HR: 0800h
AN: T11B-1264    [Abstracts]
TI: A Potential Role for Slab/Lithosphere Decoupling and Edge Driven Convection in the Tectonic Evolution of the Western US
AU: * Patel, P
EM: warlord@mail.utexas.edu
AF: University of Texas, Austin, 1 University Station C1140, Austin, TX 78712
AU: Lavier, L
EM: luc@utig.ig.utexas.edu
AF: University of Texas, Austin, 1 University Station C1140, Austin, TX 78712
AU: Grand, S
EM: steveg@maestro.geo.utexas.edu
AF: University of Texas, Austin, 1 University Station C1140, Austin, TX 78712
AB: About 30 mya, shortly after the foundering of the once horizontally subducting Farallon Plate, extensive volcanism and extensional tectonics began throughout the Rocky Mountain region. It is accepted by many that the foundering of the plate drove a significant portion of the subsequent tectonics in the western US including the activation of the Rio Grande Rift. Here, we present 2D numerical experiments aimed at studying the delamination of the Farallon plate from beneath the western US. In these experiments, the lithosphere and mantle are modeled as a visco-elasto-plastic medium. 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 and friction as a function of plastic strain. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions. The experiments focus on the conditions that would lead to the delamination of a flat slab from beneath a simple representation of the western US. This simple representation assumes that the slab and the lithosphere are at thermal equilibrium and that the initial topography is flat. We find that in order to generate the instability necessary for the foundering of the plate, the presence of a low viscosity wedge between the remnant continental lithosphere and the subducting plate is necessary. The wedge serves as a proxy for possible mantle hydration and shear heating at the boundary between the subducting plate and the over-riding lithosphere. It effectively decouples the two plates, allowing a lithospheric instability to propagate into the total, or near total, removal of the slab. Depending on the geometry, density, and the viscosity of the wedge, we observe two basic modes for the delamination of the slab. In the first mode, the foundering plate is entrained in an edge-driven convection cell which conveys it laterally eastward and then down beneath the cratonic region of the lithosphere. In the second mode, the plate is decoupled such that it deforms and "ponds" to the west before finally detaching and sinking. We explore various geometries and rheological properties for the wedge with the goal of generating a final thermal and chemical configuration similar to that inferred via seismic tomography for the presumed remnants of the Farallon slab beneath the western US. Furthermore, we examine the predicted evolution of topography in the simulations for consistency with existing tectonic models for the evolution of the RGR and the Colorado Plateau. We posit that the existence of a low viscosity wedge generated the instability leading to the foundering of the Farallon plate. Moreover, the extent and the physical properties of the wedge control the delamination pattern.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8162 Rheology--mantle
DE: 3210 Modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting