Active tectonics at Wheeler Ridge, southern San Joaquin Valley, California E. A. Keller* Environmental Studies Program and Department of Geological Sciences, University of California, Santa Barbara, California 93106 R. L. Zepeda 1342 Grove Street, Alameda, California 94501 T. K. Rockwell Department of Geological Sciences, San Diego State University, San Diego, California 91282 T. L. Ku Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 W. S. Dinklage Department of Geological Sciences, University of California, Santa Barbara, California 93106 ABSTRACT INTRODUCTION where geomorphic surfaces are folded over the anticlinal axis (Zepeda et al., 1986). In addition, Wheeler Ridge is an east-west–trending Buried reverse faults associated with actively the 1952 Kern County earthquake was centered anticline that is actively deforming on the up- deforming folds are known to produce large earth- below Wheeler Ridge, but not on the Wheeler per plate of the Pleito–Wheeler Ridge thrust- quakes. Several recent, large California earth- Ridge thrust fault; therefore the potential earth- fault system. Holocene and late Pleistocene de- quakes are examples: M = 6.7, Northridge, 1994; quake hazard in the area is clear. formation is demonstrated at the eastern end M = 6.1, Whittier Narrows, 1987; MS = 6.4, The primary goals of the research at Wheeler of the anticline where Salt Creek crosses the Coalinga, 1983, and MS = 7.7, Kern County, Ridge are (1) to characterize the tectonic geomor- anticlinal axis. Uplift, tilting, and faulting, as- 1952. Detailed investigations of folding associ- phology; (2) to develop the Pleistocene chronol- sociated with the eastward growth of the anti- ated with concealed reverse faults are therefore ogy; and (3) to test the hypothesis that climatic cline, are documented by geomorphic surfaces necessary in order to better understand earthquake perturbations are responsible for most Pleistocene that are higher and older to the west. hazards associated with these areas. and Holocene aggradational events that produce Faulting and associated folding is propagat- The goals of earthquake hazards research are alluvial fan segments, and that tectonic perturba- ing eastward, as indicated by increases in both to characterize earthquake sources likely to pro- tions primarily deform the fan segments. the degree of surface dissection and the degree duce future events by determining fault-slip rate, Testing the hypothesis concerning climatic of soil development from east to west. Distinct average recurrence intervals, and timing of past and tectonic perturbations is important in under- topographic areas having distinct degrees of events. Active faults have generally been studied standing the origin and significance of landform surface dissection, bounded by tear faults, where geologically young surficial deposits are evolution in tectonically active regions (Bull, suggest that faulting and folding have propa- observed to be displaced at the surface. Land- 1991). The question is, How significant are tec- gated eastward in discrete segments. forms associated with strike-slip, normal, and re- tonic perturbations? At the regional scale over Numerical dates indicate (1) the anticline is verse faults may include fault scarps, offset several million years, tectonic processes may propagating eastward at a rate of about 30 streams, sag ponds, or offset terraces. However, produce mountain ranges that cause climatic mm/yr (about 10 times the rate of uplift); in areas where active reverse faults do not break change, for example, through the development (2) folding was initiated about 400 ka; (3) a through to the surface, young surficial deposits of a rain shadow. At a more local scale, a study prominent wind gap was formed during Q3 are not ruptured. Therefore, the underlying fault of terraces of the Ventura River, California, indi- time (about 60 ka) when an antecedent stream may be either unknown or incorrectly identified cated that the terrace sequence has a climatic sig- was defeated, forcing the stream east around as inactive, unless independent geophysical data nature, whereas deformation of the terraces re- the nose of the fold; today drainage through demonstrate that activity (Stein and King, 1984). sulted from tectonic activity (Rockwell et al., the ridge is by way of two antecedent streams The recent earthquakes identified here have re- 1984). As a second example, tectonic tilting of (water gaps) east of the wind gap; and (4) the sulted in a concerted effort by researchers to eval- alluvial fan segments may cause a threshold of rate of uplift at the easternmost and youngest uate the potential seismic hazards in areas of ac- fan slope stability to be exceeded (Schumm, (past 1 k.y.) part of the fold is at least 3 mm/yr. tive folding (Stein and King, 1984; Zepeda et al., 1977) and fan-head entrenchment to occur Investigations of the tectonic geomorphology 1986; Yeats, 1986; Keller and Pinter, 1996). (Rockwell et al., 1985). In turn, entrenchment of Wheeler Ridge support the hypothesis that The Wheeler Ridge anticline, located in the will cause a new fan segment to develop. How- climatic perturbations are primarily responsi- southern San Joaquin Valley, California (Figs. 1 ever, the development of thick alluvial deposits ble for producing geomorphic surfaces such as and 2), was chosen for study because it is possi- through aggradational events may be the result alluvial fan segments and river terraces—tec- ble to determine rates of uplift associated with ac- of climatic perturbations. The study of landscape tonic perturbations deform the surfaces. tive folding above a buried reverse-fault system. evolution at Wheeler Ridge spans a sufficient Holocene and latest Pleistocene activity has been length of time that both tectonic and climatic *e-mail: [email protected] demonstrated at the east end of Wheeler Ridge, perturbations have influenced the landform evo- GSA Bulletin; March 1998; v. 110; no. 3; p. 298–310; 9 figures; 2 tables. 298 ACTIVE TECTONICS AT WHEELER RIDGE, CALIFORNIA CLIMATE There are large topographically controlled variations in climate in the vicinity of Wheeler Ridge (Soil Conservation Service, 1988). The av- erage climate at Wheeler Ridge (~300 m eleva- tion) is warm and semiarid to arid with one wet season during the year (winter). Of all precipita- tion, 90% falls between October and April. Sum- mers are generally cloudless, hot, and dry. Tem- peratures often exceed 38 °C and rarely fall below 20 °C. Minimum temperatures during the winter months sometimes are below freezing. The mean annual temperature is about 19 °C. Precipitation ranges from 15 to 16 cm in the San Joaquin Valley to 27 cm in the San Emigdio and Tehachapi Mountains south of Wheeler Ridge. GEOLOGIC SETTING The east-west–trending Wheeler Ridge anti- cline is located along the north flank of the San Emigdio Mountains at the boundary between the Transverse Ranges and the San Joaquin Valley (Figs. 1 and 2). The San Emigdio Mountains are part of the east-west–trending, western Trans- verse Ranges, in contrast to the predominantly northwest-southeast structural grain of Califor- nia. The western Transverse Ranges are bounded on the north and south edges by fold-and-thrust belts (Keller et al., 1987). Wheeler Ridge is located in the “Big Bend” re- gion of the San Andreas fault, where the generally northwest-southeast–trending fault strikes more east-west (Fig. 1). Within the Big Bend, north- south compression and resulting shortening are observed (Savage, 1983; King and Savage, 1984). Figure 1. Location of study area and generalized geologic setting at Wheeler Ridge. SAF— The driving force for the uplift of the San Emigdio San Andreas fault, BB—“Big Bend” of the fault. After Davis (1983); Dibblee (1973); Morton and Mountains and Wheeler Ridge is thought to be Troxel (1962). compression generated by the left step in the San Andreas fault produced by the Big Bend and ac- companying strain partitioning, resulting in dis- lution. As a result, this study helps delineate the Sample preparation of pedogenic carbonate placement on reverse faults along range fronts relative importance of climatic versus tectonic rinds for radiocarbon and uranium-series analy- (Rodgers, 1980; Lettis and Hanson, 1991; and perturbations in an area characterized by rela- ses included (1) removal of soft outer carbonate; Working Group on California Earthquake Proba- tively high rates of tectonic activity. (2) scraping and collection of the hard, innermost bilities, 1995). carbonate rinds from undersides of clasts; and Nodal planes for five earthquakes in the METHODS (3) viewing with a microscope for signs of leach- Wheeler Ridge area are consistent with the ori- ing or reprecipitation (not included for radiocar- entation of the generally east-west–striking The approach to the investigation included bon-dated carbonate rinds). faults mapped in the area (Webb and Kanamori, photogeologic analysis to characterize the struc- Soil properties were described according to the 1985). The focal mechanisms for these events ture and geomorphology of the anticline; geo- methods and terminology described by the Soil agree well with those expected from the local logic mapping; surveying with engineering level Survey Staff (1975). Stages of carbonate mor- geology and the general north-south compres- to document deformation (folding) of geomor- phology were described using the nomenclature sion. Shallow-dipping nodal planes associated phic surfaces;