Island Arcs and Convection

Abstract

Abstract : If island arcs are the outcrops of compressive faults, they cannot be products of a uniform field of compressive stress because their symmetry is lower. The polarity of the curvature, however, is provided if the compression is of convective origin. Incipient faulting is then followed by a convergence of the convective flow towards the fault, causing a convergence of the initially parallel stress trajectories. Since the fault propagates at right angle to the trajectories of maximum compressive stress, its outcrop becomes an arc convex towards the upstream side. In oceanic compressive faults due to convection the two conjugate fault planes are not equivalent because overthrusting of the downstream side by the upstream side would lead to the accumulation of a thick crust which would inhibit the progress of faulting along the plane, and because underthrusting is libricated by decomposing serpentine, fluxed basalt etc. This seems the reason why apparently in all long-lived oceanic compressive faults the upstream (usually the oceanward) side underthrusts the downstream side. (Author)