ABSTRACT This study presents three regional cross sections, a structural map analysis, and a schematic map restoration. The sections are constrained by surface geology and petroleum wells and were developed using model-based methods to be consistent with the regional tectonic context and balancing concepts. Together, these products depict the geometry and kinematics of the major fault systems. Insights from this research include the following. Franciscan complex blueschist-facies rocks in the Mount Diablo region were unroofed west of their current location and subsequently thrust beneath the Great Valley sequence in the mid-Eocene. East Bay structures are complicated by overprinting of Neogene compression and dextral strike-slip motion on a Paleogene graben system. Net lateral displacement between the Hayward fault and the Central Valley varies from 26 km toward 341° to 29 km toward 010° in the southern and northern East Bay Hills, respectively. Uplift above a wedge thrust generates the principal Neogene structural high, which extends from Vallejo through Mount Diablo to the Altamont Ridge. Anomalous structural relief at Mount Diablo is due to strike-parallel thrusting on the crest of a fault-propagation fold formed on the west-verging roof thrust. Uplift that exposes the Coast Range ophiolite in the East Bay Hills is formed by oblique thrusting generated by slip transfer at the northern termination of the Calaveras fault. The Paleogene extensional fault system likely extends farther west than previously documented. An east-dipping branch of that system may underlie the Walnut Creek Valley. Three-dimensional restoration should be applied to constrain geologic frameworks to be used for seismic velocity modeling.

ABSTRACT The basic stratigraphic and structural framework of Mount Diablo is described using a revised geologic map, gravity data, and aeromagnetic data. The mountain is made up of two distinct stratigraphic assemblages representing different depocenters that were juxtaposed by ~20 km of late Pliocene and Quaternary right-lateral offset on the Greenville-Diablo-Concord fault. Both assemblages are composed of Cretaceous and Cenozoic strata overlying a compound basement made up of the Franciscan and Great Valley complexes. The rocks are folded and faulted by late Neogene and Quaternary compressional structures related to both regional plate-boundary–normal compression and a restraining step in the strike-slip fault system. The core of the mountain is made up of uplifted basement rocks. Late Neogene and Quaternary deformation is overprinted on Paleogene extensional deformation that is evidenced at Mount Diablo by significant attenuation in the basement rocks and by an uptilted stepped graben structure on the northeast flank. Retrodeformation of the northeast flank suggests that late Early to early Late Cretaceous strata may have been deposited against and across a steeply west-dipping basement escarpment. The location of the mountain today was a depocenter through the Late Cretaceous and Paleogene and received shallow-marine deposits periodically into the late Miocene. Uplift of the mountain itself happened mostly in the Quaternary.

ABSTRACT Franciscan subduction complex rocks of Mount Diablo form a 8.5 by 4.5 km tectonic window, elongated E-W and fault-bounded to the north and south by rocks of the Coast Range ophiolite and Great Valley Group, respectively, which lack the burial metamorphism and deformation displayed by the Franciscan complex. Most of the Franciscan complex consists of a stack of lawsonite-albite–facies pillow basalt overlain successively by chert and clastic sedimentary rocks, repeated by faults at hundreds of meters to <1 m spacing. Widely distributed mélange zones from 0.5 to 300 m thick containing high-grade (including amphibolite and eclogite) assemblages and other exotic blocks, up to 120 m size, form a small fraction of exposures. Nearly all clastic rocks have a foliation, parallel to faults that repeat the various lithologies, whereas chert and basalt lack foliation. Lawsonite grew parallel to foliation and as later grains across foliation. The Franciscan-bounding faults, collectively called the Coast Range fault, strike ENE to WNW and dip northward at low to moderate average angles and collectively form a south-vergent overturned anticline. Splays of the Coast Range fault also cut into the Franciscan strata and Coast Range ophiolite and locally form the Coast Range ophiolite–Great Valley Group boundary. Dip discordance between the Coast Range fault and overlying Great Valley Group strata indicates that the northern and southern Coast Range fault segments were normal faults with opposite dip directions, forming a structural dome. These relationships suggest accretion and fault stacking of the Franciscan complex, followed by exhumation along the Coast Range fault and then folding of the Coast Range fault.

ABSTRACT Late Cenozoic growth of the Mount Diablo anticline in the eastern San Francisco Bay area, California, USA, has produced unique 3D exposures of stratigraphic relationships and normal faults that record Late Cretaceous uplift and early Tertiary extension in the ancestral California forearc basin. Several early Tertiary normal faults on the northeast flank of Mount Diablo have been correlated with structures that accommodated Paleogene subsidence of the now-buried Rio Vista basin north of Mount Diablo. Stepwise restoration of deformation at Mount Diablo reveals that the normal faults probably root into the “Mount Diablo fault,” a structure that juxtaposes blueschist-facies rocks of the Franciscan accretionary complex with attenuated remnants of the ophiolitic forearc basement and relatively unmetamorphosed marine forearc sediments. This structure is the local equivalent of the Coast Range fault, which is the regional contact between high-pressure Franciscan rocks and structurally overlying forearc basement in the northern Coast Ranges and Diablo Range, and it is folded about the axis of the Mount Diablo anticline. Apatite fission-track analyses indicate that the Franciscan rocks at Mount Diablo were exhumed and cooled from depths of 20+ km in the subduction zone between ca. 70−50 Ma. Angular unconformities and growth relations in the Cretaceous and Paleogene stratigraphic sections on the northeast side of Mount Diablo, and in the Rio Vista basin to the north, indicate that wholesale uplift, eastward tilting, and extension of the western forearc basin were coeval with blueschist exhumation. Previous workers have interpreted the structural relief associated with this uplift and tilting, as well as the appearance of Franciscan blueschist detritus in Late Cretaceous and early Tertiary forearc strata, as evidence for an “ancestral Mount Diablo high,” an emergent Franciscan highland bordering the forearc basin to the west. This outer-arc high is here interpreted to be the uplifted footwall of Coast Range fault. The stratigraphic and structural relations exposed at Mount Diablo support models for exposure of Franciscan blueschists primarily through syn-subduction extension and attenuation of the overlying forearc crust in the hanging wall of the Coast Range fault, accompanied by (local?) uplift and erosion of the exhumed accretionary prism in the footwall.

ABSTRACT The Franciscan Complex of California, the type example of an exhumed accretionary complex, records a protracted history of voluminous subduction accretion along the western margin of North America. Recent geochronological work has improved our knowledge of the timing of accretion, but the details of the accretionary history are disputed, in part, due to uncertainties in regional-scale correlations of different units. We present new detrital zircon U-Pb ages from two sites on opposite sides of San Francisco Bay in central California that confirm previously proposed correlations. Both sites are characterized by a structurally higher blueschist-facies unit (Angel Island unit) underlain by a prehnite-pumpellyite-facies unit (Alcatraz unit). The Angel Island unit yields maximum depositional ages (MDAs) ranging from 112 ± 1 Ma to 114 ± 1 Ma (±2σ), and the Alcatraz unit yields MDAs between 94 ± 2 Ma and 99 ± 1 Ma. Restoration of post-subduction dextral displacement suggests these sites were originally 44–78 km apart and much closer to other Franciscan units that are now exposed farther south in the Diablo Range. Comparison with detrital zircon dates from the Diablo Range supports correlations of the Bay Area units with certain units in the Diablo Range. In contrast, correlations with Franciscan units in the northern Coast Ranges of California are not robust: some units are clearly older than those in the Bay Area whereas others exhibit distinct differences in provenance. Integration of age data from throughout the Franciscan Complex indicates long-lived and episodic accretion from the Early Cretaceous to Paleogene. Although minor, sporadic accretion began earlier, significant accretion occurred during the interval 123–80 Ma and was followed by minor accretion at ca. 53–49 Ma. Periods of accretion and non-accretion were associated with arc magmatism in the Sierra Nevada–Klamath region, cessation of arc activity, and reorganization of paleodrainage systems, which implicates plate dynamics and sediment availability as major controls on the development of the Franciscan Complex.

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 Franciscan subduction complex formed over a protracted, ~150 m.y. period, during Late Jurassic to late Cenozoic subduction of oceanic lithosphere beneath the western margin of the North American continent. Growth of the complex occurred chiefly by progressive accretion, in which voluminous sediment was eroded from the magmatic arc and continent, deposited in the trench region, and then progressively subducted and accreted soon after deposition. The Yolla Bolly terrane, a major Franciscan subunit, has stood out as a possible exception to a progressive-accretion model. Yolla Bolly clastic rocks are almost barren of fossils, but there are ~13 localities with Late Jurassic and Early Cretaceous Buchia specimens, ~3 with mid-Cretaceous ammonites or Inoceramus , plus several with mid-Cretaceous youngest detrital-zircon populations. These ages had suggested that sediments may not have been deposited into an active trench, but instead were deposited into a relatively stable Yolla Bolly basin, which was both long-lived (ca. 150 Ma through ca. 95 Ma) and far-traveled (exotic). This basin was then accreted and metamorphosed at perhaps 92 Ma. It is surprising, however, that such a basin could have survived for ~50 m.y. along a subduction margin before being accreted. We determined detrital-zircon U-Pb ages from 31 new sandstone samples, including from key Buchia sites, and they indicate that Yolla Bolly clastic deposition actually occurred almost entirely between ca. 115 and 98 Ma. All of the Buchia specimens in the main parts of the Yolla Bolly terrane have been redeposited and the arc- and continent-sourced clastic rocks that comprise almost all of the terrane are much younger than once thought. This makes evolution of the Yolla Bolly terrane compatible with a progressive-accretion model, in which its constituent packets of clastic rocks were deposited in a native trench setting and then rapidly subducted, accreted, and metamorphosed.

ABSTRACT The Upper Cretaceous Las Tablas unit of the Franciscan Complex, a conglomerate-breccia containing a diverse array of clasts, is located in the central California Coast Ranges. The Las Tablas unit was originally deposited in southern California, where significant amounts of the western half of the Sierra Nevada batholith and coeval Great Valley forearc basin and basement are missing. The most likely explanation for this absence is that forearc and western arc assemblages were removed through a combination of surface and tectonic erosion that accompanied Laramide shallow subduction. Petrographic analysis of rounded to subrounded gabbro, quartz diorite, tonalite, granodiorite, and andesite clasts from the Las Tablas unit reveals a prehnite-pumpellyite–grade overprint of primary igneous textures. Furthermore, zircon grains derived from these clasts yield generally Late Jurassic to Early Cretaceous U-Pb ages and positive Hf isotopic values, with one sample yielding a Late Cretaceous age and a negative Hf value. These relations strongly suggest that the analyzed clasts experienced subduction zone metamorphism and were derived principally from the western and axial Sierra Nevada batholith, with possible additional input from forearc basement (the Coast Range ophiolite). The presence of western arc–derived detritus in the Las Tablas unit suggests that surface plus tectonic erosion removed a significant amount of these units and incorporated them into the subduction complex. Granitic clasts of the Las Tablas unit were likely introduced into previously subducted and exhumed Franciscan materials by sedimentary rather than tectonic processes.

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.

ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.