TRANSITION FROM SLAB TO SLABLESS - RESULTS FROM THE 1993 MENDOCINO TRIPLE JUNCTION SEISMIC EXPERIMENT

Three seismic refraction-reflection profiles, part of the Mendocino tr iple junction seismic experiment, allow us to compare and contrast cru st and upper mantle of the North American margin before and after it i s modified by passage of the Mendocino triple junction, Upper crustal velocity models reveal an asymmetric Great Valley basin overlying Sier ran or ophiolitic rocks at the latitude of Fort Bragg, California,nd o verlying Sierran or Klamath rocks near Redding, California, In additio n, the upper crustal velocity structure indicates that Franciscan rock s underlie the Klamath terrane east of Eureka, California, The Francis can complex is, on average, laterally homogeneous and is thickest in t he triple junction region, North of the triple junction, the Gorda sla b can be traced 150 km inboard from the Cascadia subduction zone, Sout h of the triple junction, strong precritical reflections indicate part ial melt and/or metamorphic fluids at the base of the crust or in the upper mantle, Breaks in these reflections are correlated with the Maac ama and Bartlett Springs faults, suggesting that these faults extend a t least to the mantle, We interpret our data to indicate tectonic thic kening of the Franciscan complex in response to passage of the Mendoci no triple junction and an associated thinning of these rocks south of the triple junction due to assimilation into melt triggered by upwelli ng asthenosphere, The region of thickened Franciscan complex overlies a zone of increased scattering, intrinsic attenuation, or both, result ing from mechanical mixing of lithologies and/or partial melt beneath the onshore projection of the Mendocino fracture zone, Our data reveal that we have crossed the southern edge of the Gorda slab and that thi s edge and/or the overlying North American crust may have fragmented b ecause of the change in stress presented by the edge.