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Tectonic Geomorphology of the San Andreas Fault Zone in the Southern Indio Hills, Coachella Valley, California

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Tectonic Geomorphology of the San Andreas Fault Zone in the Southern Indio Hills, Coachella Valley, California Tectonic geomorphology of the San Andreas fault zone in the southern Indio Hills, Coachella Valley, California E A KELLER • M S BONKOWSKI I Department of Geological Sciences, University of California, Santa Barbara, California 93106 R. J. KORSCH* Armidale College of Advanced Education, Armidale, New South Wales 2350, Australia R. J. SHLEMON P.O. Box 3066, Newport Beach, California 92663 ABSTRACT INTRODUCTION Geomorphic investigation of the San Andreas fault zone in the Tectonic geomorphology studies along the San Andreas fault Indio Hills indicates many tectonically produced landforms, includ- in the Indio Hills began in the winter of 1978-1979. Objectives of ing beheaded streams, right-lateral deflected and offset streams, this paper are: (1) to delineate patterns of recent tectonic deforma- sags, shutter ridges, pressure ridges, and fault scarps. Near Biskra tion and possible relations to simple shear; (2) to evaluate evidence Palms, an alluvial fan-pediment complex has an apparent cumula- for paleoseismicity in the area; and (3) to estimate the slip rate for tive offset of about 0.7 km along the Mission Creek fault zone this section of the San Andreas fault. Two areas were mapped in (north branch, San Andreas fault). Many of the tectonic landforms, detail: Pushawalla Canyon and an uplifted, deformed, and offset as well as the fracture pattern that has developed during the Pleisto- pediment-fan complex. cene, are explainable by simple shear or uplift associated with a The Indio Hills trend northwest along the northeast flank of small left bend in the main trace of the Mission Creek fault. The the Coachella Valley, California (Fig. 1). The hills are cut by and ratio of vertical to horizontal displacement in the vicinity of the uplifted along several branches of the San Andreas fault zone, most bend is about 0.04. notably the Banning and Mission Creek faults that intersect near Exposed in Pushawalla Canyon, 5 km northwest of the alluvial Biskra Palms (Fig. 2). fan-pediment complex, are: (1) a sequence of stream terraces, (2) The southern Indio Hills southeast of Thousand Palms folded Plio-Pleistocene fanglomerates,, and (3) an example of Canyon are pri marily composed of the Pleistocene Ocotillo Forma- stream capture following a right lateral deflection or offset of sev- tion (Fig. 2), a deformed sequence of sandstone and fanglomerates eral hundred metres. A left step of the Mission Creek fault in (Dibblee, 1954; Popenoe, 1959). Pushawalla Canyon is a probably cause of folds and sporadic uplift Topography of the Indio Hills is characteristic of arid regions that produced the stream terraces. Possible cause of recent stream (rainfall is 7 to 10 cm/yr) undergoing recent tectonism and fluvial capture are: (1) juxtaposition of Pushawalla Canyon with a relict erosion. The major fluvial landforms are steep canyons and coalesc- canyon moving northwestward along the Mission Creek fault, or ing alluvial fans along the north west-southfeast trending front of the (2) right-lateral deflection or offset of Pushawalla Canyon along the hills. Upland surface and older alluvial fans are covered with fault, with simultaneous headward erosion of a shorter, steep moderate- to well-developed desert pavement and desert varnish. stream flowing toward the Coachella valley. However, the most impressive aspect of the topography is an Estimation of a slip rate and identification of paleoseismicity assemblage of "classic" tectonically produced landforms associated for the San Andreas fault in the Indio Hills is difficult. However, with the San Andreas fault zone. Landforms observed are: fault degree of topographic dissection, formation, and preservation of scarps; beheaded, deflected, or offset drainages; tectonically desert pavement, and relative soil profile development suggest that induced stream capture; sags; shutter ridges; and pressure ridges. the age of the offset fan may be as old as 70,000 yr but that most Paleoseismicity of the San Andreas fault in the Indio Hills is likely it is on the order of 20,000 to 30,000 yr. These age estimates unknown. The main fault traces are shown on Figure 2, and these for the offset fan indicate a minimum slip rate for the San Andreas are being studied in detail (Bonkowski, unpub. data) to gather fault of 10 to 35 mm/yr, with about 23 to 35 mm/yr the most likely. information concerning recent earthquake activity. The main fault The pattern of observed offset drainages is complex, but suggests trace is locally delineated by fan palms (Washingtonia filifera) that that during the past few thousand years creep events or moderately form linear oases along the water-bearing fault zones. Offset drain- large earthquakes have periodically produced several metres of ages at several locations along the Mission Creek fault indicate that right-lateral displacement. creep or moderate magnitude earthquakes have occurred in the past few thousand years. The offset is generally 1 to 4 m but the history •Present address: Geology Department. Victoria University, Welling- is complex, making the data difficult to interpret. ton, New Zealand. Most of the smaller earthquakes (M <3.5) in recent years Geological Society of America Bulletin, v. 9!!, p. 46-56, 10 figs., 3 tables, January 1982. 46 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/1/46/3434348/i0016-7606-93-1-46.pdf by guest on 30 September 2021 TECTONIC GEOMORPHOLOGY OF SAN ANDREAS FAULT ZONE 47 oi o -+- 34°00' Figure 1. Index map, southern Indio Hills. CA q Desert Hot Springs 0'/} apparently occurred north of the main trace of the San Andreas fault (Fig. 3), and the area has no known history of great earth- quakes. There have been moderate to large seismic events, including the 1948 Desert Hot Springs earthquake, which had a magnitude of 6.5 (Fig. 3). Therefore, the large active fault system does represent a potential hazard to rapidly urbanizing local areas, including Palm strongly developed, typified by reddish, clay-rich B (argillic) horiz- Springs, Desert Hot Springs, Palm Desert, and Indio. ons, and often by two or more discrete carbonate (calcic) horizons (Buol and Yessilsoy, 1964; Elam, 1974; Gile, 1968; Gile and others, OFFSET PEDIMENT-ALLUVIAL FAN COMPLEX 1965, 1966). Soils of this relative development are usually pre-late Pleistocene in age, and deemed "relict paleosols," for they usually An uplifted, deformed, and offset pediment-alluvial fan com- occur on high-level, remnant surfaces, and were never buried; plex is located southeast of Biskra Palms near the junction of the rather, they attained most of their development during environ- Banning and Mission Creek faults (Figs. 2 and 4). The area contains ments of the past (Ruhe, 1965; Valentine and Dalrymple, 1976). a spectacular assemblage of fault-related landforms, including fault Some soils in the Mojave Desert and adjacent areas have been scarps; beheaded, deflected, and offset streams; shutter ridges; sags; dated radiometrically, by radiocarbon or U-Series (Bischoff and and microtopography (horst and graben?). These are shown dia- others, 1978; Ku and others, 1979). Others have been dated rela- grammatically on Figure 5. tively, generally by association to Pleistocene climatic change, as The minimum cumulative offset of the fan Qf2 is about 0.7 km recored by the number, depth, and morphology of pedogenic car- (Fig. 5, x-x^. The northern part of the modern fan (Qfi) is appar- bonate horizons (Arkley, 1963; Gile and others, 1965, 1966; Morri- ently offset several hundred metres, suggesting that the right lateral son, 1978; Morrison and Frye, 1965; Nettleton and others, 1975; slip continued into the Holocene. Shlemon, 1978b). Soil ages at one locality, however, cannot usually The surface [Qf2 and Qf2(?), Figs. 2 and 5] is designated a be directly applied to another, owing to differences in parent mate- pediment-fan complex because the upper segments are a true ero- rial lithology and grain size, precipitation, slope, and other basic sion surface with only a thin veneer of deposits a metre or so thick soil-forming factors. Nevertheless, nonsodic, well-drained soils in overlying the Ocotillo Formation. The upper pediment and lower fan the Mojave Desert less than about 15,000 yr old are often typified surfaces are tentatively correlated by relative soil development, by disseminated carbonates in the upper part of the profile, but lack topographic dissection, and formation of desert pavement and other distinctive pedogenic horizons. In contrast, relict profiles on desert varnish. surfaces more than about 70,000 or 80,000 yr old usually have an Soil (pedogenic profiles) are useful indicators to estimate the argillic horizon (Bt), two or more carbonate horizons (Bca; Cca), age of desert fan and pediment surfaces (Bull, 1974, and unpub. and often a stone-free, vesicular (A2 or "Av") horizon (Bischoff and data; Gile, 1975, 1977; Gile and Hawley, 1966; Nettleton and others, others, 1978; Bull, unpub. data; Peterson, 1980; Shlemon, 1978a; 1975; Shlemon, 1978a). Many desert soils are moderately to Springer, 1958). Accordingly, such differences in relative profile Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/1/46/3434348/i0016-7606-93-1-46.pdf by guest on 30 September 2021 48 KELLER AND OTHERS development make it possible to date, at least approximately, fans Pit no. 1; Fig. 5) is characterized by incipient development (A-C and pediments in the study area. Thus, several soil profiles were profile), and only a very thin, vesicular (Av) horizon. The desert measured and described from pits excavated on various surfaces pavement is moderately developed. However, the fan surface is now (Fig. 5). Tables 1, 2, and 3 present three detailed profiles. The inactive, having been offset in a right-lateral direction by the San profile terminology and field classification are standard, following Andreas fault. The age of this fan, based mainly on the weakly that of the U.S.
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