Tectonics of the Mendocino triple junction

Interpretation of reflection profiles and of the magnetic anomaly pattern over the Gorda Basin and Escarpment gives broad agreement with the triple junction model of McKenzie and Morgan (1969). However, the basin has undergone internal deformation, a local departure from rigid plate tectonics, and the escarpment has had a component of underthrusting by the Gorda block. Faults in the Gorda Basin which disturb young turbidites parallel the trends of magnetic anomalies, suggesting deformation of the oceanic crust along lines of primary weakness. The northeast trends of the faults give a constraint on first-motion solutions for earthquakes within the basin and suggest left-lateral slip on the faults. Analysis of the geometry and timing of the Gorda Basin deformation based on the magnetic pattern gives an average gross tectonic strain rate of 10⁻¹⁴/sec. These observations give a measure of the mechanics of deformation of oceanic lithosphere very close to a spreading rise crest.