FROM LITHOSPHERIC THICKENING AND DIVERGENT COLLAPSE TO ACTIVE CONTINENTAL RIFTING

P.F REY*

*School of GeoSciences, The University of Sydney, Edgeworth Building F05, NSW 2006, Australia

In  Miller, J.A., Buick, I.S., Hand, M., and Holdsworth, R.E., (eds).Continental Reworking and Reactivation, Geological Society of London Special Publication, Nº184, pp. 372-398. (1996).

ABSTRACT

The Aegean Sea, the Alboran Sea, and the Basin and Range Province suggest that continental lithosphere following gravitational collapse may end up being thinner than it was before convergence and thickening. In order to assess the condition leading to the development of finite lithosphere thinning following convergence and convective thinning, the strength of the continental lithosphere, the gravitational force, and the rate of gravity-driven flow (spreading rate) are calculated during and after continental collision. One dimensional numerical experiments, presented here, assume that the deformation is homogeneous, that erosion is a function of strain rate and elevation and that thermal relaxation involves no lateral conduction of heat. Results show that if 43% of the lower lithospheric mantle is dragged into the convective mantle (convective thinning), gravitational collapse may lead to a lithosphere thinner than the initial lithosphere (pre-thickening lithosphere), provided that gravitational collapse is accommodated by the passive displacement of the surrounding lithosphere (free boundary collapse). When a slightly larger volume of lithospheric mantle is removed, a phase of extension leading to a necking instability and the formation of an active rift follows collapse. The presence of fixed boundaries and/or horizontal compressive stresses strongly reduces the spreading rate and opposes finite lithosphere thinning and therefore active rifting. It is suggested that back-arc extension occurring in continental settings could exemplify post-collapse active rifting.

ACKNOWLEDGMENTS

This paper has benefited from helpful and constructive reviews from O. Vanderhaeghe and P. Ryan. Many thanks to G. Houseman, S. Costa, M. Jessell, and W. Schellart, for countless conversations and arguments. Thanks to S. Van Huet for having improved the english, and J. Miller for editing the text. This work was supported by ARC Large Grant Nº: A10017138..

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