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 southwest-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.