PLATE TECTONIC VS. MANTLE PLUME PROCESSES: THE TECTONIC REORGANIZATIONS OF THE REYKJANES RIDGE—INITIAL RESULTS OF THE 2019 R/V NEIL ARMSTRONG CRUISE

The Reykjanes Ridge intersects the Iceland “hotspot” and extends for nearly 1000 km to the south. The ridge underwent major tectonic reorganizations in the Cenozoic Era, initiating as a linear orthogonally-spreading axis at ~55 Ma, abruptly formed stair-step segments offset by transform faults following a plate motion change at ~37 Ma, and then progressively evolved back to its original linear geometry, although it had to spread obliquely to do this. Several studies have attributed its anomalous characteristics and evolution to mantle plume effects whereas other studies explain them by lithospheric and upper mantle processes. “Mantle plume” hypotheses propose that changes in plume temperature induce brittle to ductile lithospheric transitions that either allow transform faults to form or inhibit their formation. Tectonic explanations propose that plate tectonic processes, including changes in plate motion, propagating ridges, asymmetric spreading and the inherited linear geometry of the melting regime control the tectonic reorganizations and that a gradient in mantle melting with distance from Iceland modulates these effects. In June-July 2019 the R/V Neil Armstrong conducted a geophysical survey (gravity, magnetics, multibeam and echosounder sediment profiling) of the ridge flanks spanning the change in opening direction and extending from near the Iceland shelf to the Bight Fracture Zone to examine the detailed kinematics and geophysical effects of the plate boundary reorganizations. Here we present early cruise results bearing on the “plate vs. plume” controversy for the Reykjanes Ridge tectonic reorganizations of the North Atlantic.