Geosphere, published online on 14 August 2013 as doi:10.1130/GES00927.1 Geosphere Initiation of Sierra Nevada range front−Walker Lane faulting ca. 12 Ma in the Ancestral Cascades arc Cathy J. Busby, Jeanette C. Hagan and Paul Renne Geosphere published online 14 August 2013; doi: 10.1130/GES00927.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geosphere Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Hagan1, and Paul Renne2 1Department of Earth Science, University of California–Santa Barbara, Santa Barbara, California 93106, USA 2Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA ABSTRACT graben-vent complex because we have found arc volcanism, under a Walker Lane trans- no vents for high-K lava fl ows here. However, tensional strain regime, and that this con- The eastern escarpment of the Sierra we show that these faults localized the high-K trolled the siting of the Little Walker caldera. Nevada (USA) forms one of the most promi- Little Walker caldera. nent topographic and geologic features in We demonstrate that the range-front INTRODUCTION the Cordillera, yet the timing and nature of faults at Sonora Pass were active before and fault displacements along it remain relatively during the ca. 11.5–9 Ma high-K volcanism. The eastern escarpment of the Sierra Nevada poorly known. The central Sierra Nevada We show that these faults are dominantly (USA) is one of the most prominent topo- range front is an ideal place to determine approximately north-south down to the east graphic and geologic boundaries in the Cordi- the structural evolution of the range front normal faults, passing northward into a sys- llera (Surpless et al., 2002). The southern part of because it has abundant dateable Cenozoic tem of approximately northeast-southwest this boundary is relatively simple, straight and volcanic rocks. The Sonora Pass area of the sinistral oblique normal faults that are on the narrow along the southern Sierra Nevada range- central Sierra Nevada is particularly good southern end of the ~24-km-wide northeast front fault zone, but it becomes more complex for reconstructing the slip history of range- transfer zone in the Sierra Crest graben-vent in the central Sierra Nevada (between Mono front faults, because it includes unusually complex. At least half the slip on the north- Lake and Lake Tahoe; Fig. 1). There, it has been widespread and distinctive high-K volcanic south normal faults on the Sonora Pass range interpreted to form a northwest-trending zone rocks (the ca. 11.5–9 Ma Stanislaus Group) front occurred before and during eruption of of en echelon escarpments produced by normal that serve as outstanding strain markers. the TML, prior to development of the Little or oblique faulting (Wakabayashi and Sawyer, These include the following, from base to top. Walker caldera. It has previously been sug- 2001; Schweickert et al., 2004), with modern (1) The Table Mountain Latite (TML) con- gested that the range-front faults formed a focal plane mechanisms suggestive of oblique sists of voluminous trachyandesite, trachy- right-stepping transtensional stepover that normal faulting (Unruh et al., 2003). The long- basaltic andesite, and basalt lava flows, controlled the siting of the Little Walker term fault history of the southern Sierra Nevada erupted from fault-controlled fi ssures in the caldera; we support that interpretation by escarpment is not well understood because Neo- Sierra Crest graben-vent system. (2) The showing that synvolcanic throw on the faults gene volcanic rocks are generally lacking there, Eureka Valley Tuff consists of three trachy- increases southward toward the caldera. The whereas the long-term history of the central part dacite ignimbrite members erupted from Sonora Pass–Little Walker caldera area is is more easily established and better understood the Little Walker caldera. These ignimbrites shown here to be very similar in structural due to the presence of Cenozoic volcanic and are interstratifi ed with lava fl ows that con- style and scale to the transtensional stepover sedimentary rocks (Schweickert et al., 1999, tinued to erupt from the Sierra Crest graben- at the Quaternary Long Valley fi eld. Fur- 2000, 2004; Henry and Perkins, 2001; Surpless vent system, and include silicic high-K as thermore, the broader structural setting of et al., 2002; Cashman et al., 2009). The central well as intermediate to mafi c high-K lavas. both volcanic fi elds is similar, because both Sierra Nevada range front is an ideal place for The graben-vent system consists of a single are associated with a major approximately determining the long-term history of the range- ~27-km-long, ~8–10-km-wide approximately northeast-southwest sinistral oblique nor- front faults because the area contains extensive north-south graben that is along the mod- mal fault zone. This structural style is typi- dateable Cenozoic strata (Figs. 1, 2, and 3; ern Sierran crest between Sonora Pass and cal of central Walker Lane belt transtension. Busby et al., 2008a, 2008b; Busby and Putirka, Ebbetts Pass, with a series of approximately Previous models have called for westward 2009; Hagan et al., 2009). north-south half-grabens on its western mar- encroachment of Basin and Range extension The Sonora Pass area is particularly advanta- gin, and an ~24-km-wide northeast transfer into the Sierra Nevada range front after arc geous for reconstructing the slip history of Sierra zone emanating from the northeast bound- volcanism ceased (ca. 6–3.5 Ma); we show Nevada range-front faults, because the dateable ary of the graben on the modern range front instead that Walker Lane transtension is Cenozoic volcanic strata include widely distrib- south of Ebbetts Pass. In this paper we focus responsible for the formation of the range uted, compositionally distinctive volcanic units; on the structural evolution of the Sonora Pass front, and that it began by ca. 12 Ma. We con- these are the high-K volcanic rocks of the Stanis- segment of the Sierra Nevada range front, clude that Sierra Nevada range-front fault- laus Group (Fig. 2C) (for a detailed description which we do not include in the Sierra Crest ing at Sonora Pass initiated during high-K of the stratigraphy, see Busby et al., 2013a). Geosphere; October 2013; v. 9; no. 5; p. 1–22; doi:10.1130/GES00927.1; 10 fi gures; 2 tables. Received 11 March 2013 ♦ Revision received 7 June 2013 ♦ Accepted 18 July 2013 ♦ Published online 14 August 2013 For permission to copy, contact [email protected] 1 © 2013 Geological Society of America Geosphere, published online on 14 August 2013 as doi:10.1130/GES00927.1 Busby et al. BASIN AND RANGE ^ Susanville Nevada Honey Lake California Honey Lake f.z. Mohawk Valley f.z. Pyramid Lake N 40º N Dixie Mtn Reno^ Truckee^ Carson Range ^ Genoa fault Carson City Pine Nut Mtns Lake Tahoe Yerington SIERRA NEVADA Singatse^ Range ^ ^Gardnerville Stateline Hope Valley flt Wassuk Range Grover Hot Red Lake flt Springs flt ^ Carson Pass Figure 2 Silver Mtn flt Ebbetts Pass ^ Noble C yn flt Map Key on flt Chan Faults go Lake flt 119º W ^ Lost Cann Sierran crest Sonora Pass Little Walker Center St Mary’s Pass flt Roads ^Bridgeport Rivers and Lakes 03150 Km Mono Lake 38º N Figure 1. Faults of the central to northern Walker Lane belt, which has abundant Cenozoic volcanic rocks, shown in gray (all other rocks and sediments shown in white). F.z.—fault zone; fl t—fault; Mtn—mountain. The brown dotted line represents the present-day Sierra Nevada range crest. Sources include Koenig (1963), Stewart and Carlson (1978), Wagner et al., (1981), Wagner and Saucedo (1992), Henry and Perkins (2001), Saucedo (2005), Busby et al. (2008a, 2008b), Hagan et al. (2009),