HR: 16:30h
AN: T54B-03    [Abstracts]
TI: Continental subduction, surface processes, P-T-t-z conditions and unstable vs. stable plate dynamics: Insights from thermo-mechanical modelling
AU: * BUROV, E
EM: burov@lgs.jussieu.fr
AF: lab of Tectonics, University of Paris 6, 4 Place Jussieu, Paris, 75252, France
AU: Yamato, P
EM: philippe.yamato@univ-rennes1.fr
AF: Geosciences Rennes, University of Rennes 1, Bāt 15, Campus de Beaulieu, Rennes, 35042, France
AB: We analyze major mechanisms of shortening of continental lithosphere (simple shear subduction, pure shear collision, folding, Rayleigh-Taylor instabilities). We use a thermo- dynamically coupled thermo-mechanical numerical model that accounts for brittle-elastic-ductile rheology, surface processes and metamorphic phase changes. The model also traces P-T-t-z paths of metamorphic facies that can be compared with petrology data. The experiments suggest that continental subduction occurs in case of relatively strong lithospheres with a competent mantle part (T_Moho < 550° C) , at relatively high initial convergence rates (> 1.5-5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the entire (upper and lower) crust or only the lower crust can be involved in subduction. Pure shear collision is dominant when T_Moho > 550° C or convergence rates are lower than 1.5-3 cm/yr (subduction number, S > 0.5). Large-scale folding is favored in case of T_Moho=500-650° C and is more effective in case of mechanical coupling between the crust and mantle (e.g., strong diabase lower crust). Gravitational R-T instabilities overcome other mechanisms for very high values of T_Moho (>800° C) and lead to the development of subvertical "cold spots." In case of weak metamorphic rheologies, phase changes improve chances for stable subduction. In general, exhumation of UHP-HP rocks to the surface is favored if the crustal rheological profile is characterized by two internal ductile decolement levels (between the upper and lower or intermediate crust and the lower crust and mantle lithosphere). Finally, we investigate the impact of surface processes (erosion/sedimentation) on the amount of continental subduction. The maximal amount of subduction is achieved for intermediate erosion rates when tectonic uplift rates are fine-balanced by denudation rates. In case of India-Asia collision-like scenario (fast convergence > 5 cm/y, stiff lower plate), the optimal balance is achieved for k ~ 3000 m²/yr. The experiments suggest that both extra slow (k < 50-100 m²/yr) and extra rapid erosion (k > 6000-8000 m²/yr) limit, by up to 50%, the total amount of subduction, if not totally prevent it. We suggest that most orogenic belts could have started their formation from continental subduction, yet, in case of slow convergence (< 3 cm/yr) or weak lithosphere the subduction channel locks up after about few Myr, and subduction is then relayed by a different deformation mode such as pure shear collision. In case of India-Asia-like convergence settings, continental subduction may continue for tens of Myr allowing for subduction of 600-800 km continental "slab".
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 4255 Numerical modeling (0545, 0560)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8160 Rheology: general (1236, 8032)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting