Content Previous Next

GREAT VALLEY AND SIERRAN FOOTHILLS

Rocks of the Great Valley are known from exposures in an upturned section on the east side of the Diablo Range and from wells. The upturned section rests structurally above the Franciscan assemblage on the low-angle Coast Range fault, although in many places this relation is obscured by younger high-angle faults. This upturned section includes, from oldest to youngest, Middle and Late Jurassic Coast Range ophiolite and a related tuffaceous unit; Upper Jurassic and Cretaceous Great Valley sequence, which is chiefly forearc flysch; lower Cenozoic postarc marine and terrestrial sedimentary rocks; and upper Cenozoic continental-arc sedimentary rocks (Maddock, 1964; Evarts, 1977; Bartow and others, 1985).

At the latitude of transect C2, the Coast Range ophiolite is interpreted to be a (rifted) island-arc assemblage because it contains abundant silicic volcanic and intrusive rocks (Bailey and Blake, 1974; Evarts, 1977; Hopson and others, 1981; McLaughlin and others, 1988b). Its contact with the overlying sedimentary rocks, though faulted in most places, is believed to be fundamentally depositional (Bailey and others, 1970); on transect C2 it is demonstrably depositional (Evarts, 1977).

The Great Valley sequence and younger rocks exposed in the upturned section in the eastern Diablo Range appear to be nearly twice as thick as the section of sedimentary rocks penetrated in wells farther east in the Great Valley (Figure. 8.4). Some of this apparent westward thickening results from the stratigraphic addition of older rocks to the basal part of the section in the west; some may be caused by imbrication along thrust faults (Wentworth and others, 1984). Similar apparent thickening west of the synclinal axis of the Great Valley has been documented in other localities as well. In the southern Great Valley, Wentworth and others (1984) indicated an apparent doubling of thickness west of the axis, and in the northern Great Valley, an apparent trebling of thickness.

Most of the basement rocks that have been penetrated by wells in the Great Valley have been identified as granitic rocks (Saleeby, 1986). Rocks exposed in the Sierran foothills, east of the Great Valley, may be related to basement rocks beneath the Great Valley, but they are not so dense or magnetic (see below; Figure. 8.4A).

Deep structure along transect C2 in the Great Valley has been elaborated in some detail by Colburn and Mooney (1986), Holbrook and Mooney (1987), and Dean Whitman and others (unpub. data, 1985) from seismic-refraction data, and by Wentworth and others (1987) primarily from seismic-reflection data. Seismic velocities in the sedimentary section range from 1.6 to 4.1 km/s where these velocities can be clearly ascribed to sedimentary rocks, such as near well No. 1 (Figure. 8.4A). In the eastern Diablo Range, velocities as high as 4.7 km/s may also be due to sedimentary rocks (Figure. 8.4A). East of the synclinal axis in the Great Valley, reflections within the sedimentary section are subparallel to the top of basement, which is marked by the disappearance of reflections (f, Figure. 8.4A). West of the synclinal axis, these reflections (d, Figure. 8.4A) diverge slightly from the inferred top of basement (e, Figure. 8.4A).

Beneath the sedimentary rocks of the Great Valley are several layers of increasing seismic velocity: a 5.5- to 5.7-km/s layer, 1.5 to 2 km thick (layer 1), a 6.0- to 6.3-km/s layer, 2.5 to 6 km thick (layer 2); a 6.6- to 6. 75-km/s layer, 4 to 7 km thick (layer 3); and a 6.9-to 7.2-km/s layer, about 7 km thick (layer 4) (Figure. 8.4A). In addition, there is a thin, laterally discontinuous 7.0-km/s layer embedded in the top of layer 3. Well No. 1 indicates that layer 1 is granitic rocks. Farther west, however, this layer may be interpretable either as granitic rocks or as Franciscan assemblage, which have similar velocities at this depth (Figure. 8.5). In the original data of Colburn and Mooney (1986) and Holbrook and Mooney (1987), there is no perceptible reflection from an interface between layers 1 and 2 (as there is, for example, between layer 1 and the overlying sedimentary rocks), and so these two layers may, in fact, grade into one another. Layer 2 could then also be granitic rocks, and layers 1 and 2 together would constitute a velocity-depth section similar, for example, to upper crust of the batholithic Salinian block (Figures. 8.4A, 8.5). Layers 3 and 4 (6.6-7.2 km/s), which are analogous to the lower-crustal layer in the Diablo Range (6.7-7.1 km/s), may represent the middle and lower crust of accreted island arc(s) and (or) oceanic crust. The Moho is well documented at about 27-km depth. Deep reflection data beneath the Great Valley (Wentworth and others 1987) indicate a conspicuous east-dipping band of reflections (g, Figure. 8.4A) and less conspicuous subhorizontal and west-dipping reflectors.

Rocks of the Sierran foothills consist of Lower to Upper Jurassic mafic to felsic volcanic and plutonic rocks and related sedimentary rocks (argillite, chert, and flysch) that were accumulated or emplaced in an island-arc setting (Clark, 1964; Schweickert and Cowan, 1975; Saleeby, 1982; Schweickert and Bogen, 1983). The basement and metamorphic wallrocks for the intrusive rocks are tectonically disrupted and polymetamorphosed Paleozoic ophiolitic rocks (approx 300 Ma; Saleeby, 1982).

The island arc(s) in which the Jurassic rocks of the Sierran foothills were formed collapsed against the margIn of the North American Continent during the Late Jurassic Nevadan orogeny (Jones and others, 1976). How this collapse occurred is problematic. Steeply east-dipping faults and upright antiforms are seen in the Sierran foothills, but a study by Moores and Day (1984) of surface relations 300 km north of transect C2 indicates obduction of the arc(s) on west-dipping thrust faults. These rocks were intruded during the Early Cretaceous by mafic to intermediate plutons belonging to the western phase of Sierra Nevada plutonism (Evernden and Kistler, 1970).

The deep structure of the Sierran foothills is known from the reconnaissance seismic-refraction experiment of Spieth and others (1981), the reflection profiling of Zoback and Wentworth (1986), and the compilation of reflection, refraction, and potential-field results by Wentworth and others (1987). The refraction data can be modeled with a 6.2-km/s basement from near the surface to about 30-km depth, a 6. 6-km/s lower crust, and a Moho at 39-km depth. Other models are possible, however, and the Moho may be as shallow as 30 km (Spieth and others 1981). We have projected the seismic-reflection results and gravity/magnetic boundary of Wentworth and others (1987) from 45 to 60 km southward onto transect C2. Two conspicuous west-dipping sets of reflections are visible as well as a few subhorizontal reflectors. The gravity/ magnetic boundary, however, has a moderate eastward dip.

Our projection of the results of Wentworth and others (1987) is uncertain not only because of the distances involved but also because their profile terminates on the east in an area that is anomalous both geologically and geophysically. In this area, batholithic rocks (trondhjemite) engulf most accreted rocks of the Sierran foothills (Jennings, 1977) and are associated with a gravity low (Oliver and others, 1980). Our projection, however, may be defensible as follows. (1) The batholithic rocks responsible for the gravity low probably do not extend below 10-km depth (R. C. Jachens, oral commun., 1988); most of the reflectors that we have projected are largely below that depth. (2) The modeled gravity/magnetic boundary is approximately similar in shape throughout the length of the Great Valley (Andrew Griscom, oral commun., 1988); in our projection, we have attempted to correct for the difference in azimuth between transect C2 and the profile of Wentworth and others (1987) by assuming a strike parallel to the Great Valley.

Given the geologic and seismic constraints discussed above, we have interpreted the cross section through the Great Valley and Sierran foothills (Figure. 8.4B), using some of the ideas of Wentworth and others (1984, 1987) for the configuration of an inferred tectonic wedge of Franciscan rocks, and some of the ideas of Saleeby (1986) for structure within crystalline rocks. The uppermost part of our cross section (to approx 2-km depth) on the east flank of the Diablo Range (Figure. 8.4A) was supplied by R.C. Evarts (written commun., 1989). Below this area, we have added a hypothetical west-dipping thrust fault to bring the Great Valley sequence beneath the easternmost block of the Coast Range ophiolite and to grossly satisfy the velocity constraints of Dean Whitman and others (unpub. data, 1985; Figure. 8.4A). East of the Coast Range ophiolite, we postulate thrust faults that largely follow bedding planes in the upturned section of the Great Valley sequence, similar to those postulated by Wentworth and others (1984) for the northern Great Valley. These "backthrust" faults are required for emplacement of the wedge and help explain the thickening of the Great Valley sequence in the western limb of the syncline (see section below entitled "Discussion-Tectonic Wedging"). From the easternmost backthrust fault in the Great Valley to the San Andreas fault, we have modeled the discontinuity between variably reflective rocks of lower velocity (Franciscan assemblage, Coast Range ophiolite, and Great Valley sequence; 1.7-5.8 km/s) and poorly reflective rocks of higher velocity (mafic rocks of the Diablo Range and crystalline basement of the Great Valley; 5.5-6.8 km/s) as the floor thrust fault of the wedge. Wentworth (1987) presented a similar interpretation.

The details of composition and structure in the crystalline rocks beneath the Great Valley and Sierran foothills are speculative. Saleeby (1986) interpreted these rocks to consist fundamentally of slabs or nappes of island-arc and oceanic rocks obducted along west-dipping Nevadan thrust faults intruded by chiefly Early Cretaceous Sierran granitic plutons. We have adopted this basic scheme and added some details, interpreting layers 1 and 2 in the basement beneath the Great Valley (5.5-6.3 km/s; see above) as post-Nevadan felsic plutonic rocks, although, as noted above, the western part of layer 1 (5.5 km/s) may be Franciscan assemblage. We interpret the east-dipping gravity/magnetic boundary of Wentworth and others (1987) as the average top of mafic crust (pre-Nevadan gabbro, diabase, or basalt) in the inferred obducted sequence. Alternatively, this boundary may be the average top of mafic, magnetic intrusions in the crust (post-Nevadan gabbro) or the average base of felsic, nonmagnetic intrusions (post-Nevadan granitic rocks). At the location where this boundary was actually modeled, it may be the average base of a large trondhjemite intrusion. We associate the east-dipping reflections beneath the central Great Valley (g, Figure. 8.4A) with the thin discontinuous 7.0-km/s layer of Holbrook and Mooney (1987), although the depth correspondence is imperfect and we interpret this feature as a gabbroic dike. Alternatively, these east-dipping reflections may represent an east-dipping fault zone. Following Saleeby (1986), we correlate the upper and lower west-dipping bands of reflections in the eastern Great Valley and Sierran foothills (h, j, Figure. 8.4A) with the Bear Mountain and Melones fault zones, which may represent Cenozoic reactivations of inferred west-dipping Nevadan thrust faults.