Fracture of a Superplate: The Proximal Cause of the Hawaiian-Emperor Seamount Bend?

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—A change in plate motions of the Pacific basin - as inferred from the abrupt change in the strike at 43 Ma of the Hawaiian-Emperor Seamount chain - is modeled as the direct consequence of a plate fracture associated with the separation of Australia and Antarctica during the late Eocene. The postulated fracture was that of a large, and for the most part oceanic plate composed of the present Australian and Pacific plates. Volcanism in the Tasman Sea region with emphasis on the Balleny Islands hotspot is the proposed cause of the fracture, while plate tectonics in general is modeled in association with a mesospheric Rayleigh-Benard convection mode whose specific kinematics are determined from the global distribution of hotspots. Mesospheric convection is assumed to descend along a network of polygon boundaries shared by neighboring convection cells, and to approach those boundaries as radially advecting flow arising from hotspot plumes. Tractions due to the resulting shear strain across the asthenosphere both stabilize and drive the plate motions. The model yields unique and stable Euler poles for the motions of arbitrary plates, and specifically for the postulated pre-43 Ma ‘superplate’ and for the Pacific and Australian plates of present day to within about 10° to 20° (central angle) of observation. Plate tectonic torque magnitudes due to the underlying Benard convection are computed and found to be comparable to, or potentially larger than torques due to lithospheric density gradients, subsidence and subduction which may act in concert with, but are not necessarily required by the model. The analysis evaluates a unified lithosphere-bulk mantle approach to plate tectonics and addresses an ambiguity in the potential causes of plate motions.