Sean Paul Sandifer Gulick

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 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 ;N208E near the Gorda ridge to ;N758E 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.

Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Stratigraphic development

The Eel River forearc basin, northern California, lies at the southern end of the Cascadia subduction zone and at the leading edge of the migrating Mendocino triple junction. Stratigraphic relationships within the Eel River forearc basin suggest that the current outer-arc high formed between ca. 3 and 2 Ma when the margin switched from a nonaccretionary to an accretionary phase and then uplifted to attain critical taper. Between ca. 2 and 1 Ma, an influx of sedimentation from the ancestral Klamath and Eel River systems increased the width of the northern California margin and caused continued uplift followed by widespread erosion of the western margin of the basin at ca. 1 Ma. In the northeastern part of the forearc basin, localized erosion of the shelf occurred at ca. 500 ka. The arrival of the northward-migrating Mendocino triple junction at ca. 500 ka is documented by uplift, northward tilting, erosion of the margin as much as 20 km north of Cape Mendocino, and reduced deposition within the forearc basin as much as 80 km north of the current position of the triple junction. Terrestrial sediments delivered to the continental margin and eroded sediments near the triple junction largely bypassed the southern part of the basin and were likely deposited in northern areas of the basin or flowed down the Eel Canyon to be deposited within the Gorda Fan.

Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Structural evolution

Offshore northern California, the Gorda plate is subducting obliquely beneath North America; the resulting complicated tectonic setting forms the southern end of the Cascadia subduction zone. The southern Cascadia subduction zone and overlying Eel River forearc basin lie just north of the unstable Mendocino triple junction. The Neogene strata of the Eel River basin record structural deformation caused by the underthrusting of the Gorda plate as well as deformation generated by northward migration and encroachment of the Mendocino triple junction. Three distinct deformation regimes are present in the Eel River forearc basin. (1) Along the western margin of the forearc basin and within the foreslope of the accretionary prism, thrust faults and anticlines record Pliocene– Pleistocene shortening caused by subduction of the Gorda plate. (2) The southern part of the basin rotated counterclockwise in the late Pleistocene, resulting in modern transpressional deformation offshore Humboldt Bay. The rotation and deformation are caused by north-south convergence across the boundary between the Pacific plate and the southernmost part of the forearc basin at the triple junction. (3) The northeastern margin of the Eel River basin is deformed by high-angle faults with a component of strike-slip motion that may represent the incipient northward propagation of the Pacific–North American transform system north of the triple junction.