Internal deformation of the southern Gorda plate: Fragmentation of a weak plate near the Mendocino triple junction

North-south compression across the Gorda-Pacific plate boundary caused by t he northward-migrating Mendocino triple junction appears to reactivate Gord a plate normal faults, originally formed at the spreading ridge, as left-la teral strike-slip faults. Both seismically imaged faults and magnetic anoma lies fan eastward from similar to N20 degreesE near the Gorda ridge to simi lar to N75 degreesE near the triple junction. Near the triple junction, the Gorda plate is faulted pervasively and appears to be extending east-southe ast as it subducts beneath North America. Continuation of northeast-southwe st-oriented deformation in the southern Gorda plate beneath the continental margin contrasts with the northwest-southeast-trending structures in the o verlying accretionary prism, suggesting partial Gorda-North American plate decoupling. Southeast of the triple junction, a slabless window is generate d by removal of the subducting Gorda plate. Southwest of the triple junctio n, the Pacific plate acts as a rigid barrier forcing southern Gorda crust t o rotate clockwise, fragment, and flow into the slabless window. Net clockw ise rotation of the southern Gorda crust forms a boundary with the nonrotat ing northern Gorda plate, which is observed as a bend in the magnetic anoma lies. This boundary, which is compressional on the we-stern end and extensi onal to the cast, may separate the stress regime of the southern Gorda plat e from the remainder of the Cascadia subduction zone.