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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