HR: 16:15h
AN: T34A-02    [Abstracts]
TI: From Crustal Thinning to Continental Break-up in Magma-Poor Rifted Margins: How Important are Detachment Faults?
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, 1 rue Blessing, Strasbourg, 67084, France
AU: Lavier, L L
EM: luc@utig.ig.utexas.edu
AF: UTGI, 10100 Burnet Rd, Austin, 78759, United States
AU: Péron-Pinvidic, G
EM: gwenn.peron-pinvidic@illite.u-strasbg.fr
AF: CGS-EOST, 1 rue Blessing, Strasbourg, 67084, France
AU: Jammes, S
EM: Suzon.Jammes@illite.u-strasbg.fr
AF: UTGI, 10100 Burnet Rd, Austin, 78759, United States
AU: Mohn, G
EM: Geoffroy.Mohn@illite.u-strasbg.fr
AF: CGS-EOST, 1 rue Blessing, Strasbourg, 67084, France
AU: Muntener, O
EM: othmar.muntener@unil.ch
AF: IMG Lausanne, Anthopole, Lausanne, 1015, Switzerland
AB: Detachment faults are widely regarded as playing an important role in crustal thinning and mantle exhumation at magma-poor rifted margins. However, how important are these structures and what is their role in crustal thinning and mantle exhumation? In our presentation, we review the pertinent observations made along the Iberia-Newfoundland, Western Pyrenees-Bay of Biscay and the Alpine margins. A reconstruction of the future distal margins 10 to 20 myr before continental break-up shows that: (1) extension was localized and the crust was thinned to less than 10 km despite subdued high-angle normal faulting, (2) detachment faults responsible for mantle unroofing are late and shallow crustal structures, (3) the mantle lithosphere was locally omitted and replaced by infiltrated asthenospheric mantle, and (4) shallow marine sediments were deposited over thinned crust suggesting a retardation of tectonic subsidence. These observations scrutinize our present knowledge of rift processes and ask for a paradigm shift in the way to interpret rifted margins. To further investigate the role of detachment faulting during crustal thinning and mantle exhumation, we developed a numerical model that: (1) uses initial conditions, strain distribution, rheology and deformation modes constrained from the study of Alpine margins, and (2) is able to replicate the well-documented tectonic evolution of the Iberia and Alpine margins, starting with pure-shear dominated crustal stretching and ending with mantle exhumation along detachment faults. The most interesting result of the experiment is the way the model evolves from the prescribed initial distributed stretching (e.g. pure-shear) to final localized exhumation (e.g. simple shear). During this transitional phase, referred to as thinning phase, extension is accommodated by a system of superimposed but decoupled concave-downward faults that simultaneously exhume middle crust to the seafloor and deeper mantle at the base of the crust. The two rolling hinges eventually merge to form one concave downward fault that unroofs the mantle to the seafloor. These results suggest that exhumation associated with detachment faulting is an important process. It can explain crustal thinning to less than 10 km; the lack of major fault bounded topography; and the emplacement of deep and infiltrated mantle rocks beneath thinned crust 10 to 20 myr before continental break-up. This has important consequences for the isostatic, thermal and rheological evolution of deep magma-poor margins that need to be constrained by data. At present, the thinning mode is neither understood nor constrained by geological or geophysical data. Because the proximal and distal margins are mainly preserving structures of the initial stretching and the final exhumation modes, we started to investigate the transition zone between the proximal and distal margins in the Alps and the zone ahead of a propagating ocean in the Western Pyrenees – Bay of Biscay. In our presentation, we present evidence for the existence of the thinning mode within these two domains and show how these results help to develop new conceptual ideas to re-interpret seismic data from present-day magma-poor rifted margins.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting