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OVERVIEW Late Cenozoic tectonics of the central and southern Coast Ranges of California Benjamin M. Page* Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 George A. Thompson† Department of Geophysics, Stanford University, Stanford, California 94305-2215 Robert G. Coleman Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115 ABSTRACT within the Coast Ranges is ascribed in large Taliaferro (e.g., 1943). A prodigious amount of part to the well-established change in plate mo- geologic mapping by T. W. Dibblee, Jr., pre- The central and southern Coast Ranges tions at about 3.5 Ma. sented the areal geology in a form that made gen- of California coincide with the broad Pa- eral interpretations possible. E. H. Bailey, W. P. cific–North American plate boundary. The INTRODUCTION Irwin, D. L. Jones, M. C. Blake, and R. J. ranges formed during the transform regime, McLaughlin of the U.S. Geological Survey and but show little direct mechanical relation to The California Coast Ranges province encom- W. R. Dickinson are among many who have con- strike-slip faulting. After late Miocene defor- passes a system of elongate mountains and inter- tributed enormously to the present understanding mation, two recent generations of range build- vening valleys collectively extending southeast- of the Coast Ranges. Representative references ing occurred: (1) folding and thrusting, begin- ward from the latitude of Cape Mendocino (or by these and many other individuals were cited in ning ca. 3.5 Ma and increasing at 0.4 Ma, and beyond) to the Transverse Ranges. This paper Page (1981). Additional significant papers are (2) subsequent late Quaternary uplift of the deals with the portion of the province that lies noted in this article. ranges. The ranges rose synchronously along southeast of San Francisco, a subprovince (Fig. 1) Probably the earliest study that specifically fo- the central California margin and are still ris- that we call the central and southern Coast Ranges, cused on the uplift of the present-day ranges was ing; their long axes are quasiparallel to the encompassing the Diablo Range, East Bay Hills, that of Christensen (1965), who recognized that plate boundary and strike-slip faults. The up- Santa Cruz Mountains, Gabilan Range, Santa Lu- the Coast Ranges as we know them are younger per crustal internal and marginal structures of cia Range, La Panza Range, San Rafael Range, than 3–4 Ma. Subsequent writings on uplift of the the ranges are contractional, dominated by Sierra Madre Range, Caliente Mountain, and ranges include Page (1981, p. 415–416); Zandt folds and thrusts resulting from the convergent Temblor Range (Fig. 1). Exactly the same area has and Furlong (1982), Montgomery (1993), component of plate motion. Newly constructed been called the southern Coast Ranges in the past Anderson (1994), and Burgmann et al. (1994). transects using seismic reflection and refrac- (e.g., Page, 1981; see also Jennings, 1977). Many other authors have contributed important tion, plus gravity and magnetic studies, reveal The central and southern Coast Ranges have data, as noted herein. Our review is an attempt to lower crustal basement(s) at depths of 10–22 been studied for more than a century, attention integrate an earlier paper (Page, 1981) with new km. The upper surface of the basement and being largely devoted to stratigraphy, petrology, and significant geophysical studies that have en- Moho show no effect of the folding and thrust- structure, and resources potential. In recent years, hanced our knowledge of the crust and mantle ing observed in the upper crust. We conclude geophysical investigations have provided funda- within the Coast Ranges. that horizontal shortening is accommodated at mental subsurface information. In this paper we depth by slip on subhorizontal detachments, examine the main Cenozoic tectonic events and GEOMORPHIC FEATURES OF and by ductile shear and thickening. The the culminating rise of the ranges in their present TECTONIC SIGNIFICANCE ranges are marked by high heat flow; weak configuration, the timing, manner of uplift, pos- rocks of the Franciscan subduction complex; sible causes, and mechanics. The ranges and major intervening structural high fluid pressure; bounding high-angle re- A review of the geology of the central and valleys are subparallel with the continental margin verse, strike-slip, or thrust faults; and uplift at a southern Coast Ranges and references to many and the Pacific–North America plate boundary. rate of 1 mm/yr beginning about 0.40 Ma. studies may be found in Page (1981), and conven- The San Andreas fault is parallel with some of the Transverse compression manifested in folding ient summary papers are available in Wahrhaftig ranges, but is slightly oblique to the Coast Ranges and Sloan (1989). Pioneering groundwork was province as a whole, and crosses it with a prevail- *Deceased. done by such notables as A. C. Lawson (e.g., ing trend of about N41°W. The orientation of the †E-mail: [email protected]. 1893, 1914), R. D. Reed (1933), and N. L. ranges with respect to the plate boundary pre- GSA Bulletin; July 1998; v. 110; no. 7; p. 846–876; 18 figures; 5 tables. 846 CENTRAL AND SOUTHERN COAST RANGES, CALIFORNIA _ _ 0 0 such as the presence of low-density roots or local 122 122 causative thermal conditions associated with indi- _ 0 38 0 vidual mountain ranges. The scarcity of through- 38 going transverse river valleys, coupled with the weak nature of most Coast Ranges rocks, is sug- H SF gestive of (but not proof of) geologically recent, EBH SN ongoing uplift. Except for their mutual parallelism and rela- SB tively level crests, the overall morphologic as- D pects of the several ranges differ widely. Some SC (e.g., the northern Santa Lucia Range, Fig. 4, and northern Gabilan Range) show eroded fault- block features including steep scarp-like flanks, GV faceted spurs, and V-shaped canyon mouths at the apices of alluvial fans. Several ranges, including the two just mentioned, have remnants of upland G M pediments or vestiges of gently rolling uplands, D presumably formerly near sea level, which are K SV sharply incised by headward-growing modern N canyons. Nearly level upland remnants suggest C even uplift. However, the southern part of the SG SL 0 Gabilan Range is a tilted fault block that has _ 36 0 KH 36 deeply eroded, gently sloping upland surface S A remnants that dip gently toward their juncture F with the floor of Salinas Valley. Some ranges (or PR foothill ranges, such as the one visible in Fig. 2 bordering the Santa Clara Valley south of San Jose, and the East Bay Hills) look like linear PO arched ductile welts that have marginal faults but L T do not have commanding scarps. In contrast, SO CP parts of the Diablo Range and parts of the Santa CR Lucia Range have somewhat depressed wide central portions bordered on both sides by crestal SM CV ridges. The northeast flank of the Diablo Range is locally accompanied by broad, gently sloping, SR 100 km alluviated, plateau-like tracts that have been only slightly uplifted along unspectacular marginal 100 miles faults. Each of these diverse morphologic aspects 0 0 must tell us something about the tectonics of the 120 122 subprovince. _ It is important to note that parts of the flanks of Figure 1. Physiographic map of the central and southern Coast Ranges, a subprovince (within several ranges are deeply embayed by erosion and the white polygon) of the California Coast Ranges. Inset shows location within the State of Cali- lack indications of marginal faults (e.g., the north- fornia. The subprovince approximately coincides with the broad transform plate boundary, char- east flank of the Santa Lucia Range between acterized by active dextral transform faults, three of which are labeled (SAF, SG, and H). Abbre- Greenfield and Paso Robles, and much of the viations: C—Coalinga; CP—Carrizo Plain; CR—Caliente Range; CV—Cuyama Valley; southwest flank of the Gabilan Range). Hence, D—Diablo Range; EBH—East Bay Hills; G—Gabilan Range; GV—Great Valley; H—Hayward fault slip is not always necessary for the rise of the fault; K—King City fault; KH—Kettleman Hills; L—La Panza Range; M—Monterey; PO—Pa- ranges, although it has locally played a role. cific Ocean; PR—Paso Robles; SAF—San Andreas fault; SB—San Francisco Bay–Santa Clara The principal intermontane valleys, such as the Valley depression; SC—Santa Cruz Mountains; SF—San Francisco; SG—San Gregorio–Hosgri Santa Clara, Salinas, and Cuyama Valleys, and fault zone; SL—Santa Lucia Range; SM—Sierra Madre Mountains; SN—Sierra Nevada; SR— the Carrizo Plain, are relevant to the tectonics of San Rafael Range; SO—San Luis Obispo; SV—Salinas Valley; T—Temblor Range. the province, as they are clearly of structural ori- gin and are presumably products of the same events that caused uplift of the ranges. Their cludes an origin by wrench tectonics (i.e., folding their trend and in their quasilevel crests, which floors are aggraded rather than eroded, and they and faulting produced directly by horizontal shear have few conspicuous peaks (Figs. 2 and 3). They are bordered by piedmont fans. The Carrizo Plain such as in transform interaction), in which case are 120 km to more than 300 km long, 10 to 50 lacks a fluvial outlet and contains a saline playa. folds and thrusts would be expected to form at an km wide, and generally 400 to 1200 m high. The The longitudinal drainage of the Santa Clara Val- angle to the boundary, in an en echelon pattern. modest widths, together with geophysical data, ley reverses its direction of flow near the mid- The individual ranges are mutually similar in argue against some proposed modes of origin, point of the valley, indicating that the valley prob- Geological Society of America Bulletin, July 1998 847 PAGE ET AL.

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