Geodynamic and plate tectonic models for the Mendocino triple junction, a fault-fault-trench triple junction in northwestern California, predict a slab-free zone south of the triple junction in which asthenospheric mantle upwells to the base of the crust. A variety of geological and geophysical data support this model, although fine-scale (<20 km) details of the lithospheric structure have been unknown previously. Seismic investigations in the onshore transform regime south of the Mendocino triple junction region reveal very strong short-offset reflections from the lower crust and at the crust-mantle boundary beneath the entire width of the Coast Range, particularly near Lake Pillsbury, California. Seismic analysis suggests that these reflections are from discrete zones of fluid. The reflector geometry implies that the source of the fluid is within the upper mantle. In this tectonic context it is likely that the fluids are largely partial melt, segregated from asthenospheric mantle upwelling into the slab-free zone. The tectonic setting and the location of Lake Pillsbury relative to the estimated position of the southern edge of the Gorda slab and the Clear Lake volcanic field suggest that volcanism may initiate in this region within the next 400 k.y.