ABSTRACT Field relationships in the Franciscan Complex of California suggest localization of subduction slip in narrow zones (≤300 m thick) at the depths of ~10–80 km. Accretionary and non-accretionary subduction slip over the ca. 150 Ma of Franciscan history was accommodated across the structural thickness of the complex (maximum of ~30 km). During accretion of a specific unit (<5 Ma), subduction slip (accretionary subduction slip) deformed the full thickness of the accreting unit (≤5 km), primarily on discrete faults of <20 m in thickness, with the remainder accommodated by penetrative deformation. Some faults accommodating accretionary subduction slip formed anastomosing zones ≤200 m thick that resulted in block-in-matrix (tectonic mélange) relationships but did not emplace exotic blocks. Mélange horizons with exotic blocks range in thickness from 0.5 m to 1 km. These apparently formed by sedimentary processes as part of the trench fill prior to subsequent deformation during subduction-accretion. Accretionary subduction slip was localized within some of these mélanges in zones ≤300 m thick. Such deformation obscured primary sedimentary textures. Non-accretionary subduction faults separate units accreted at different times, but these <100-m-thick fault zones capture a small fraction of associated subduction slip because of footwall subduction and likely removal of hanging wall by subduction erosion. Most exhumation was accommodated by discrete faults ≤30 m thick. Structural, geochronologic, and plate motion data suggest that of the ~13,000 km of subduction during the ca. 150 Ma assembly of the Franciscan Complex, ~2000 km was associated with accretion.

Abstract The tectonic framework of NW Himalaya is different from that of the central Himalaya with respect to the position of the Main Central Thrust and Higher Himalayan Crystalline and the Lesser and Sub Himalayan structures. The former is characterized by thick-skinned tectonics, whereas the thin-skinned model explains the tectonic evolution of the central Himalaya. The boundary between the two segments of Himalaya is recognized along the Ropar–Manali lineament fault zone. The normal convergence rate within the Himalaya decreases from c. 18 mm a −1 in the central to c. 15 mm a −1 in the NW segments. In the last 800 years of historical accounts of large earthquakes of magnitude M w ≥ 7, there are seven earthquakes clustered in the central Himalaya, whereas three reported earthquakes are widely separated in the NW Himalaya. The earthquakes in central Himalaya are inferred as occurring over the plate boundary fault, the Main Himalayan Thrust. The wedge thrust earthquakes in NW Himalaya originate over the faults on the hanging wall of the Main Himalayan Thrust. Palaeoseismic evidence recorded on the Himalayan front suggests the occurrence of giant earthquakes in the central Himalaya. The lack of such an event reported in the NW Himalaya may be due to oblique convergence.

ABSTRACT Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.