EARLIEST CRETACEOUS PACIFICWARD OFFSET OF THE KLAMATH SALIENT, NW CALIFORNIA-SW OREGON

Although Late Triassic igneous rocks are present, the Sierra Nevada-Klamath calc-alkaline arc began forming in earnest along the continental margin at ~170 Ma during oblique underflow of paleo-Pacific oceanic lithosphere; intense activity continued throughout the volcanic-plutonic belt until at least ~140 Ma. This Andean arc supplied detritus to the proximal Mariposa-Galice overlap sequence by ~165-160 Ma. After onset of uppermost Jurassic Myrtle sedimentation on the western flank of the Klamaths, but before Hornbrook and Valanginian Great Valley Group overlap deposition on the eastern and SE sides, the Klamath Mountains were displaced ~200 km westward relative to the Sierran arc. In so doing, the Klamath salient moved off the deep-seated magmagenic zone underlying the arc, so did not participate in the massive Sierra Nevada igneous flare-up between ~125 and ~85 Ma. Speculatively, beginning at ~140 Ma, underflow of a young, warm, thin oceanic plate segment beneath the Klamaths caused ductility-enhanced rotation of the superjacent stack of allochthons into a gently east-rooting section. This process may have been accompanied by local back-arc spreading. Subduction and collision of much thicker oceanic lithosphere on both the north and south caused contraction and eastward displacement of the continental margin arc relative to the west-projecting Klamath Mountains. After a magmatic lull, heightened igneous activity in the Sierra Nevada commenced at ~125 Ma. The earliest Cretaceous oceanward plate junction rollback lay directly offshore from the Klamath imbricate orogen, but to the south trapped the ~165 Ma Coast Range Ophiolite on the North American side of the suture. After ~140 Ma, first-cycle arc detritus began to accumulate on the mafic igneous basement flooring the Great Valley forearc, and turbiditic clastics also were carried oceanward across the forearc and into the coeval Franciscan trench.