Strain patterns within the forearc at a convergent margin adjacent to a passing fault-fault-trench triple junction show a systematic evolution. We describe late Neogene and Quaternary deformation in northern coastal California in the last 3 m.y. associated with the migration of the Mendocino triple junction. Deformation in the North American forearc near Humbolt Bay consists of a 30-km-wide zone of on-land contraction with a narrower zone of translation to the east. The zone of subaerial contraction is part of the relatively wide, southernmost extent of forearc contraction within the Cascadia subduction zone. Net Quaternary northeast-southwest contraction across forearc thrust faults is at least 7.9 km, and minimum fault slip rates on the six major thrust faults are 0.8--2.3 mm/yr. Net right slip within the zone of translation is a minimum of 3 km. South of the forearc, translation is predominant in the North American plate and is accommodated along two major right-lateral fault zones of the San Andreas transform boundary, the Lake Mountain, and Garberville fault zones. Discontinuous exposures of late Neogene sediments along these faults near Covelo and Garberville, respectively, indicate that crustal contraction, expressed by thrust faults and associated folding, predated translational deformation at each site. Inception of contraction near Covelo was related to inception of internal deformation of Gorda plate at 3 Ma. This contraction, due to coupling of the convergent plates, migrated northward in the forearc in advance of and at the rate of migration of the triple junction.

Present-day deformation near Humboldt Bay thus reflects contraction associa with subduction of the Gorda plate and translation farther east associated with oblique convergence at the plate boundary. As the triple junction migrates, the strike-slip faults of the San Andreas transform boundary will extend to the north-northwest into the present forearc, and forearc strike-slip faults will become faults of the San Andreas system. Contractional structures near Humboldt Bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south. ¿ American Geophysical Union 1988