Viscous dissipation, slab melting, and post-subduction volcanism in south-central Baja California, Mexico

Volcanic activity continued to occur along the length of the Baja California Peninsula (northwestern Mexico) even after the cessation of subduction during the middle Miocene. This volcanism occurred mainly in monogenetic volcanic fields, erupting lavas with a wide variety of compositions, including: adakites, niobium-enriched basalts, high-niobium basalts, and high-magnesian andesites. The chemical compositions of these magmas suggest an origin in partially melted basaltic oceanic crust that was subsequently subducted below the peninsula. Several attempts have been made to explain the origin and compositional diversity of post-subduction volcanism in Baja California. Many of these attempts rely on the hypothesis that the magmas were formed through adiabatic decompression of upwelling asthenosphere in direct response to the formation of a window or tear in the subducted slab. This process, however, cannot offer a satisfactory explanation for all existing observations, particularly the lithospheric structure, of Baja California. Here, we present a physical model that sheds light onto the origin of the post-subduction volcanism in Baja California. The model calls upon viscous dissipation as the causative agent of volcanism. Our starting conjecture is that shearing along a low-viscosity channel confined between the stalled oceanic slab and continental crust of Baja California peninsula caused partial melting at moderate depths following cessation of subduction. Our modeling results show that it is indeed possible for rocks to reach their solidus temperature by means of this mechanism. Numerical results indicate that shear heating could lead to a temperature increase of close to 200 °C at a depth of 30 km, sufficient to produce more than 30% melt by volume.