Geological Society of America Memoir 195 2002 Stratigraphy and geochemistry of volcanic rocks in the Lava Mountains, California: Implications for the Miocene development of the Garlock fault Eugene I. Smith Alexander Sa´nchez Deborah L. Keenan Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, USA Francis C. Monastero Geothermal Program Office, Naval Air Weapons Station, China Lake, California 93555-6001, USA ABSTRACT Volcanism in the Lava Mountains occurred between 11.7 and 5.8 Ma and was contemporaneous with sinistral motion on the Garlock fault. Volcanic rocks, equiv- alent in age and chemistry to those in the Lava Mountains, crop out 40 km to the southwest in the El Paso Mountains across the Garlock fault. Three chemical groups of volcanic rocks erupted in the Lava Mountains over a period of 5 m.y. These are (1) andesite of Summit Diggings, Almond Mountain volcanic section, and Lava Moun- tains Andesite, (2) basalt of Teagle Wash, and (3) tuffs in the northeastern Lava Moun- tains and dacite in the Summit Range. Volcanic rocks of each group have distinctive chemical signatures useful for correlation of units across the Garlock fault. Our work demonstrated that tuffs in the Almond Mountain volcanic section may be equivalent to a tuff in member 5 of the Miocene Dove Spring Formation, El Paso Mountains. The basalt of Teagle Wash probably correlates with basalt flows in member 4, and tuffs in the northeast Lava Mountains may be equivalent to tuff of member 2. Cor- relation of these units across the Garlock fault implies that the Lava Mountains were situated south of the El Paso Mountains between 10.3 and 11.6 Ma and that 32–40 km of offset occurred on the Garlock fault in ϳ10.4 m.y., resulting in a displacement rate of 3.1 to 3.8 mm/yr. Projecting this rate to the total offset of 64 km on the Garlock suggests that left-lateral slip began at ca. 16.4 Ma. INTRODUCTION Valley (Fig. 1). Total sinistral displacement on the Garlock fault is ϳ64 km (Smith, 1962; Smith and Ketner, 1970; Davis and The Lava Mountains lie just south of the active Garlock Burchfiel, 1973; Monastero et al., 1997). Estimates of the ini- fault, a major continental strike-slip fault that separates the tiation of faulting vary from 10 to 9 Ma (Burbank and Whistler, southwestern Basin and Range to the north from the Mojave 1987; Loomis and Burbank, 1988) to after 17 Ma (Monastero block to the south. The fault extends 250 km from the San et al., 1997). Faulting continued to the present with a minimum Andreas fault to the Avawatz Mountains just south of Death of 18 km of displacement occurring across the central Garlock Data Repository item 2002### contains additional material related to this article. Smith, E.I., Sa´nchez, A., Keenan, D.L., and Monastero, F.C., 2002, Stratigraphy and geochemistry of volcanic rocks in the Lava Mountains, California: Implications for the Miocene development of the Garlock fault, in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Boulder, Colorado, Geological Society of America Memoir 195, p. 151–160. 151 152 E.I. Smith et al. Christmas Canyon Fm. Bedrock Spring Fm. Lava Mountains Andesite Other Tertiary volcanic rocks v v Almond Mountain volcanic rocks Tertiary rhyolites & tuffs v v 36'N vv ° Andesite of Summit Diggings & Mesozoic granites dacite in the Black Hills Basalt of Teagle Wash & gravels 010km SA CR GF Figure 1. Geologic map of the Lava EP Mountains and neighboring Summit EC 31'N 35 Range and Black Hills modified from ° LM Garlock fault Smith (1964), Dibblee (1967), and 35 Keenan (2000). WLMV—Western Lava Mountains Volcano. Inset map of Cali- fornia shows the following locations: GF—Garlock fault, SA—San Andreas fault, LM—Lava Mountains, EC—Ea- Summit Range gle Crags volcanic field, EP—El Paso v v v Mountains, and CR—Coso Range. Box v v Black Hills v v v v v v v v v v v v v indicates area covered by Figure 3. v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v 26'N v v v v v v v v v v v v v v v v v v Blackwater fault ° v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v 35 v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v WLMVv v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v N v v v v v v v v v v v v v v v v v v Trona Rd. v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v + v v v v v v v v v v v v vvvAlmondv Mtn. v v v v v v Figure 3 v v v v v v v v v v v v + v v v v v Red Mtn. 117°38'W 117°28'W 117°18'W fault since the Pleistocene (Carter, 1980). Volcanismin the Lava as conglomerate clasts in the eastern Lava Mountains (Carter, Mountains began at ca. 11.7 Ma, ended at 6.4 Ma, and is unique 1982, 1987, 1994). in that it represents a period of major volcanic activity astride This paper presents the results of new stratigraphic, geo- the Garlock fault that occurred during lateral displacement. The chronologic, and geochemical investigations of the Miocene Lava Mountains, originally described by Smith (1964) and Dib- volcanic rocks of the Lava and El Paso Mountains and relies blee (1967), were remapped in part to clarify stratigraphic re- on earlier studies by Smith (1964) and Carter (1980, 1982, lationships, to determine the petrogenesis of the volcanic rocks, 1987, 1994). The principal objective is the correlation of vol- and to calculate offset on the Garlock fault. Smith (1964) es- canic units between the Lava and El Paso Mountains across the tablished the basic stratigraphic framework for the Lava Moun- Garlock fault with the purpose of estimating slip rate on the tains and generally located source areas for the Almond Moun- fault. Field studies described a volcanic center in the western tain volcanic section and the Lava Mountains Andesite in both Lava Mountains, the source of a significant volume of andesite the western and eastern Lava Mountains. and dacite lava and pyroclastic material. The close proximity of volcanic centers in the Lava Moun- tains to the Garlock fault and the potential for locating units VOLCANIC STRATIGRAPHY that either flowed or were transported across the fault provide the opportunity to more fully understand the development of On the basis of detailed and reconnaissance mapping, the Garlock fault during Tertiary time. Units coeval and geo- Smith (1964) developed a stratigraphy for the Lava Mountains chemically identical to those in the Lava Mountains occur north composed of six volcanic units and basin-fill gravels and sand- of the Garlock fault in the El Paso Mountains in the Miocene stones. Although he lacked isotopic dates, volcanic units were Dove Spring Formation (Loomis and Burbank, 1988). Further- assigned ages varying from pre–middle Pliocene to Quaternary more, lithologies characteristic of the El Paso Mountains occur on the basis of paleontological and stratigraphic observations. Stratigraphy and geochemistry of volcanic rocks in the Lava Mountains, California 153 Our work based on geochemistry, geochronology, and field Rocks of the second episode (10.4 to 9.54 Ma; Table 1) studies required the revision of this stratigraphy and demon- are volumetrically the most important and are separated from strated that volcanic activity in the Lava Mountains occurred volcanic rocks of the first episode by the sandstones of the in three episodes spanning about 5 m.y. in the middle Miocene Bedrock Spring Formation (Fig. 2). The following units were to late Miocene (Fig. 2). produced during the second episode: (1) volcanic rocks in the Rocks of the first episode were mapped by Smith (1964) uppermost part of the Bedrock Spring Formation, (2) the Al- as volcanic units older than the Bedrock Spring Formation. mond Mountain volcanic section, (3) “Quaternary” andesite These units are shown herein to range in age from 11.7 to 10.7 (Smith, 1964) herein termed “the andesite of Summit Dig- Ma, are lithologically diverse, and include (1) dacite (11.7 Ma; gings,” and (4) the “Quaternary” basalt (Smith, 1964) herein Table 1) with large (2 cm) phenocrysts of plagioclase, herein termed “the basalt of Teagle Wash.” termed “the dacite of Summit Range”; (2) dacite containing The Almond Mountain volcanic section erupted from the quartz, hornblende, biotite, and plagioclase dated at 10.73 Ma 9-km-diameter western Lava Mountains volcano.