Neotectonics of the San Cayetano fault, Transverse Ranges, California THOMAS ROCKWELL Department of Geological Sciences, San Diego State University, San Diego, California 92182 ABSTRACT INTRODUCTION used to assess the earthquake hazard associated with this fault. Study of late Quaternary alluvial deposits The San Cayetano fault is a major, north- cut by the San Cayetano reverse fault indi- dipping reverse fault on the north flank of Ven- GENERAL GEOLOGY AND cates that the dip-slip rate of faulting in- tura Basin, southern California (Fig. 1), which STRATIGRAPHY creases from 1.05 ± 0.2 mm/yr at Sisar Creek displaces Tertiary and Quaternary rocks with as near the western end of the fault eastward to much as 9 km of stratigraphic separation. The The bedrock geology in the study region is 1.35 ± 0.4 mm/ yr at Bear Canyon and 2.35 ± fault has been extensively studied and drilled in fairly well known. The field work of many 0.55 to 4.15 ± 0.85 mm/yr at Mud Creek. At conjunction with exploration for petroleum theses has been completed in the vicinities of Timber Canyon near the central part of the (Cahill and others, 1976; Schlueter, 1976; Fillmore (Cemen, 1977; Eschner, 1957; John- fault, a minimum slip rate of 3.6 ± 0.4 mm/yr Cemen, 1977; Eldridge and Arnold, 1907; Fine, son, 1959; Robinson, 1955), Santa Paula is suggested by fanhead segmentation; how- 1954; Nagle and Parker, 1971), but this is the (Schlueter, 1976; Bertholf, 1967; McCullough, ever, a higher slip rate of 8.75 ± 1.95 mm/yr first study, other than the early general work on 1957; Loofbourow, 1941), Upper Ojai Valley is suggested by stratigraphic evidence. Strati- the fault by Putnam (1942), to specifically ad- (Schlueter, 1976; Bush, 1956), and along the graphic separation of 7.5 km in less than 1 dress its late Quaternary activity. Ventura Basin Ventura River (Putnam, 1942; Dembroff, 1983). m.y. across the fault between Fillmore and lies within the western Transverse Ranges prov- Oil company geologists have extensively Piru indicates a slip rate of at least 7.5 mm/yr ince of southern California, an east-west-trend- mapped the bedrock throughout the area, as for the eastern section of the fault. Recent ing zone of north-south-directed convergence. well. The above studies have generally ignored activity is shown by faulted Holocene or Many workers have attributed the north-south the varying nature of the surficial soils and left latest Pleistocene fan alluvium and colluvium convergence to compression generated by the the upper Quaternary deposits mostly undiffer- at Bear Canyon and the Silverthread oil field, "big bend" of the San Andreas fault which lies entiated, however. respectively, and inferred at Timber Canyon directly to the north (Hamilton, 1961; Yeats, More than 12,400 m (40,000 ft) of sedimen- by fanhead deposition. 1981; Rodgers, 1975; Bird and Rosenstock, tary rocks of Eocene through Pleistocene age are Active folding of the footwall is demon- 1984). Most recently, Weldon and Humphreys exposed at the surface or penetrated by drill in strated by tilted and faulted Holocene and (1986) suggested that shortening across the Ventura Basin. The terminology for some of the Pleistocene alluvial fans: present tilt rates ap- western Transverse Ranges results from a major formations is confusing and varies among proach 1°/1Q ka yr, and associated bedding step in dextral motion from the Elsinore, Rose workers. In this paper, I use Sisquoc Formation plane faulting rates are about 0.5 to 1.5 Canyon, and southern California borderland instead of Santa Margarita Formation (after mm/yr. The hanging wall, however, does not faults south of the Transverse Ranges to the Thomas Dibblee, unpub. maps), San Fernando appear to be involved in active deformation, Hosgri and related coastal faults north of it. This Formation instead of Repetto, Pico, and Santa inasmuch as Pleistocene deposits as old as second model would place up to one-third of the Barbara Formations (after Jennings and Strand, several hundred thousand years are not tilted Pacific-North American plate motion on the 1969), and Saugus Formation for both marine or affected by bedding-plane faulting. The coastal system of faults and would result in up to and nonmarine units where many have used San only exception is in the Silverthread field 2 cm/yr of convergence across the western Pedro Formation. Because the primary purpose area, where hanging-wall deformation is di- Transverse Ranges. This value is in close agree- of this paper is to present evidence of late Qua- rectly relatable to fault geometry, not folding ment with those of Yeats (1983) and Rockwell ternary activity on the San Cayetano fault, the of the hanging-wall rocks. (1983) who reported 22 mm/yr and 17 ± 4 reader is referred to Rockwell (1983) for a com- mm/yr, respectively. The San Cayetano fault The lack of documented moderate- to plete discussion of the nomenclature. results from this north-south convergence and is large-magnitude historical earthquakes on accommodating a significant part of the shorten- this fault is in strong contrast with its demon- CHRONOLOGY OF THE MIDDLE ing. This paper focuses on the late Quaternary strated late Quaternary activity. Based on its AND UPPER QUATERNARY style, rates, and recency of movement on the San length, slip rate, overall geometry, and seg- ALLUVIAL DEPOSITS Cayetano fault, as well as deformation asso- mentation into two parts, a recurrence inter- ciated with the hanging-wall and foot-wall val of between 200 and 600 yr for the western blocks of the fault. This information, in conjunc- The San Cayetano fault displaces alluvial segment and about 230 yr for the eastern tion with the historical record of seismicity, is deposits of varying age where older deposits, and segment is suggested. their corresponding surfaces, display greater dis- Geological Society of America Bulletin, v. 100, p. 500-513,12 figs., 2 tables, April 1988. 500 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/4/500/3380245/i0016-7606-100-4-500.pdf by guest on 30 September 2021 NEOTECTONICS OF SAN CAYETANO FAULT, CALIFORNIA 501 VENTURA RIVER TOPATOPA MOUNTAINS OJAt / ' n v F,9- 2 SESPE CREEK ARROYO PARIDA SM*— j v AYERS CREEK SYÑCLINE \ j £ RED PACIFIC OCEAN STUDY AREA' Figure 1. Major structural elements in central Ventura basin. Sections A-A', B-B', and C-C' are shown in Figures 5,9, and 11. placements than do younger ones. The chronol- others, 1984; Hatch and Rockwell, 1986; fault were mapped in detail by age based upon ogy of the alluvial deposits is based on a dated Vaughan and Rockwell, 1986; Millman and the strength of the soil profile developed within soil chronosequence developed for and within Rockwell, 1986; Harden and others, 1986). The them, much like sedimentary rocks are mapped Ventura Basin (Keller and others, 1980; Rock- chronosequence presented in Table 1 and out- based on their similarity to type sections. Active well, 1983; Rockwell and others, 1985). Soil lined briefly below (discussed in depth in channel deposits with no soil development chronosequences, or groups of soils with mem- Rockwell and others, 1985) is utilized in this (Typic Xerofluvents) are designated Qi depos- bers that are differentiated by their properties as paper to assess rates of faulting along the San its. Flood-plain and recently inactive terrace a function of age, have been used to evaluate Cayetano fault. deposits with A/C soil profiles (Fluventic Hap- rates of faulting in several studies (Rockwell and The alluvial deposits along the San Cayetano loxerolls) are designated Q2 deposits. The Q3 TABLE 1. RELATIVE AND ABSOLUTE AGE OF SOIL PROFILES Geomorphic Brightest moist mixed Clayf Clay fTlm§ Estimated age in years surface color in B horizon XB/XA index before present and soil Color* Hue Chroma index Ql AC profile, no B horizon noB 0 10-20 Q2 AC profile, no B horizon noB 0 <250 Qt3 10 YR 3 4 0.6 0 500-5,000»* Qt4 10 YR 4 5 1.3 3.0 8,000-12,000^ Qt5a 10 YR 4 5 ND 4.2 I5,000-20,000tt Qt5b 10 YR 4 5 ND 6 30,000tt Qt6a 7.5 YR 5 7 1.4 7.25 38,000§S Qt6b 7.5 YR 6 8 1.5 5.5 54,000*" ± 10,000 Qt6c 7.5 YR 7 9 1.6 7.0 92,000*** ± 13,000 Qt7 5 YR 6 9 ND 8.0 160,000-200,OOOttt Note: data in this table from Rockwell and othere (1984,1985). *Color index is computed by adding chroma number to hue (of moist mixed sample), where 10 YR = 1,7.5 YR = 2,5 YR - 3. Indices from different profiles on same geomorphic surface are averaged. To determine color, a large air-dried bulk sample was passed through a 2~mm sieve, then fractionated in a mechanical splitter, moistened, hand homogenized to a putty consistency, and rolled to a sphere; then it was pulled into halves, and color was noted from one freshly broken surface. tRatio of the mean percentage of clay in 8 horizon to that in A horizon, computed from particle-size data. 'Index based on day film information contained in the profile descriptions and computed by adding the percent frequency of clay film occurrence to their thickness, as follows. Percent frequency: very few = 1; few = 2; common = 3; many = 4; continuous - 5. Thickness.- thin = 1; moderately thick ~ 2; ¿lick - 3. For example, if in the Bt horizon there are many to continuous (4.5) moderately thick and thick (2.5) clay films, the index is 7.0.