FLUIDS IN THE LOWER CRUST FOLLOWING MENDOCINO TRIPLE JUNCTION MIGRATION - ACTIVE BASALTIC INTRUSION

Geodynamic and plate tectonic models for the Mendocino triple junction , a fault-fault-trench triple junction in northwestern California, pre dict a slab-free zone south of the triple junction in which asthenosph eric 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. Se ismic investigations in the onshore transform regime south of the Mend ocino triple junction region reveal very strong short-offset reflectio ns from the lower crust and at the crust-mantle boundary beneath the e ntire width of the Coast Range, particularly near Lake Pillsbury, Cali fornia. Seismic analysis suggests that these reflections are from disc rete 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 asthe nospheric mantle upwelling into the slab-free zone. The tectonic setti ng and the location of Lake Pillsbury relative to the estimated positi on of the southern edge of the Gorda slab and the Clear Lake volcanic field suggest that volcanism may initiate in this region within the ne xt 400 k.y.