HR: 11:45h
AN: T42A-05    [Abstracts]
TI: Modeling Polyphase Rifting in Magma Poor Margins
AU: * Lavier, L L
EM: luc@ig.utexas.edu
AF: University of Texas Institute for Ceophysics, 4412 Spicewood Springs Rd #600, Austin, TX 78704 United States
AU: Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, Universit‚ Louis Pasteur, 1 rue Blessig, Strasbourg, 67064 France
AB: In the past few years there has been an increase in the acquisition of higher-resolution data from passive continental margins and rift basins. This has been concurrent with the development of numerical techniques to dynamically model lithospheric deformation. These developments give us the tools to peer into the rheological evolution and the physical processes involved in controlling continental break-up. We present numerical experiments of lithospheric extension in which the initial thermal and lithological structure of the lithosphere is constrained by geological reconstructions of the Alpine Tethyan and Iberia/Newfounland conjugate margins. Geological reconstructions points to three consecutive phases or modes of deformations that spans initial break-up of the continent to the initiation of an ocean basin. (1) A stretching mode during which the deformation is distributed over several basins. The crust is initially 30 km thick and in each of these basins the rate of subsidence is greater than that the flank uplift. (2) A thinning mode during which the crust is thinned down to 10 km. Little or no evidence of upper crustal extension is found in the stratigraphic record. (3) An exhumation mode during which downward-concave faults exhumed lower crustal and mantle rocks to the seafloor. During these 3 phases the rift flanks have undergone little or no uplift. This suggests that the strength of the lithosphere decreased all along the evolution of the rift. We find that in order to consistently model the rift evolution during the first two stages, we need to simulate the progressive weakening of the continental crust with the formation of an attenuated middle crustal layer composed of anorthosite and quartz. This layer essentially decouples upper/middle crust from lower crust and mantle. During the first stage the presence of a preexisting strong gabbroic layer (Ivrea lower crustal body) in the lower crust helps distribute the deformation by strengthening the crust. In the second stage this lower crust allow for the coupling of the crust and mantle along a system a downward concave faults that thins the crust with no distributed faulting. During the exhumation phase the downward concave fault exhumes lower crust and mantle. In that last phase serpentinization provides a mechanism to weaken (attenuate) the lithospheric mantle.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 9325 Atlantic Ocean
SC: Tectonophysics [T]
MN: 2005 Joint Assembly