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Lithospheric thinning and localization of deformation during Rayleigh‐Taylor instability with nonlinear rheology and implications for intracontinental magmatism
Author(s) -
Harig Christopher,
Molnar Peter,
Houseman Gregory A.
Publication year - 2010
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2009jb006422
Subject(s) - geology , lithosphere , asthenosphere , magmatism , geophysics , downwelling , rayleigh–taylor instability , subduction , shear thinning , instability , upwelling , seismology , mechanics , viscosity , tectonics , physics , oceanography , quantum mechanics
Thinning of mantle lithosphere due to Rayleigh‐Taylor instability can be a mechanism for triggering continental magmatism near active or recently active plate boundaries. We consider whether it is also plausible as a mechanism for intracontinental magmatism, several hundred kilometers from active subduction or rifting. We perform two‐dimensional Rayleigh‐Taylor experiments and find that a shear stress‐free top and non‐Newtonian flow permit two types of instability to develop, largely dependent on how the viscosity coefficient varies with depth. For small variation with depth, with the e ‐folding depth scale (the interval across which the coefficient changes by a factor of e ) greater than a third to a half of the thickness of the unstable layer, deformation concentrates at the ends of the layer in localized thinning and thickening zones; the middle part moves horizontally toward the region of thickening as a coherent block undergoing minimal strain. When the viscosity coefficient decreases more rapidly with depth, thinning of the layer is distributed laterally over a wide zone. Between the regions of thickening and thinning, shear strain and vertical gradients in horizontal velocity prevent this area from moving as a coherent block. The rheological exponent, n , that relates strain rate to stress in the constitutive equation controls the degree of localization of the downwelling and upwelling: the width varies as ≈ n −1/2 . In intraplate settings where a shear stress‐free top condition could be applicable, high‐stress crystalline plasticity could provide a mechanism for the narrow zones of thinning and upwelling, which would facilitate decompression related volcanism.

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