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Home , Franciscan Complex, Great Valley Sequence

Structural interpretation of the southern part of the northern Coast Ranges and , : Summary

JOHN SUPPE Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540

The surface of the northern Coast Ranges of California lief on the contacts between these major tectonostratigraphic units is dominated by structurally complex Upper and Lower is one of the most striking features of the surface geology of the rocks of the , some of which have Coast Ranges. This relief is produced in the present solution by im- undergone high-pressure in the Late or bricate thrust faults that flatten within the upper crust and account . Strata of the coeval, but generally little-deformed, for about 175 km of Cenozoic horizontal shortening of the conti- (~10 km thick) are buried under Upper nental margin, as is shown by a restored section published with the Cenozoic nonmarine sedimentary rocks in the Sacramento Valley complete article. The shortening is large relative to the present but are fully exposed in the great homocline forming the boundary width of the Coast Ranges, which is about 110 km; nevertheless it between the Sacramento Valley to the east and the Coast Ranges to is small relative to the several thousand kilometres of Cenozoic the west (Fig. I).1 The Great Valley sequence and its ophiolitic to the west with which this thrusting was presumably are also exposed across the entire Coast Ranges in coeval (Atwater, 1970). Subduction stopped at the latitude of the numerous small fragments immersed in the predominantly Francis- cross section about 5 m.y. ago as the Mendocino triple junction can terrain; these fragments have been generally interpreted as ero- passed northward (Atwater, 1970). The thrusting took place at sionally isolated klippen of a regional "Coast Range thrust" that high-fluid pressure (Berry, 1973). places the Great Valley sequence over the more metamorphosed The section (Fig. 1) was constructed as part of the Cross-Sections and deformed coeval Franciscan Complex (see Bailey and others, Project of the Plate Margins Group, U.S. Geodynamics Committee, 1964). the purpose of which is to summarize present knowledge and to In contrast with the dominance of Mesozoic Franciscan Rocks in point the way toward future research. As a guiding principle, the the surface geology of the Coast Ranges, a new structural interpre- cross section was made as detailed and specific as possible, even at tation that involves a relatively small amount of Mesozoic Francis- some expense of certainty. In this spirit, a number of the details, can rocks at depth has been developed during the construction of a particularly at greater depth, are merely permissible solutions in deep, retrodeformable cross section of the Coast Ranges and Sac- light of present knowledge and may be proved or disproved with ramento Valley ~100 km north of (Fig. 1). By this future data. new solution, much of the upper crust of the Coast Ranges is oc- The confidence with which the cross-sectional solution is appro- cupied by rocks of the Great Valley sequence and of the Upper Cre- priately viewed should reflect an understanding of the quality of the taceous and Cenozoic "Coastal-Belt" Franciscan, which lie struc- primary data and the methods and assumptions used in the con- turally below relatively thin (2 to 5 km) west-vergent thrust sheets struction of the section. These are discussed in the primary publi- of Mesozoic Franciscan rocks. cation, but it is important that the methods and assumptions be The thrust sheets of the new solution are not the result of any summarized here. Mesozoic subduction and metamorphism of the Franciscan Com- The near-surface structure was constructed by direct projection plex because the structure involves Lower Cenozoic strata both along the dip or plunge. Much of what is shown in the upper 5 km within the "Coastal-Belt" Franciscan and overlying the Great Val- and above the ground surface may be seen directly in distorted ley sequence. The Cenozoic thrusting is superposed on the major form if the map is viewed down the regional southeasterly plunge. Mesozoic structure for which the Franciscan is better known. The Some of the imbricate structure may be seen directly in map view main episode of thrusting represented on the cross section is later (see discussion of Suppe and Foland, 1978). In addition to the than the emplacement of the Great Valley sequence over the thrust faults, young high-angle faults, roughly parallel with the San Mesozoic Franciscan Complex and the emplacement of the Andreas fault, are exposed in the western Coast Ranges, but they Mesozoic Franciscan over the "Coastal-Belt" Franciscan because produce only nominal apparent offset in the plane of the section. the young thrusting deforms these contacts. The large structural re- Thus, they can be largely ignored in cross section, even though some may have substantial strike slip. 1 This article provides a summary discussion of the text, cross section, Deeper parts of the cross section were constructed with the con- restored section, geologic map, and gravity models published in color under straint that it be retrodeformable or "balanced"; the primary the title "Cross Section of the Southern Part of the Northern Coast Ranges and Sacramento Valley, California," Geological Society of America Map geometric constraints for the major features of the section come and Chart Series, MC-28B. Contribution of Plate Margins Group, U.S. from the volume and wavelength of the structural relief (Laubscher, Geodynamics Committee. 1965; Dahlstrom, 1969). The wavelength of the structural relief is

Geological Society of America Bulletin, Part 1, v. 90, p. 327-330, 1 fig., April 1979, Doc. no. S90403.

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• Calculated gravity o Calculated gravity with Franciscan at depth — Observed gravity

Jurassic Ophiolite Jurassic and Cretaceous Franciscan

_KJg Ts

Uppermost Cretaceous and Tertiary Coastal-Belt Franciscan

High-Velocity Lower Crust MOHO

30 km

UPPER MANTLE

60 km LOW-VELOCITY ZONE

Figure 1. Simplified east-west geologic cross section of the northern (to the left) and Sacramento Valley (to the right) ~ 100 km north of San Francisco.

relatively short (s5 km), which requires that the associated thrust the low-velocity zone is determined from seismograph P-wave re- faults flatten within the upper crust. siduals (Nuttli and Bolt, 1969). This relief on the low-velocity zone The lower crust is detached from the upper crust and includes is caused by the northward motion of the Mendocino triple junc- seismically determined high-velocity (~6.8 km/s) non-Franciscan tion, as discussed in the complete article, and is the only feature of material (Stewart and Peselnick, 1977), which could be subducted the cross section capable of explaining the uplift of the Coast oceanic lithosphere. The dip of the Moho and lower crustal layer Ranges relative to the Great Valley. was estimated from the regional gradient in gravity. The relief on The geologic solution arrived at in cross section was tested

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-75 mgal Complex

TERTIARY MARINE AND NONMARINE SEDIMENTS • - sea level

(-6.8 km/s) MOHO 30 km

LITHOSPHERE

- 60 km

Figure 1. (Continued).

against the observed gravity by calculating detailed models. The Thus adjustment of neither geometry nor density was necessary in geometry specified by the cross section was not allowed to vary, the Coast Ranges and western Sacramento Valley. with two exceptions: (1) the slope of the Moho and mafic lower The most remarkable aspect of the observed gravity is the crust, and (2) the source of the Great Valley anomaly. The densities smooth gradient across the Coast Ranges, with the Coast Range- of all of the major units are known from measurement and Great Valley contact having no gravitational expression. This give satisfactory agreement between calculated and observed grav- smoothness is surprising because the traditional view of the struc- ity after the slope of the Moho is adjusted to the regional gradient. ture places more dense Franciscan rocks in the upper crust of the

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Coast Ranges and less dense Great Valley sequence in the Sac- thrust faults intersected by cross section segment B-B' shown as KJf ramento Valley; thus an edge anomaly, which is not observed, should also be Js. In segment A-A' of the cross section, the colors of should mark the contact between the geologic provinces. If the hid- KJfm, 3 and 10 cm northe ast of A, and of KJf, 1 cm northeast of A, den mass of Great Valley sequence in the eastern one-third of the are incorrect; formation symbols for all three areas are correct. Coast Ranges (p ~ 2,570 kg/m3) is replaced by Franciscan (p > 3 2,670 kg/m ), a positive anomaly of at least 15 to 20 mgals is REFERENCES CITED created (Fig. 1). This anomaly is not present in the observed grav- ity; thus the traditional view may not be possible. The present cross Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic section is permissible. evolution of western North America: Geological Society of America The 50-mgal positive gravity anomaly in the Great Valley corre- Bulletin, v. 81, p. 3513-3536. Bailey, E. H., Irwin, W. P., and Jones, D. L., 1964, Franciscan and related lates with a high amplitude magnetic anomaly; both are due to a rocks, and their significance in the geology of western California: mafic and ultramafic complex lying immediately below the Upper California Division of Mines and Geology Bulletin 183, 177 p. Cretaceous unconformity (Cady, 1975). The present cross section Berry, F.A.F., 1973, High-fluid potentials in California Coast Ranges and provides further constraint on the source of the anomaly; the struc- their tectonic significance: American Association of Petroleum Geologists Bulletin, v. 57, p. 1219-1249. ture in the Coast Ranges requires that the basement of the Great Cady, J. W., 1975, Magnetic and gravity anomalies in the Great Valley and Valley be truncated at depth by a major . Thus the western metamorphic belt, California: Geological So- source of the Great Valley anomaly is constrained between the fault ciety of America Special Paper 168, 56 p. below and the unconformity above. In the model shown, the source Dahlstrom, C.D.A., 1969, Balanced cross sections: Canadian Journal of of the anomaly is a west-dipping slab of and mantle, Earth Sciences, v. 6, p. 743-757. first obducted and then tilted westward in pre— Laubscher, H. P., 1965, Ein kinemalisches Modell der Jurafaltung: Eclogae Geologicae Helvetiae, v. 58, p. 231-318. time. Nuttli, O. W., and Bolt, B. A., 1969, P-wave residuals as a function of azimuth, 2. Undulations of the low-velocity layer as an explanation: ACKNOWLEDGMENTS Journal of Geophysical Research, v. 74, p. 6594-6602. Stewart, R., and Peselnick, L., 1977, Velocity of compressional waves in dry Franciscan rocks to 8 kbar and 300°C: Journal of Geophysical Re- Acknowledgments and a list of sources of data are given in the search, v. 82, p. 2027-2039. complete article accompanying the maps. Writing of this summary Suppe, G., and Foland, K. A., 1978, The Goat Mountain and Pacific was supported by the U.S. National Science Foundation, Grant Ridge Complex: A redeformed but still-intact late Mesozoic Francis- EAR77-09700, and the John Simon Guggenheim Memorial Foun- can schuppen complex; in Howell, D., and McDougall, L., eds., Mesozoic paleogeography of the western United States: Society of dation. Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Symposium 2, p. 431—451. Note added in proof: Errors on MC-28B (GSA Map and Chart Series).

The writer was in Taiwan and could not examine proof of MC-28B before it went to press. Several errors have been noted on the geologic map: an area shown as water 3 cm west of long MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 8, 1978 122°30'W and lat 38°45'N is (Js). An area between MANUSCRIPT ACCEPTED DECEMBER 12, 1978

Printed in US A.

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