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DISCUSSION - TECTONIC WEDGING
GEOLOGIC HISTORY

Wentworth and others (1984) interpreted the juxtaposition of Franciscan assemblage and a coeval section consisting of Coast Range ophiolite and Great Valley sequence as having occurred during landward movement of the Franciscan assemblage as a tectonic wedge. They reinterpreted the "Coast Range thrust fault" of Bailey and others (1970), a subduction megathrust between the Coast Range ophiolite and the Franciscan assemblage, as the roof thrust of the wedge. More recently, the thrust nature of the "Coast Range thrust fault" has been reevaluated. Jayko and others (1987), testing an hypothesis by Platt (1986), produced abundant evidence that the contact between Franciscan assemblage and Coast Range ophiolite is a detachment surface along which the upper plate was extended during uplift of the Franciscan assemblage. Their evidence is the consistent attenuation, as opposed to repetition, of geologic section across this discontinuity and associated faults above it. They proposed the term "Coast Range fault" for this discontinuity which we adopt here. Evidence of attenuation is present even on transect C2, in that the two outcrops of the Coast Range ophiolite in the eastern Diablo Range (Figure. 8.4A) represent an abridged section of ophiolite: The western outcrop is partially serpentinized ultramafic rock of the basal part of an ophiolite, whereas the eastern outcrop is the sill complex and volcanic flows of the upper part of an ophiolite. These two parts of the ophiolite are now juxtaposed across the crooked, steeply dipping Tesla-Ortigalita fault. Although this fault now offsets the Coast Range fault, it may represent reactivation of a normal fault that originally soled into the Coast Range fault (compare Raymond, 1973).

The extensional nature of the Coast Range fault poses several problems for emplacement of the Franciscan assemblage as a tectonic wedge. Where is the roof thrust fault of the wedge? How did the Franciscan assemblage reach its current position with an extended overlying section of the Coast Range ophiolite and Great Valley sequence? Was the Franciscan assemblage uplifted from beneath the western Great Valley? The apparent continuity between the Great Valley basement and the 6.7- to 7.1-km/s layer in the Diablo Range indicates a negative answer to the last question.

These problems can be solved if the extensional event was separated in time and space from the compressional event, or tectonic wedging. Jayko and others (1987) reviewed the published evidence regarding the geologic history of extensional faulting. In one place, the Coast Range fault and associated faults are overlapped by sedimentary rocks of Oligocene and younger age, and in another place by sedimentary rocks of Paleocene and younger age. The occurrence of detritus derived from the Franciscan assemblage in Paleocene and Eocene strata of the Coast Ranges (Dickinson, 1966; Berkland, 1973) indicates that the lower plate was exposed by the early Tertiary. Jayko and others (1987) inferred that uplift of the Franciscan assemblage and associated extensional faulting in the upper plate occurred during the Late Cretaceous and (or) early Tertiary.

The history of compressional tectonics in the Coast Ranges is sparse and varies from place to place. In the northern Coast Ranges, thrust faulting and folding began during the early Tertiary (Blake and others, 1987; M.C. Blake, Jr., oral commun., 1989), and compressional deformation is continuing today in rocks of the northern Great Valley (Harwood and Helley, 1987). In the southern Coast Ranges, at least four Cenozoic deformations or uplifts, indicated by unconformities or eastward-migrating depocenters, have ages of late Paleocene, late Eocene to early Miocene, late Miocene, and late Pliocene (Namson and Davis, 1988; Namson and others, 1990; Rentschler and Bloch, 1988). Modern thrust faulting and folding still is occurring, as indicated by the 1983 Coalinga earthquake (see chap. 5; Eaton, 1990).

Landward movement of the Franciscan assemblage as a wedge may have even begun in the Mesozoic. In the northern Coast Ranges, several northwest-striking faults (Paskenta, Elder Creek, and Cold Fork faults) offset rocks structurally above the Franciscan assemblage (but not the Franciscan assemblage itself) and represent major discontinuities in the depositional environment of the Great Valley sequence (Jones and Irwin, 1971). These faults, which have displacements of tens of kilometers to as much as 100 km, are interpreted to have moved primarily during the Cretaceous (Jones and Irwin, 1971), although the latest limit on the time of movement is about 3.4 Ma (Hardwood and Helley, 1987; M.C. Blake, Jr., oral commun., 1989). Wentworth and others (1984) and Jayko and others (1987) interpreted these faults as tear faults in the plate structurally above a wedge of Franciscan assemblage.

In light of the above data and interpretations, we postulate (1) that uplift of the Franciscan assemblage and extension of the upper plate, consisting of Coast Range ophiolite and Great Valley sequence, occurred during the Cretaceous (or, at the latest, during the early Tertiary, if Cretaceous movement on the Paskenta-Cold Fork fault system is not linked to landward wedge transport) well west of the present Diablo Range; and (2) that a tectonic wedge of Franciscan assemblage was subsequently driven landward, with the extended upper plate riding passively atop it. This wedge is interpreted to have moved along a floor thrust fault aligned with the contact between the Great Valley sequence and its crystalline basement. To the west of the present Diablo Range, where movement initiated, the basement was an outboard part of the Coast Range ophiolite. Beneath the Great Valley, where the movement is presently occurring, the basement is similar to the Coast Range ophiolite but contains numerous younger plutons. A roof thrust fault apparently developed only near the east tip of the wedge (Figure. 8.4B); presumably, erosion kept pace with uplift near the tip. Differential vertical or horizontal movements of the wedge may have produced tear faults, such as the Paskenta, Elder Creek, and Cold Fork faults and may have reactivated extensional faults to produce complex faults, such as the Tesla-Ortigalita fault.