Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

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Production of voluminous igneous arc rocks, high-pressure/low-temperature (HP/LT) metamafic rocks, westward relative migration of the Klamath Mountains province, and U–Pb ages of deposition, sediment sources, and spatial locations of Jurassic and younger, detrital zircon-bearing clastic rocks constrain geologic development of the northern California continental edge as follows: (1) At ~175 Ma, transpressive plate underflow began to generate an Andean-type Klamath-Sierran arc along the margin. (2) Oceanic crustal rocks were metamorphosed under HP/LT conditions in an inboard, east-inclined subduction zone from ~170-155 Ma. Except for the Red Ant blueschists, such lithologies remained stored at depth; most HP/LT mafic tectonic blocks returned surfaceward only during mid- and Late Cretaceous time, chiefly entrained in circulating, buoyant Franciscan mud-matrix melange. (3) By ~165 Ma and continuing to ~150-140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (4) At ~140, immediately prior to the onset of paired Franciscan and Great Valley Group (GVG) + Hornbrook deposition, the Klamath salient was deformed and displaced ~100–150 km westward relative to the Sierran arc, stranding pre-existing oceanic crust on the south as the Coast Range Ophiolite (CRO). (5) After the end-of-Jurassic seaward step-out of the Farallon-North American convergent plate junction, terrigineous debris began to be deposited in the outboard Franciscan trench and intervening Great Valley forearc. (6) Voluminous sedimentation and accretion of Franciscan Eastern + Central belts and GVG detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ~125-80 Ma. (7) Sierran arc volcanism-plutonism ceased by ~80 Ma in northern California, signaling a transition to shallow, nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east. (8) Presently exposed Paleogene-lower Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm unaffected by HP/LT subduction. (9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan section. (10) Judging from petrofacies and zircon U–Pb data, the Franciscan Eastern Belt contains debris derived principally from the Sierra Nevada and Klamath ranges; detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in some Central Belt sandstones, whereas clasts from the Idaho Batholith, Challis volcanics, and Cascade Range appear in progressively younger Paleogene-lower Miocene Coastal Belt sediments. (11) Gradual NW dextral offset of the Franciscan trench deposits of as much as ~1,600 km may have occurred relative to the native GVG forearc and basement terranes of the American Southwest.