FLATTENING THE SLAB: FARALLON PLATE SUBDUCTION AND THE LARAMIDE OROGENY

The Laramide orogeny (~80-50 Ma) was an anomalous period of mountain-building in the western United States that took place >1000 km inboard of the Farallon Plate subduction margin. The orogeny was preceded by the development of the continental-scale Western Interior Seaway and eastward migration of the volcanic arc. Thus, it is widely believed that this marked a time of flat subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat (subhorizontal) subduction are not well-understood. Three mechanisms have been proposed: (1) an increase in westward North America Plate 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. In this study, we use 2D upper-mantle-scale numerical models to investigate these mechanisms. The models show that trenchward continental motion provides 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 much denser than the mantle. The transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau which does not undergo metamorphic densification, and (2) a slab break-off at the landward side of the plateau. The break-off event removes the dense frontal slab, and flat subduction develops as the buoyant plateau ‘pulls’ the slab upward to a subhorizontal trajectory. In our preferred model, the plateau is 400 km wide with 18 km thick metastable crust. With a continental velocity of 4 cm/yr, flat subduction develops within 15 Ma of plateau subduction. The flat slab segment underthrusts the continent at ~200 km depth, eventually extending >1500 km inboard of the trench. Future work will examine variations in the strength of the continental plate and subduction interface, in order to explore how flat subduction may induce Laramide-style deformation in the continental interior.