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

¹ Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA
² Department of Geology and Geophysics, Rice University, Houston, Texas 77005, USA

North-south compression across the Gorda-Pacific plate boundary caused by the northward-migrating Mendocino triple junction appears to reactivate Gorda plate normal faults, originally formed at the spreading ridge, as left-lateral strike-slip faults. Both seismically imaged faults and magnetic anomalies fan eastward from N20°E near the Gorda ridge to N75°E near the triple junction. Near the triple junction, the Gorda plate is faulted pervasively and appears to be extending east-southeast as it subducts beneath North America. Continuation of northeast-southwest–oriented deformation in the southern Gorda plate beneath the continental margin contrasts with the northwest-southeast–trending structures in the overlying accretionary prism, suggesting partial Gorda–North American plate decoupling. Southeast of the triple junction, a slabless window is generated by removal of the subducting Gorda plate. Southwest of the triple junction, the Pacific plate acts as a rigid barrier forcing southern Gorda crust to rotate clockwise, fragment, and flow into the slabless window. Net clockwise rotation of the southern Gorda crust forms a boundary with the nonrotating northern Gorda plate, which is observed as a bend in the magnetic anomalies. This boundary, which is compressional on the western end and extensional to the east, may separate the stress regime of the southern Gorda plate from the remainder of the Cascadia subduction zone.

Key Words: Mendocino triple junction • Gorda plate • plate boundary • deformation • fragmentation

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