| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4691431 | Tectonophysics | 2016 | 15 Pages |
•Flat subduction of the old Farallon plate is believed to have caused the Laramide orogeny.•Slab flattening may result from continental motion, slab suction, or oceanic plateau subduction.•2D numerical models test each of these factors.•None of the individual factors results in flat subduction on its own.•Flat slab develops in 15 Ma with trenchward continental motion and subduction of a buoyant plateau.
The Laramide orogeny (~ 80–50 Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000 km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. 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. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (> 100 Myr) and denser than the mantle. 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. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. 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. With a continental velocity of 4 cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15 Ma after plateau subduction. The flat slab underthrusts the continent at ~ 200 km depth, eventually extending > 1500 km inboard of the trench.
