MID-MESOZOIC-PALEOGENE EVOLUTION OF THE NORTHERN AND CENTRAL CALIFORNIAN ACCRETIONARY MARGIN

Distributions of igneous arc rocks and HP/LT metamafic rocks, combined with U-Pb ages of detrital zircons and Jurassic-Miocene clastic sediment sources constrain geologic development of the Californian continental margin: (1) At ~175 Ma, transpressive plate underflow initiated construction of an Andean-type Klamath-Sierran arc astride the crustal margin. (2) Paleo-Pacific oceanic plate rocks recrystallized under HP/LT in an inboard, east-dipping subduction zone beneath the arc at ~170-155 Ma. Stored at depth, these metamafic blocks returned surfaceward in mid- and Late Cretaceous time as olistoliths + tectonic fragments entrained in circulating Franciscan mud-matrix mélanges. (3) By ~165 Ma and continuing to ~150 Ma, erosion of the volcanic arc supplied upper crustal debris to proximal Mariposa-Galice + Myrtle overlap strata. (4) At ~140 Ma, before onset of paired Franciscan and GVG + Hornbrook deposition, the Klamath salient moved ~80-100 km west relative to the Sierran arc, trapping old oceanic crust on the south as the CRO. (5) After end-of-Jurassic seaward step-out of the Farallon-American convergent plate junction, terrigeneous debris accumulated in the outboard Franciscan trench + inboard Great Valley forearc. (6) Voluminous deposition and accretion of Franciscan Eastern + Central belts and GVG detritus occurred during vigorous Sierran igneous activity on rapid, nearly orthogonal plate underflow starting at ~125 Ma. (7) Grenville-age detrital zircons are absent from post-120 Ma Franciscan sections. (8) Sierra Nevada volcanism-plutonism ceased by ~85 Ma, signaling transition to subhorizontal eastward plate underflow and Laramide orogeny farther east. (9) Exposed Paleogene Franciscan Coastal belt clastic strata accreted in a tectonic realm unaffected by HP/LT. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain debris derived from the Sierran + Klamath ranges; zircons from the Sierra Nevada ± Idaho batholith grains are present in Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstones. (11) Up to ~1600 km of post-Cretaceous NW dextral offset of Franciscan trench deposits may have occurred relative to native GVG + older terranes of the Californian margin.