Cenozoic palaeocanyon evolution, Ancestral Cascades arc volcanism,and structure of the Hope Valley–Carson Pass region, Sierra Nevada,CaliforniaJeanette C. Hagana*, Cathy J. Busbya, Keith Putirkaband Paul R. RennecdaDepartment of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106,USA;bDepartment of Earth and Environmental Sciences, California State University, Fresno, CA93710, USA;cBerkeley Geochronology Center, 2455 Ridge Rd, Berkeley, CA 94709, USA;dDepartment of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA( Accepted 20 April 2009)We use new geologic mapping, geochronological, and geochemical data on Tertiaryvolcanic, volcaniclastic, and intrusive rocks to investigate the volcanic, stratigraphic,and structural evolution of the Carson Pass region south of Lake Tahoe in the centralSierra Nevada. Volcanic and volcaniclastic rocks were deposited in east –west-trendingpalaeocanyons carved into Mesozoic granitic and metamorphic basement rocks;sediments were transported westwards towards the present-day Central Valley ofCalifornia. In the Carson Pass– Hope Valley area, two palaeotributaries are preservedin faulted terrane east of the present-day Sierran crest (Hope Valley area); these mergeat the crest to form one large, . 7 km wide palaeocanyon that is undisrupted by faultswest of the crest (Carson Pass –Kirkwood area). This single, large palaeocanyonroughly coincides with the present-day Mokelumne River drainage. New40Ar/39Ardates and stratigraphic data east of the crest, integrated with previously published datawest of the crest, constrain the ages of strata and unconformities within the HopeValley– Carson Pass– Kirkwood palaeocanyon system. We interpret three majorerosional unconformities to record uplift events at ca. 23–16, 13.5 –11, and 10 –7 Ma.In other parts of the central Sierra, these uplift events are inferred to correspond torange-front faulting events. We propose the term ‘Hope Valley Graben’ for thestructural feature mapped immediately east of the Sierran crest at Carson Pass. It is anearly symmetrical full graben that offsets volcanic rocks as young as 6 Ma at least400 m (1300 ft) on each of its bounding faults – herein named the Red Lake Fault onthe west and the Hope Valley Fault on the east. However, we infer that faulting beganbefore eruption of the 6 Ma volcanic rocks for three reasons: (1) the graben-localizedemplacement of one of the largest volcanic centres in the Sierra, the 6.34 ^ 0.14–6.18 ^ 0.14 Ma Markleeville Peak Center; (2) andesite lava flows erupted at6.22 ^ 0.14 Ma from the Red Lake Fault, and abut it within the graben; and (3)brecciated granite along the Red Lake Fault is intruded by altered andesite, indicatingthat the fault started slipping before magmatism ceased. Our stratigraphic andgeochronologic data do not permit an estimate of the amount of pre-6 Ma displacementin the Hope Valley graben. The geometry of the palaeocanyon system indicates that adextral component of slip demonstrated for transtensional faults in other parts of theregion did not operate in the Hope Valley graben.Keywords: Sierra Nevada; Ancestral Cascades arc; Walker Lane BeltISSN 0020-6814 print/ISSN 1938-2839 onlineq 2009 Taylor & FrancisDOI: 10.1080/00206810903028102http://www.informaworld.com*Corresponding author. Email: [email protected] Geology ReviewVol. 51, Nos. 9–11, September– November 2009, 777–823IntroductionIn the central Sierra Nevada (Figure 1), Tertiary deposits of dominantly fragmentalvolcanic–volcaniclastic andesitic rocks of the Ancestral Cascades arc were deposited into,and are preserved in palaeochannels (Lindgren 1911; Curtis 1954; Wagner et al. 2000).Volcanic and volcaniclastic rocks from Carson Pass in the central Sierra Nevada recordpart of this palaeocanyon fill. These rocks record the late Cenozoic volcanotectonic historyof the Sierra Nevada, including the deposition of Oligocene ignimbrites erupted in centralNevada, in situ Ancestral Cascades arc magmatism, initiation of Walker Lanetranstensional faulting, and onset of Basin and Range extensional faulting.In the Late Cretaceous, the Sierra Nevada formed the western edge of a Tibetan-typeplateau, termed the ‘Nevadaplano’ (Coney and Harms 1984; Chase et al. 1998; Wolfe et al.1998; DeCelles 2004), with a drainage divide in central Nevada (Henry 2008).Palaeorivers drained westwards across western Nevada and eastern California into thepresent-day Central Valley, carving wide canyons into the Sierra Nevada basement rock(Lindgren 1911; Bateman and Wahrhaftig 1966; Huber 1990; Wakabayashi and Sawyer2001; Garside et al. 2005). During Oligocene time, large-volume caldera-formingeruptions in central Nevada produced ash flows that were funnelled down the channelsacross western Nevada and the Sierra Nevada (Davis et al. 2000; Henry et al. 2003). Fromthe late Oligocene to early Miocene, the arc front swept westwards across Nevada,accompanying slab fallback (Wernicke 1992; Dilles and Gans 1995; Schweickert et al.2004; Dickinson 2006; Cousens et al. 2008). Debris from calc-alkaline andesitic eruptionswas transported westwards, down the palaeocanyons, as debris flows and streamflows. Thecalc-alkaline Ancestral Cascades arc swept westwards into the present-day northern andcentral Sierra Nevada by 16 Ma (Putirka and Busby 2007; Busby et al. 2008a,b; Cousenset al. 2008; Busby and Putirka 2009). New40Ar/39Ar age data suggest that arc magmatismin the central Sierra occurred from , 16 to 6 Ma (Busby et al. 2008a,b; this paper).A westward encroachment of normal faulting may have accompanied the westwardmagmatic sweep, although the timing and nature of onset of Sierran range-front faultingremains controversial (Dilles and Gans 1995; Trexler et al. 2000; Henry and Perkins 2001;Stockli et al. 2002; Surpless et al. 2002). Our recent research at Sonora Pass (Figure 1(a))shows that dextral transtension began at that latitude by , 10 Ma, triggering high-Kmagmatism from the Little Walker Center (Putirka and Busby 2007; Busby et al. 2008b).We proposed that this centre formed at a releasing stepover of dextral transtensional faultsat the inception of the Walker Lane Belt, and may record the birth of the future Pacific–North American plate boundary (Busby and Putirka 2009).The Sierra Nevada lies within a microplate bounded to the west by the San AndreasFault and to the east by the Walker Lane Belt (Figure 1), an , 150