LATE JURASSIC-EARLY MIOCENE CLASTIC SEDIMENTATION AND PLATE-TECTONIC EVOLUTION OF NORTHERN CALIFORNIA—A CLASSIC ACCRETIONARY MARGIN

Subduction metamorphism, landward arc construction, westward relative migration of the Klamaths, and U-Pb sediment ages, sources, and areal disposition of zircon-bearing strata constrain the geologic development of northern California: (1) At ~175 Ma, transpressive underflow initiated the Klamath/Sierra Nevada igneous arc along the continental margin. (2) Oceanic crust recrystallized at HP/LT in an east-dipping subduction zone from ~170-155 Ma. Most rocks remained stored at depth; tectonic blocks of eclogite + mafic blueschist chiefly returned surfaceward during mid- and Late Cretaceous entrainment in circulating, buoyant Franciscan mud-matrix mélange. (3) By ~165 Ma, continuing to ~150-140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (4) At ~140, prior to the onset of paired Franciscan and Great Valley Group (GVG) + Hornbrook deposition, the Klamath salient was deformed/displaced ~100-200 km seaward relative to the Sierran arc, stranding the pre-existing Coast Range Ophiolite on the south. (5) After an end-of-Jurassic seaward step-out of the Farallon-North American convergent plate junction, clastics began to accumulate in the outboard Franciscan trench and medial GVG. (6) At ~125-80 Ma, massive sedimentation/accretion of Franciscan Eastern + Central Belt and GVG detritus occurred during rapid, nearly orthogonal plate convergence and paroxysmal arc activity. (7) Sierran magmatism ceased by ~80 Ma, signaling transition to shallow, nearly subhorizontal eastward plate underflow attending the Laramide orogeny. (8) Paleogene-lower Miocene Franciscan Coastal Belt strata accreted in a tectonic realm unaffected by HP/LT subduction. (9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan clastics. (10) Based on petrofacies and zircon U-Pb ages, the Franciscan Eastern Belt was sourced mainly from the Klamath/Sierran ranges; detritus from the Idaho + Sierra Nevada batholiths is 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. This suggests possible NW dextral offset of Franciscan trench deposits of up to ~1600 km relative to the native GVG forearc and North American basement.