OCEANIC INUNDATION AS THE TRIGGER FOR THE RIFT TO DRIFT TRANSITION

The process of how a continental rift transitions to self-sustaining continuous generation of new seafloor at a spreading axis has remained a central unanswered question of plate tectonics for decades. The variety of rift structures and transition timings at rifted margins do not easily lend themselves to some specific degree of strain and/or magmatism as the tipping point. A potential solution to the conundrum is the asthenospheric isostatic response that comes with ocean inundation. Ocean mass pouring into a rift drives flow in the asthenosphere outward much the same way a growing icecap drives a corresponding outward mantle flow. This flow alters mantle tractions of the rift system with disappearance of basal resistance and adds a push to the rifting process. Crucially, this outward directed flow remains after initial inundation as differential thermal subsidence continually draws ocean mass towards the rift axis resulting in a corresponding outward asthenosphere flow to balance the mass flux. This continual outward flux gives the overlying plate a perpetual resistance-free basal traction (and even slight push), facilitating plate separation. Geological evidence for ocean inundation facilitating the transition to seafloor spreading is observed across the globe including in the Atlantic, Afar, and Iceland. These examples show that through isostasy the ocean is not simply incidental to the creation of oceanic basins, but actively participates in the process.