We propose that the gently eastward dipping double planed seismic zone observed at 15--25 km depths in the southern Cascadia subduction zone, just north of the Mendocino triple junction, is a direct consequence of the thermally controlled rheology. As the oceanic lithosphere subducts to a depth of about 15 km, the temperature regime causes a brittle-plastic transition to occur within the oceanic crust. Thus, a ductile layer forms in the lower oceanic crust, sandwiched between the brittle upper crust and brittle upper mantle. The very high strain rates near the triple junction caused by the northward push of the Pacific plate on the Gorda plate increases the seismicity and thus accentuate the double seismic zone in this region. This model explains the focal mechanisms observed in the seismic zone and their spatial change. The double seismic layers clearly define the position of the subducting Gorda plate, previously uncertain in the Cape Mendocino region. ¿ American Geophysical Union 1994