On the Competing Roles of Fault Reactivation and Brittle Failure in Generating Plate Tectonics from Mantle Convection

Abstract

Fault reactivation plays a fundamental role in the generation of plate tectonics from mantle convection. Converging and transform plate margins are mechanically weak due to both preexisting faults and preserved shear zones within the crust and lithosphere, on the one hand, and continuous brittle failure of lithosphere, on the other hand. Transform margins are the site for nucleating new converging margins. Lithospheric earthquakes demonstrate that oceanic lithosphere within a trench can deform as fast as the upper mantle. Models demonstrate that faulted converging plate margins contribute to producing plate tectonic-like motion. Preexisting faults and a power law plastic rheology dynamically interact and together give rise to plate-like motion. Faults have an important influence over outer rise and trench topography and by comparing models with observations, interplate stress is about 10-30 MPa. Subduction can initiate on preexisting faults when interplate coupling is as high as 10-30 MPa if the oceanic plate already has slabs attached to it - as was the case for the Pacific plate when the Marianas nucleated. Modeled plates only change velocity slowly in response to the initiation of new subduction zones; the initiation of subduction in the Western Pacific at ≈ 45 Ma was likely the result of a change in Pacific plate motion as opposed to causing the change in the direction of plate motion. The locations of the greatest amount of present day brittle failure within oceanic lithosphere are not located at arbitrary positions on oceanic plates, rather brittle failure occurs in close proximity to long lived zones of preexisting weakness. Old weak structures are reused by the convecting system because it takes less energy to reactivate a preexisting structure than it does to create an entirely new plate margin from pristine, intact lithosphere.