G33634 2nd pages GEOLOGY | January 2013 | www.gsapubs.org 1 INTRODUCTION Volcanic chronostratigraphy is essential in understanding the timing of human origins, migration, and exchange. Radiometric dating of volcanic rocks down to the historical realm is possible in favorable cases by K-Ar ( 40 Ar/ 39 Ar) geochronology (e.g., the 79 CE [Common Era] eruption of Vesuvius, Italy; Renne et al., 1997; Lanphere et al., 2007), but often accuracy is compromised by excess 40 Ar, parent-daughter mobility, or diffusive fractionation of 40 Ar/ 36 Ar (e.g., Cerling et al., 1985; Esser et al., 1997; McDougall and Harrison, 1999; Morgan et al., 2009; Flude et al., 2010). (U-Th)/He geochronology of zircon, as one of the first radiometric dating techniques (Strutt, 1908), has had a revival as a thermo- chronometer, but it is equally suitable for rapidly cooled volcanic rocks without sub- sequent thermal disturbance (e.g., Tagami et al., 2003; Davidson et al., 2004; Schmitt et al., 2006; Blondes et al., 2007). Compared to K-Ar ( 40 Ar/ 39 Ar) geochronology, it offers the advantages of reduced excess 4 He due to rapid diffusion (Reiners et al., 2004), high daughter isotope production rates ( 4 He/ 40 Ar >20 per par- ent nucleus), and negligible atmospheric con- tamination (air Ar/He ~2000). Its application, however, has been limited by U-series disequi- librium effects, which, in the case of youthful zircon, typically cause severe age underesti- mation (Farley et al., 2002). To overcome this limitation, a novel combined U-Th ion micro- probe crystal rim analysis and (U-Th)/He bulk crystal degassing analysis technique has been developed (to equal extent at the University of California, Los Angeles [UCLA] by Schmitt and Lovera, and at the University of Kansas by Stockli). Here we apply this technique to docu- ment an eruption in the Salton Trough (South- ern California, United States) as recently as 0–940 BCE (Before Common Era). This age overlaps with human occupation of the Colo- rado Desert and adjacent regions, and agrees with the late onset of obsidian use from the Obsidian Butte resource in the Salton Trough (e.g., Schaefer and Laylander, 2007). VOLCANISM IN THE SALTON TROUGH The Salton Buttes together with Cerro Pri- eto and Roca Consag are morphologically the most pristine volcanoes in the northern Gulf of California and Salton Trough rift zone (Fig. 1). They form an ~7-km-long, northeast-southwest– trending lineament of 5 rhyolite domes (<1 km in diameter) along the dilatational San Andreas– Imperial fault step-over, immediately north of the active Brawley seismic zone (Fig. 1). The two largest domes are Red Island and Obsidian Butte; they consist of aphyric, glassy to devit- rified rhyolite lava with rare oligoclase-anor- thoclase phenocrysts surrounded by remnants of pyroclastic deposits. Where lavas developed obsidian textures, they were extensively quar- ried for production of lithic tools during late prehistoric and early post-Columbian time (Treganza, 1942; Hughes and True, 1985; Koer- per et al., 1986; Schaefer and Laylander, 2007). They also contain diverse xenoliths comprising unconsolidated sediment, metasediment, basalt, and granophyre (Robinson et al., 1976; Schmitt and Vazquez, 2006). Granophyre xenoliths are leucocratic and phaneritic with interstitial glass. Their radiogenic and oxygen isotopic composi- tions demonstrate that they are cogenetic with the rhyolite host, and derived from remelting of juvenile mafic crust in an incipient oceanic spreading center (Schmitt and Vazquez, 2006), analogous to other transform fault–bounded rift segments within the Gulf of California (e.g., Lizarralde et al., 2007). *E-mail: axelk@argon.ess.ucla.edu. (U-Th)/He zircon and archaeological ages for a late prehistoric eruption in the Salton Trough (California, USA) Axel K. Schmitt 1 *, Arturo Martín 2 , Daniel F. Stockli 3 , Kenneth A. Farley 4 , and Oscar M. Lovera 1 1 Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, California 90095-1567, USA 2 Departamento de Geología, CICESE, Km 107 Carretera Tijuana, Ensenada, B.C. C.P. 22800, México 3 Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA 4 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA GEOLOGY, January 2013; v. 41; no. 1; p. 1–4; Data Repository item 2013003 | doi:10.1130/G33634.1 | Published online XX Month 2012 © 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. 117° 114°W 115° 115° 116° 116° Colorado Colorado River Cerro Prieto SAF IF Salton Trough CPF BSZ Lake Cahuilla shoreline Roca Consag WB Peninsular Ranges Baja California N 1 2 3 5 6 Salton Buttes Sonora 4 A B Salton Sea Red Island Obsidian Butte 1 0 2 km Gila River Desert 32°N 32°N 32°N ABSTRACT U-Th and (U-Th)/He zircon geochronology redefines the timing of volcanic activity in the Salton Trough (Southern California, USA), the subaerial extension of the incipiently oceanic Gulf of California. U-series disequilibrium corrected (U-Th)/He zircon analyses for a granophyre ejecta clast from the Red Island rhyolite dome indicate an eruption age of 2480 ± 470 a (calendric dates between 0 and 940 Before Common Era, BCE; error at 95% confidence). This eruption age is supported by U-Th zircon crystallization ages for two obsidian-bearing lavas: Red Island (the host for the granophyre) and Obsidian Butte, a pre- historic quarry for obsidian that is widely distributed in southern California and northern Mexico archaeological sites. Lavas and granophyre display overlapping zircon crystalliza- tion age distributions that support field and compositional evidence that they are cogenetic and contemporaneous. The (U-Th)/He eruption age is younger and significantly more pre- cise than previous ages for these volcanoes, and is the first indication that the eruption of obsidian flows coincided with human presence in the region. A late prehistoric eruption age agrees with the absence of the Obsidian Butte lithic source among early prehistoric cultural artifacts, previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient Lake Cahuilla. Figure 1. Map of northern Gulf of California (United States) and Salton Trough showing Qua- ternary volcanoes (triangles) and major faults (solid lines; SAF—San Andreas; IF—Impe- rial; CPF—Cerro Prieto). Maxi- mum extent of Pleistocene–Ho- locene Lake Cahuilla (i.e., fill line at 12 m above sea level) is indicated (after Brothers et al., 2011). BSZ—Brawley seismic zone; WB—Wagner Basin. In- set A shows study region with archaeological locations (1— San Diego; 2—Anza-Borrego; 3—Coachella Valley; 4—Zara- goza, 5—Channel Islands; 6— Coso). Inset B details locations for Salton Buttes domes (red).