GEODYNAMIC MODELS OF FARALLON PLATE SUBDUCTION: CREATING AND REMOVING A FLAT SLAB (Invited Presentation)

The Laramide Orogeny is an enigmatic event that affected the western United States (US). It is marked by the >1500 km migration of deformation and volcanism from the Farallon Plate subduction margin to the continental interior starting in the Late Cretaceous. Deformation reached its maximum eastward extent by the Early Eocene and then migrated westward over the next ~20 Ma. This pattern is consistent with a change in the Farallon Plate subduction trajectory, from a relatively steep angle to subhorizontal (i.e., flat-slab), followed by slab retreat and steepening.

We use two-dimensional thermal-mechanical numerical models to investigate the development and demise of flat-slab subduction. The models use lithospheric properties and plate velocities consistent with those of the western US from 90 to 30 Ma. Within the model domain, the oceanic plate evolves dynamically in response to imposed plate convergence and internal buoyancy forces. Previous work has shown that three key factors may induce flat-slab subduction: (1) trenchward motion of the upper plate, (2) upward suction on the oceanic plate due thick, high viscosity upper plate lithosphere; and (3) subduction of anomalously buoyant material, such as an oceanic plateau. Here we demonstrate that the same factors affect the removal of a flat-slab segment.

The Laramide orogeny was preceded by westward acceleration of North America and subduction of the Conjugate Shatsky Rise (CSR) oceanic plateau below the thick Colorado Plateau lithosphere. The models show that CSR subduction is the most important control on slab geometry, as the Farallon Plate was old, and therefore cold and dense. To deflect the slab to a subhorizontal trajectory, the CSR crust must remain metastable and buoyant during subduction (i.e., does not undergo immediate eclogitization). Termination of flat-slab subduction coincides with metamorphic densification of the CSR crust. At this time, the flat slab segment sinks and the oceanic plate evolves toward steep-angle subduction. Slab steepening is aided by (1) a decrease in plate convergence and continental westward motion starting in the Eocene, and (2) rheological weakening of the continental mantle lithosphere, perhaps due to hydration from the underlying flat slab, which allows the slab to decouple from the continent.