UPLIFTED NEOGENE MARGIN, SOUTHERNMOST CASCADIA-MENDOCINO TRIPLE JUNCTION REGION, CALIFORNIA

Along a 10-km length of coast north of the Mendocino triple junction, a Neogene accretionary complex has been uplifted greater than or equal to 2 km and tilted northward in response to the interaction of the so uthern Juan de Fuca (Gorda) plate with the older North American and Pa cific plates. These plate interactions were accompanied by tectonic in tercalation of Miocene to Pliocene deposits of the southern Cascadia f orearc (Wildcat Group), with penetratively deformed Oligocene to Mioce ne accretionary prism deposits of the False Cape terrane (new name) an d the Cretaceous to Eocene Coastal terrace of the Franciscan Complex. North of Cape Mendocino, more than 2 km of stratigraphic section of th e accretionary False Cape terrane crops out at beach level in an east- west trending anticline and recumbent syncline. The folded False Cape rocks are juxtaposed with forearc deposits to the north along the subv ertical Russ shear zone. To the south, a second subvertical shear zone truncates the False Cape terrane near the mouth of the Bear River. Th is North Bear River shear zone interleaves rocks of the False Cape and Coastal terranes with additional Neogene forearc deposits (the Bear R iver beds). South of the North Bear River shear zone, the Bear River b eds crop out for about 5 km in a continuous folded and imbricated sect ion that is again truncated to the south by a third subvertical shear zone. This South Bear River shear zone interleaves rocks of the Coasta l terrane, the Bear River beds, and Pliocene to Pleistocene shelf depo sits. Folding, tilting, and shear zone development reflect ongoing nor th-south crustal shortening that has occurred in response to interplat e coupling and wedge thickening in the deforming Miocene and younger a ccretionary complex. The strain recorded in the accreted terranes and in the forearc overlap assemblage reflects a stress regime unlike that in Cascadia fold-and-thrust belt north of the Eel River syncline. Thu s the deformation pattern may be unique to the triple junction area. T he False Cape terrane is one of only two localities along the Cascadia margin where the Oligocene to Miocene accretionary complex is exposed on land. The other locality is that of the Hoh melange on the southwe st Olympic Peninsula. On-land exposure of accretionary rocks in these two areas is a consequence of high uplift rates, focused compression, and triple-junction tectonism. Framework grain composition data for sa ndstones in the Hoh melange and coeval continental deposits in norther nmost California (the Oligocene to Miocene Weaverville Formation) sugg est a provenance link, while sandstone of the False Cape terrane, pres ently situated directly west of the Weaverville Formation, is dissimil ar. Given the northward component of Farallon (Juan de Fuca) plate tra nslation with respect to North America for the past 20 m.y., provenanc e links of Hoh and Weaverville sandstones might reflect a substantial component of northward translation between the Farallon and North Amer ican plates that was accommodated within the accretionary prism.