The Development of Flat-Slab Subduction: Implications for the Laramide Orogeny

English Abstract:

Flat or subhorizontal subduction occurs in approximately 10% of modern subduction zones. It is also believed to be important in the geologic past. An example is the western U.S. Laramide orogeny (~80-50 Ma), which occurred >1000 km inboard of the Farallon Plate subduction margin. It is widely believed that deformation was induced by flat subduction. However, the Farallon plate age was >100 Ma at this time, and it is not clear how such a cold, dense oceanic plate could evolve from a steep-angle trajectory to a flat geometry. Here, we use 2D thermal-mechanical numerical models to explore the development of flat subduction. The models show that trenchward continental motion provides the primary control on subduction geometry, with decreasing slab dip as velocity increases. For a >100 Ma oceanic plate, this creates low-angle subduction. The transition to a flat-slab requires: (1) subduction of an oceanic plateau which does not undergo metamorphic densification, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and the buoyant plateau pulls the slab upward to a subhorizontal trajectory. For parameters consistent with Farallon Plate subduction, a flat geometry develops within 15 Ma after oceanic plateau subduction, eventually extending >1500 km inboard of the trench. Additional models with a younger (40 Ma) oceanic plate show that flat subduction can develop with only continental advance, especially if subduction occurs below thick contienental lithosphere. The additional buoyancy of the younger (warmer) oceanic plate is sufficient to induce flat subduction and slab break-off is not required. This may explain the origin of flat-slab segments at the present-day South America subduction zone.