Uplifted Neogene margin, southernmost Cascadia‐Mendocino triple junction region, California

Abstract

Along a 10‐km length of coast north of the Mendocino triple junction, a Neogene accretionary complex has been uplifted ≥2 km and tilted northward in response to the interaction of the southern Juan de Fuca (Gorda) plate with the older North American and Pacific plates. These plate interactions were accompanied by tectonic intercalation of Miocene to Pliocene deposits of the southern Cascadia forearc (Wildcat Group), with penetratively deformed Oligocene to Miocene accretionary prism deposits of the False Cape terrane (new name) and the Cretaceous to Eocene Coastal terrane of the Franciscan Complex. North of Cape Mendocino, more than 2 km of stratigraphic section of the 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 subvertical Russ shear zone. To the south, a second subvertical shear zone truncates the False Cape terrane near the mouth of the Bear River. This North Bear River shear zone interleaves rocks of the False Cape and Coastal terranes with additional Neogene forearc deposits (the Bear River beds). South of the North Bear River shear zone, the Bear River beds crop out for about 5 km in a continuous folded and imbricated section that is again truncated to the south by a third subvertical shear zone. This South Bear River shear zone interleaves rocks of the Coastal terrane, the Bear River beds, and Pliocene to Pleistocene shelf deposits. Folding, tilting, and shear zone development reflect ongoing north‐south crustal shortening that has occurred in response to interplate coupling and wedge thickening in the deforming Miocene and younger accretionary complex. The strain recorded in the accreted terranes and in the forearc overlap assemblage reflects a stress regime unlike that in the Cascadia fold‐and‐thrust belt north of the Eel River syncline. Thus the deformation pattern may be unique to the triple junction area. The 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 southwest 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 sandstones in the Hoh melange and coeval continental deposits in northernmost California (the Oligocene to Miocene Weaverville Formation) suggest a provenance link, while sandstone of the False Cape terrane, presently situated directly west of the Weaverville Formation, is dissimilar. Given the northward component of Farallon (Juan de Fuca) plate translation with respect to North America for the past 20 m.y., provenance links of Hoh and Weaverville sandstones might reflect a substantial component of northward translation between the Farallon and North American plates that was accommodated within the accretionary prism.