ABSTRACT The California Coast Ranges and Central Valley form a single transpressional “orogenicfloat,” with strike-slip and thrust wedging mainly decoupled from each other, but linked through a deep master decollement. In the Coast Ranges, imbricate thrusting has shuffled Mesozoic and Cenozoic rocks into a complex structural stack, which is also transected by strike-slip faults by the San Andreas system. In the northern Coast Ranges, thrusts and related ramp anticlines strike north-south to north-northwest-south-southeast, plunge south-southeast, and are cut locally by west-vergent thrusts. The amplitude of these structures diminishes eastward. Surface and subsurface data lead us to interpret the Sacramento Valley and Coast Range structures as a unified system in which strike-slip and thrust motions are largely decoupled. West-vergent thrusts are backthrusts from a master, east-vergent blind detachment that underlies the entire Coast Ranges and the western Sacramento Valley. The easternmost backthrusts in the Sacramento Valley rise from near the tip of the detachment, and together with it form a thrust wedge. Geophysical, radiometric, structural, and stratigraphic data suggest that this and similar wedges may have been active since at least the Paleocene. At present, the wedge is propagating eastward into the Sacramento Valley, forming rising anticlines above its ramps and backthrusts. Strike-slip faults of the San Andreas system also are detached at the level of themaster decollement, which therefore is an oblique-slip fault with strike-slip displacement increasing westward. Strike-slip movement is mainly decoupled from thrust movement, as shown by the general absence of high topography along the most active strike-slip strands.

ABSTRACT The northern Coast Ranges and Sacramento Valley of California are underlain by Mesozoic and Cenozoic forearc-basin rocks of the Great Valley Group, the underlying Coast Range ophiolite, and the underthrust Franciscan subduction complex, along with overlying Cenozoic shallow-marine to continental and volcanic rocks. West- to soutbwest-vergent imbricate thrusting has shuffled these rocks into a complex structural stack. Thrusts and ramp anticlines strike N-S to NW-SE and generally verge west-and southwest-ward, and exhibit attendant ramp anticlines, lateral ramps, and backthrusts. Structural relief ranges from 1 to about 15 km and generally diminishes eastward. This structural assemblage is typical of fold-and-thrust-belts throughout the world; in California, it is similar to that in the Transverse Ranges, southern Coast Ranges, and western San Joaquin valley. Data from earthquake seismology, seismic reflection profiling, and structural geology, as well as the analogy to the San Joaquin Valley, suggest that the surface west-vergent thrusts are back-thrusts rising from a deep, east-vergent master thrust that passes entirely beneath the Coast Ranges. This thrust system thus constitutes a crustal-scale thrust wedge beneath a system of multiple, imbricated roof thrusts. The wedge tip is propagating eastward into the Sacramento Valley, forming young folds and causing uplift, tilting, and active seismicity; the wedge tip presently lies beneath the central part of the Valley. We suggest that the northern Coast Ranges are underlain by a west-dipping, crustal-scale thrust ramp that rises eastward from a deep detachment to the more gently west-dipping base of the thrust wedge visible at depths of 4-6 km in seismic-reflection profiles beneath the west side of the Sacramento Valley. The base of this crustal-scale ramp lies near the coast, and its top approximately beneath the abrupt mountain front of the northern Coast Ranges. Active thrust deformation, as determined by modern seismicity, is largely, although not completely, confined to the region near the coast and to the western Sacramento Valley. We suggest that relative movement between thrust slices is largely limited to the regions near the base of the crustal ramp and to the eastward-propagating wedge tip. We speculate that, in the Coast Range interior, rocks within the wedge are presently being carried up the crustal ramp without much internal shortening, continuously regenerating the young, rugged topography of the Coast Ranges with little attendant seismicity. The Central Valley of California, once a forearc basin, is now an active foreland basin that is subsiding under thrust loading by the Coast Range wedge. Syn-orogenic sediments shed eastward into the Valley from the Coast Ranges are being involved in the eastward-propagating thrusting. Thrust loading by the wedge may cause flexural normal faulting in the basement beneath the Valley. In the Coast Ranges, strike-slip faults of the San Andreas system locally disrupt the thrust structures, but do not appear to be responsible for most structural or topographic relief. Within the wedge, the relationship between the strike-slip faults and the imbricate thrusts is unclear, but hypocenter plots and cross-sections suggest that the strike-slip faults coincide with the thrusts in the near-surface, but diverge downward from them and dip more steeply. Geometric considerations lead us to suggest that the strike-slip faults are also detached along the basal fault of the wedge, which thus forms a gently dipping, oblique-slip segment of the boundary separating the Pacific and North American plates. In this interpretation, the Sacramento Valley and northern California Coast Ranges form a transpressional “orogenic float”, a unified system of faults in which strike-slip and thrust motions are largely decoupled from each other at the surface but are linked through a deep decollement. There is stratigraphic, structural, and radiometric evidence for earlier uplift of similar style within the region, and we suggest that there may have been earlier episodes of thrust wedging during the subduction regime that preceded the present transpressional environment. During this early wedging, accretionary-prism rocks were thrust into the forearc basin, a process seen in many modern subduction zones. Such wedging, when combined with out-of-sequence thrusting, provides an attractive mechanism for the uplift of Franciscan high-pressure metamorphic rocks, and an alternative to the widely espoused attenuational-normal-faulting model. There is no obvious record in the thrust structures of the transition from subduction to transpression, and it is entirely possible that wedging was continuous during the transition. The structural model we propose implies that thrust-related hydrocarbon traps may be important in the western Sacramento Valley and Coast Ranges, and moreover that significant unexplored volumes of potentially productive rocks may exist beneath Coast Range thrust faults. Our thermal studies of Coast Range rocks, originally conducted to date the young uplift and to study section omission along faults, suggest that Franciscan rocks may be the sources of much of the oil in the Coast Range interior and along the base of the Great Valley Group homocline. Natural gas in the Sacramento Valley may have been derived largely from the maturation of voluminous but finely disseminated wood. Quaternary rocks are involved in these structures, some of which have been historically seismic. Thus, the thrust structures, as well as the better-known strike-slip faults, may pose serious seismic hazards for the eastern Coast Ranges and western Sacramento Valley. “…folds, rather than the more spectacular San Andreas fault, are the key…” -- Ben Page, 1966 , p. 275.

The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage. Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here. Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc. Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny. Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.