HR: 12:05h
AN: T11E-08    [PDF]
TI: Modeling of the Role of Serpentinization and Magmatism at the Transition From Rifting to Seafloor Spreading
AU: * Lavier, L
EM: luc@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759 United States
AU: Manatschal, G
EM: manat@illite.u-strasbg.fr
AF: CGS_EOST, Universite Louis Pasteur, 1 rue Blessig, Strasbourg, 67084 France
AB: Observations from magma-poor rifted margins exposed in the Alps and drilled and seismically imaged in Iberia show that at the transition from rifting to seafloor spreading the fault geometry change from upward to downward concave. Observations and data from these to margins suggest that the fault geometry is controlled by serpentinization processes and the emplacement of igneous rocks, which are the results of temperature and reaction dependent processes and decompression melting. Here, we present 2D numerical models of the extension of a visco-elasto-plastic lithosphere. We study the effect of serpentinization and magmatism on both the evolution of strain and the force necessary to breakup the continent and form a new ocean. The brittle parts of the lithosphere are modeled as a frictional and cohesional material. The ductile lithosphere is modeled as a non-Newtonian Maxwell visco-elastic material. Faults in the brittle parts of the model are formed by locally decreasing the cohesion as a function of plastic strain. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions.To study the transition between "listric concave upward" to "concave downward" normal faulting, we start the model with a 10 km thick brittle crust and 90 km thick mantle. The serpentinized layer is modeled as a weak plastic layer. The mantle is inferred to be dominated by olivine rheology and the crust by diabase rheology. In a first set of model we study the effect of a serpentinzed layer below the brittle crust on the style of faulting and the tectonic force. With no weak serpentinized layer the deformation leads directly to the formation of large offset normal fault. Moreover the force needed to extend the lithosphere is higher than what can be expected from ridge push or slab pull. We find that the presence of a serpentinized layer leads to the localization of multiple normal faults in the brittle layer rooted in the weak serpentine and a large decrease in the tectonic force necessary to rift the layer. We then study the possible interaction between the ascent and melting of the mantle with faulting in the brittle layer. We model magma percolation and accumulation using a Darcy's law approach. The main role of melting is to help focus the area of necking and accelerate the rate of deformation. It also substantially decreases the force necessary to rift the modeled layer.
DE: 3210 Modeling
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics--extensional (0905)
DE: 8149 Planetary tectonics (5475)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting