerical study of continental collision: influence of buoyancy forces and an initial stiff inclusion
Author(s) -
Vilotte J. P.,
Madariaga R.,
Daignières M.,
Zienkiewicz O.
Publication year - 1986
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1986.tb04357.x
Subject(s) - geology , buoyancy , collision , crust , mantle (geology) , plate tectonics , context (archaeology) , wavelength , thickening , deformation (meteorology) , inclusion (mineral) , seismology , geophysics , mechanics , physics , tectonics , mineralogy , optics , materials science , paleontology , oceanography , computer security , polymer science , computer science
Summary. We study simple models of collision between two continental plates patterned after the geometry of the India–Asia collision, but which may be applicable to more general situations. The collision is modelled by the indentation of a viscoplastic plate (Asia) by a rigid punch (India). Although deformation is three‐dimensional in nature, a plate thickness of the order of 100 km permits a 2‐D approach for wavelengths longer than a few hundred kilometres. We assume an Airy compensation at the crust–mantle boundary throughout the deformation. We can then reduce the problem to a 2‐D one within the context of thin plate theory. This model makes it possible to study crustal thickening during collisions. The opposition between buoyancy forces, which tend to flatten the topography, and viscous resistance, appears clearly and we show the existence of a relaxation mechanism associated with the volumetric part of the deformation. The asymptotic behaviour of the model is of the incompressible plane strain type and thus crustal thickening tends to a maximum value. When this limit is reached no more thickening is possible and distensive phenomena can be induced. Buoyancy effects can be described by a non‐dimensional parameter, the Argand number Ar, which is the ratio between the relaxation time and the characteristic time of the viscous flow. A finite element approximation, based on a material description of the finite deformation, is used to investigate the evolution of the indentation process. The role of pre‐existing heterogeneity (stiff inclusion) is also considered. Lateral boundary conditions on the plate are chosen to approximate the boundary interactions that are believed to apply for the Asian plate, i.e. fixed boundaries to the east and the north and free boundaries to the west and the south. For an homogeneous lithosphere the main influence of the buoyancy forces is to flatten the topography and to amplify a strong rotational extrusion process. Local distension can be induced but never in front of the indenter. Crustal thickening remains slight and smooth. The presence of lithospheric heterogeneity perturbs the deformation pattern. Strong shear zones appear along the edges of the inclusions and control an intense extrusion process. Important rotational effects are observed during the extrusion as attested by a rigid rotation of the inclusion itself. The inclusion appears as a depression bounded by two thickened zones: one in front of the punch where the thickening is of the order of 100 per cent and another one in the northern part with a thickening of the order of 40–50 per cent. The importance of lithospheric heterogeneity is clear when we compare numerical results with available elevation data in Asia.

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