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Abstract

The Laramide orogeny (80-50 Ma) resulted in thick-skinned deformation of the western United States, more than 700 km inboard of the plate boundary where the Farallon Plate was subducting below North America. Most studies conclude that this event was the result of low-angle or flat subduction of the Farallon plate, whereby horizontal compressive stress from the shallow slab produced inboard crustal compression and shortening. However, it is still not clear what factors caused the Farallon plate to shallow prior to Laramide time or how stress was transferred from the flat slab to the continental interior. Three hypotheses have been proposed for triggering flat subduction: (1) an increase in the westward velocity of the North American plate; (2) subduction of an buoyant oceanic plateau with abnormally thick crust and possibly a low density harzburgite mantle lithosphere layer; and (3) slab suction produced by subduction-induced mantle wedge flow and enhanced by the presence of thick Colorado Plateau lithosphere in the backarc. In this study, we use numerical models to study the development of low-angle subduction below a continental plate with a structure similar to that of the western US. The two-dimensional, plain strain models use the SOPALE code, in which Arbitrary Lagrangian-Eulerian finite element techniques are used to compute the coupled thermal-mechanical evolution of the lithosphere-upper mantle system. We first assess what factors are needed to dynamically develop low-angle subduction. We find that the main control is the continental velocity, with enhanced slab shallowing as the continental velocity increases. In order to create a section of horizontal (i.e., flat) subduction, a further requirement is the presence of an oceanic plateau with a low-density harzburgite layer. The slab suction force seems to be less capable of creating a flat slab than the other two factors. This may be due to the low viscosity in the mantle wedge in our models (10^19~10^20Pa s), which cannot produce a sufficiently large hydrodynamic force to flatten the oceanic plate. Future work will examine variations in the strength of both the continental plate and the interface between the continent and low-angle oceanic plate, in order to explore the relationship between flat subduction and Rocky Mountain foreland deformation during Laramide time.