Some isostatic and thermal consequences of the vertical strain geometry in convergent orogens

Details Some isostatic and thermal consequences of the vertical strain geometry in convergent orogens Some isostatic and thermal consequences of the vertical strain geometry in convergent orogens

May 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990e%26psl..98..154s&link_type=abstract

Earth and Planetary Science Letters, Volume 98, Issue 2, p. 154-165.

Physics

34

Scientific paper

The thermal and isostatic consequences of continental deformation, particularly surface elevation, horizontal buoyancy forces and metamorphism in the crust, depend to a considerable extent on the way in which strain is distributed between the crust and the mantle lithosphere [1]. Since these vertical strains may be strongly ``decoupled'' by processes such as convective thinning or detachment of the lower thermal boundary layer [2], with the extent of this ``decoupling'' varying in space and time in an orogen [3], it is useful to parameterise the vertical strain on the scale of the crust, ƒ;c, and lithosphere, ƒ;1, respectively. We illustrate this parameterisation by considering the thermal and isostatic consequences of a number of ƒ;c-ƒ;1 paths which may develop during the convergent deformation of a thermally-stabilised lithosphere. Three qualitatively distinct ƒ;c-ƒ;1 paths in orogens are distinguished on the extent of ``decoupling'' between vertical strains in the crust and mantle lithosphere. Type 1 paths involve no decoupling, so the vertical strain in the lithosphere is homogeneous. For a given ƒ;c, Type 1 paths result in relatively low surface elevation and low-intermediate metamorphic temperatures appropriate for the development of glaucophane-schist and Barrovian metamorphic assemblages. Type 2 paths allow decoupling due to processes such as convective thinning of the mantle lithosphere on the orogenic timescale but only after considerable initial lithospheric thickening. Type 2 paths lead to relatively high surface elevations, for a given ƒ;c, and intermediate to high metamorphic temperatures only after the mantle lithosphere is thinned. The increase in potential energy associated with mantle lithospheric thinning (resulting in an increase in buoyancy forces by as much as 1013 Nm-1) will quickly terminate convergent deformation, with the result that metamorphic heating will postdate the thickening deformation, and may induce extensional collapse of the orogen. Type 3 paths involve efficient mantle lithosphere thinning as the crust thickens with a consequence being, particularly, the possibility of relatively high metamorphic temperatures during active crustal thickening. For a given driving force, convergent deformation along a Type 3 path will be terminated at relatively small ƒ;c providing an appropriate setting for the development of high T -low P metamorphic assemblages.

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