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Late Quaternary Activity of the Laguna Salada Fault in Northern Baja California, Mexico

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Late Quaternary Activity of the Laguna Salada Fault in Northern Baja California, Mexico Late quaternary activity of the Laguna Salada fault in northern Baja California, Mexico Karl J. Mueller* Department of Geological Sciences, San Diego State University, San Diego, California 92182 Thomas K. Rockwell } ABSTRACT able information. By studying fault scarp overs present in the Imperial and Mexicali morphology and its relation to alluvial sed- Valleys include the Brawley and Cerro Pri- Faulted alluvial fans and bajadas along imentation, weathering, and soil develop- eto spreading centers (Fig. 1), (Muffler and the central Laguna Salada fault zone in ment, it is possible to define the late Qua- White, 1969; Elders et al., 1972; Halfman et northern Baja California record a recurrent ternary slip history of active faults where al., 1984) which link the San Andreas, Im- history of oblique-slip Holocene earth- detailed subsurface information is unavail- perial, and Cerro Prieto dextral faults. Re- quakes. Alluvial surfaces, which range from able. This requires recognition and mapping straining stepovers along active dextral fault late Pleistocene to historic in age have been of offset geomorphic features and faults, zones also exist in the Salton Trough and progressively displaced along the base of along with the description, analysis, and include the Durmid, Mecca, and Indio Hills the crystalline rangefront, as well as along comparison of soil profiles developed on along the San Andreas fault east of the Sal- more basinward fault splays in alluvium. alluvial surfaces. A critical aspect of this ton Sea (Bilham and Williams, 1985); the The recurrence interval determined from method is the recognition of small-scale Ocotillo Badlands, Borrego Mountain, Su- displaced alluvial deposits that are dated by topographic features offset across fault perstition Mountain, and the Superstition soil profile development is in the range of scarps to establish the recency and sense of Hills along the San Jacinto fault zone south 1–2 ka, with a corresponding right lateral slip on recent surface ruptures. of the Santa Rosa Mountains (Clark, 1972; slip rate of ;2–3 mm/yr, similar to that of The Laguna Salada fault zone bounds the Sharp and Clark, 1972; Sibson, 1986); and the southern Elsinore fault in southern Cal- western margin of the Sierra Cucapa, an iso- the Coyote Mountains and related struc- ifornia. The most recent event along the lated northwest-trending mountain range in tures along the Elsinore–Laguna Salada fault zone is probably the widely felt earth- the southern Salton Trough (Fig. 1). This trend (Isaacs, 1987; Rockwell and Pinault, quake of February 23, 1892. At least 22 km range and the adjoining Sierra Mayor ex- 1986; Rockwell, 1989). of the fault zone ruptured during this event, tend 85 km from north of the international Well-exposed releasing bends are also along both an oblique-dextral section of the border to the southeast into Baja California present along the eastern margin of the La- northwest-striking Laguna Salada fault and and separate the southern Salton Trough guna Salada basin (Mueller and Rockwell, the linked, northeast-striking Can˜on Rojo into two regions of deep sedimentary fill, the 1991). The location of these fault stepovers normal fault. The length of ground rupture Mexicali Valley and Laguna Salada basin defines the eastern limit of deep sediment and amount of displacement (4 m of dextral (Fig. 1). The great variation in structural re- fill in Laguna Salada and has governed the slip and 3.5 m of normal slip) suggest that lief in basement rocks across these two ba- development of this basin during late Ter- the earthquake had a magnitude (Mw) of at sins and the intervening Sierra Cucapa is tiary and Quaternary time (Biehler et al., least 7.1. partly due to interaction among several 1964; Kelm, 1971; Mueller, 1984; Mueller northwest-striking dextral and oblique-slip and Rockwell, 1991). The northern Laguna INTRODUCTION faults. These include the Imperial, Cerro Salada fault may be linked to the Elsinore Prieto, Cucapa, Pescadores, Borrego, La- fault in southern California across a com- Determination of the recent slip history of guna Salada, and Chupamiertos faults (Bar- plex zone of northeast- and northwest- active fault zones commonly requires de- nard, 1968), which partition dextral strain striking faults in the Yuha basin area tailed subsurface data, which include bore- across the southern Salton Trough. These (Isaacs, 1987). Vertical separation is consid- holes, trenches, and earthquake locations. faults are part of the southern San Andreas erable in Laguna Salada, where up to 4–6 Studies of fault zones located in regions fault system and mark the transition to km of basin fill is present (Biehler et al., where these data are unavailable thus need transform faulting in the northern Gulf of 1964; Kelm, 1971). Furthermore, fault rocks to employ techniques based on readily avail- California. are exposed along parts of the Laguna Major changes in structural relief, or up- Salada fault zone that appear to have been lifts and depressions, are present along the developed under conditions of cataclastic flow (Kerrich and Allison, 1978), suggesting *Present address: Department of Geological lengths of these faults (Fuis et al., 1982, and Geophysical Sciences, Princeton University, 1984) and may coincide with stepovers or significant vertical separation and uplift of Princeton, New Jersey 08540. variations in fault strike. Releasing step- the Sierra Cucapa. Data Repository item 9501 contains additional material related to this article. GSA Bulletin; January 1995; v. 107; no. 1; p. 8–18; 6 figures; 2 tables. 8 LAGUNA SALADA FAULT, NORTHERN BAJA CALIFORNIA, MEXICO by the degree of maturity of the soils devel- oped in them and by other surface weath- ering characteristics such as clast disintegra- tion and the development of rock varnish and desert pavement. The magnitude and sense of displacement for the most recent earthquake was deter- mined along a 120 m portion of the fault where the fresh surface rupture was con- fined to a single strand, based on our map- ping (Fig. 2). In this area, the displaced fea- tures were mapped with an electronic theodolite and distance meter (total sta- tion), and a 1:1000-scale topographic map derived from more than 1000 data points was generated, usinga1mcontour interval. ALLUVIAL CHRONOLOGY The southwest margin of the Sierra Cu- capa is marked by an abrupt, steep range front of Mesozoic igneous rocks produced by oblique slip along the Laguna Salada fault (Fig. 2). Coarse clastic debris shed off this range front and from the interior of the range has been deposited as alluvial fans along the margin of Laguna Salada (Fig. 2). Antecedent streams coalesce into broad ba- jadas or form solitary alluvial fan–fan deltas depending on spacing of channels draining the interior of the range. Coarse-grained al- luvial deposits grade abruptly basinward into fine-grained mudstone and evaporite deposits. Several ages of alluvial deposits and their associated surfaces are repre- Figure 1. Simplified map of the Salton Trough, which outlines basement outcrops, Qua- sented by fans and terraces at different geo- ternary deposits, major faults, and the area mapped on Figure 2, located along the south- morphic levels (Fig. 3). western margin of the Sierra Cucapa, Mexico. Determination of the relative age of dis- placed geomorphic surfaces was based on the degree of maturity of the soils developed The total amount of slip along the Laguna slip along the Laguna Salada fault zone. The in the deposits, and the extent of surface Salada fault is presently unknown, although slip rate and recurrence interval of the fault weathering. We described 26 soil profiles on we believe it to be ,20 km based on the was determined by description of soils de- surfaces of at least five distinct ages from amount of slip documented on the linked veloped in the displaced deposits and by exposures in hand-dug pits. Examples of Elsinore fault in southern California. Total comparison to dated soils developed under typical soil profiles are listed in Table 1. A dextral slip across the northern Elsinore similar climatic conditions in southern Cal- complete set of all 26 soil descriptions is also fault does not exceed 15 km (Hull, 1990; ifornia. available1. Particle size distribution and sec- Morton and Miller, 1987), whereas parts of ondary calcium carbonate percent were de- the southern Elsinore fault may transfer METHODS termined analytically for selected profiles ;2–3 km (Lampe et al., 1988). (Mueller, 1984). The fault is well expressed at the surface The southwest margin of the Sierra Cu- Absolute age was not determined for any as a zone of well-preserved fault scarps and capa was mapped at a scale of 1:10 000, in- of the Laguna Salada soils due to a lack of displaced alluvial fans (Mueller, 1984). In cluding the central Laguna Salada fault zone datable material in the alluvial deposits. Es- particular, well-preserved alluvial free faces and the northeastern, most recently active timates of ages were made by comparing and bedrock fault surfaces are exposed segment of the Can˜on Rojo fault (Figs. 1 time dependent characteristics to dated soils along much of the fault and may be the re- and 2). Features mapped in the area include sult of an historic earthquake and associated fault traces and scarps, deflected stream 1GSA Data Repository item 9501 (complete set surface rupture. We mapped the active fault channels, displaced alluvial fans, and other of soil descriptions) is available on request from traces and studied the morphology of the alluvial surfaces. Individual alluvial surfaces Documents Secretary, GSA, P.O. Box 9140, Boul- scarps to resolve the sense and recency of and the underlying deposits were delineated der, CO 80301. Geological Society of America Bulletin, January 1995 9 MUELLER AND ROCKWELL Figure 2. Geologic map of the southwestern margin of the Sierra Cucapa defining the active and inactive fault strands of the central portion of the Laguna Salada fault zone.
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