MID-MESOZOIC TO MIOCENE CLASTIC SEDIMENTATION ALONG THE NORTHERN CALIFORNIA MARGIN—PROVENANCE AND PLATE-TECTONIC IMPLICATIONS

Based on relationships among volcanic-plutonic arc rocks, HP/LT metamafic rocks, westward relative migration of the Klamath Mountains salient, and the locus of the Mariposa-Galice, Great Valley Group (GVG), and Franciscan depo-basins, I infer the following geologic evolution for northern California: (1) At ~175-170 Ma, onset of transpressive plate underflow generated an emergent igneous arc along the margin. By ~165 Ma and continuing to ~150-140 Ma, erosion provided volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (2) Mafic rocks were metamorphosed at HP/LT in an inboard subduction zone from ~170-155 Ma. Except for the Red Ant blueschists, such rocks remained at depth; most HP/LT mafic tectonic blocks returned surfaceward only during Late Cretaceous time entrained in circulating, buoyant Franciscan mélange. (3) At the end-of-Jurassic time, before onset of Franciscan and GVG ± Hornbrook deposition, the Klamath salient was deformed and began to be offset ~100-200 km westward relative to the Sierran arc. (4) After this step-out of the Farallon-North American convergent plate junction—which stranded pre-existing oceanic crust on the south as the Great Valley Ophiolite—terrigineous debris began to arrive at the Franciscan trench + intervening Great Valley forearc by ~140 Ma. The most voluminous sedimentation/accretion of Franciscan Eastern + Central Belt and GVG detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ~125-85 Ma. (5) Andean-type arc volcanism-plutonism ceased at ~85 Ma in northern California, signaling a transition to nearly subhorizontal plate underflow attending Laramide orogeny far to the east. (6) Paleogene-Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm practically unaffected by HP/LT subduction. (7) Grenville detrital zircons are completely missing from the post-125 Ma Franciscan section, whereas detritus from the Idaho Batholith, Challis volcanics, and Cascade Range appear in progressively younger Paleogene-Miocene Coastal Belt sediments. Although not conclusive, this suggests the possibility of gradual, post-depositional NW dextral offset of Franciscan trench deposits (~1600 km) relative to the GVG forearc and basement terranes of the SW conterminous U. S.