HR: 11:05h
AN: T52B-04 INVITED     [Abstracts]
TI: A Mechanism for Thinning the Continental Lithosphere at Magma-Poor Margins.
AU: * Lavier, L L
EM: luc@ig.utexas.edu
AF: Jackson School of Geosciences, University of Texas Institute for Geophysics, 4412 Spicewood Springs Road, #600, Austin, TX 78759 United States
AU: Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, Universit‚ Louis Pasteur, 1 rue Blessig, Strasbourg, 67084 France
AB: We develop a model for thinning the continental lithosphere during break-up in magma poor margins. This model is based on the integration of detailed geological observations into numerical experiments of lithospheric extension. We model three consecutive phases of deformation that are consistent with geological and geophysical observations. (1) A stretching phase during which deformation is distributed over the whole margin and extension is rooted in a weak quartzo-feldspathic middle crust. (2) A thinning phase where extension is controlled by a system of superimposed concave-downward faults that simultaneously exhume middle crust and upper mantle. The two rolling hinges eventually merge to form one concave downward fault. (3) An exhumation phase during which serpentinized mantle is exhumed to the seafloor along a downward-concave path. In that last phase serpentinization provides a mechanism to weaken the lithospheric mantle. In the first two phases we find that a progressive weakening of the continental crust is an essential requirement to reproduce observations. Crustal weakening is accomplished by the formation of an attenuated middle quartzo-felsdpathic crustal layer. Based on these numerical experiments, we propose a new mechanism for lithospheric thinning by the simultaneous middle crustal exhumation and lower crust/mantle thinning along concave downward faults. This results in the generation of new surfaces (exhumed detachment surface of the downward-concave faults). The lack of faulted prerift basement or pre-rift sediment in the area of continental break-up leaves us with no indicator of the amount of strain incurred during thinning. This process may explain previously not understood observations at other margins such as tectonic subsidence in the absence of observable deformation in the upper crust. This mechanism allows for thinning with little uplift of the rift flank since the elastic thickness of the lithosphere is reduced by decoupling. The inferred rheological evolution of the lithosphere shows that weakening is acting to minimize the force needed to bend the crust and upper mantle throughout the rift history. In the absence of magmatic activity to weaken the lithosphere, the numerical experiment point to this evolution as key mechanism to allow for continental break-up in an initially cold and strong lithosphere.
DE: 4255 Numerical modeling (0545, 0560)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: Fall Meeting 2005