A 3-D modeling approach to melt migration and extraction pathways at the Siqueiros transform along the East Pacific Rise

Within the global mid-ocean ridge system, transform faults offset spreading centers. Conductive cooling is probably more efficient beneath transform faults, producing a thickened lithosphere that potentially reduces melting and directs melt away from the transform. However, recent observations of thick crust along transform faults at fast-spreading ridges [Gregg et al., Nature 2007] suggest melt redistribution toward transforms, intra-crustal melt production, or efficient extraction of melt. We investigate the origin of these crustal anomalies by applying a 3-D model of melt migration and extraction to an oceanic transform domain bounded by ridge segments. We consider that melt extraction occurs in three steps: First, melt rises vertically from the zone of melt production to a melt-impermeable boundary, or permeability barrier, associated with a crystallization front controlled by the conductive regime at the base of the thermal boundary layer. Second, melt travels along the permeability barrier, which is generally inclined towards the ridge axis. It is at this stage that melt focusing occurs. Third, melt reaches the surface, usually because it enters a melt extraction zone, which may be physically interpreted as the presence of faults and/or dikes leading to rapid lateral and vertical melt migration toward plate boundaries. The geometry of the permeability barrier and the melt extraction zone are both affected by the detailed geometry of the segmented ridge axis. We examine a model that represents the Siqueiros transform on the East Pacific Rise. All three steps are necessary to explain the observed crustal thickness variations within the intra-transform spreading centers along the fast-slipping Siqueiros fault. In particular, we constrain the presence of a melt extraction zone within 10 km of the transform axis.