Development of a flat slab is generally characterized by migration and termination of arc magmatism, reduced continental surface heat flow, and thick-skinned deformation in the continental interior. In these regions, the oceanic plate unbends to become subhorizontal, underplating the continental plate for several hundred kilometers inboard of the plate margin at depths of 45-150 km. However, in approximately 10% of modern subduction zones, shallow to flat-slab subduction occurs. Subduction is driven by the negative buoyancy of the downgoing oceanic plate, resulting in a steep descent angle at most subduction zones. Subduction zones are active plate margins that have an important role in mantle and lithosphere processes. The flat slab underthrusts the continent at ~. cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. However, this can only create low-angle subduction, as the Farallon Plate was old (>. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. This study uses 2D upper mantle scale numerical models to investigate these factors. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. km inboard of the Farallon Plate subduction margin. Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000.
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