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Timing of Cenozoic Volcanism and Basin and Range Extension in Northwestern Nevada: New Constraints from the Northern Pine Forest Range

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Timing of Cenozoic Volcanism and Basin and Range Extension in Northwestern Nevada: New Constraints from the Northern Pine Forest Range Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range Joseph P. Colgan† Trevor A. Dumitru Michael McWilliams Elizabeth L. Miller Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA ABSTRACT Oligocene and Miocene volcanism, and simi- widespread middle Miocene extension in the lar geologic relationships in nearby ranges center of the province (Colgan et al., 2004). High Eocene–middle Miocene volcanic rocks in suggest that a larger region of northwest- fault-bounded ranges in this region give way the northern Pine Forest Range, Nevada, are ern Nevada was also little extended during northward to the high lava plateaus of southern ideally situated for reconstructing the timing this interval. Basin and Range faulting in Oregon, and the pre-Miocene tectonic history and style of volcanism and extensional fault- northwestern Nevada appears to have begun is obscured in many places by gently dipping ing in the northwesternmost part of the Basin no earlier than 12 Ma, making it distinctly Cenozoic lavas. Previous workers suggested and Range province. A conformable sequence younger than deformation in much of central that older Oligocene–early Miocene rocks in of Cenozoic volcanic and sedimentary strata and southern Nevada, where peak extension northwestern Nevada (Fig. 2) may record early in the northern Pine Forest Range dips occurred in the middle Miocene or earlier. Miocene(?) extensional faulting (Noble et al., ~30°W, and 11 new 40Ar/39Ar ages from this 1970; Graichen, 1972), but the relevant units sequence defi ne 3 major episodes of volcanic Keywords: Basin and Range, Pine Forest have not been studied in suffi cient detail to activity. Pre-Tertiary basement and older (ca. Range, volcanism, geochronology, fi ssion- properly characterize such faulting. 38 Ma) Tertiary intrusive rocks are overlain track dating. This study aims to better document the tim- unconformably by Oligocene (ca. 30–23 Ma) ing of Basin and Range extensional faulting and basalt fl ows and dacitic to rhyolitic ash-fl ow INTRODUCTION Cenozoic volcanism in northwestern Nevada tuffs interbedded with fi ne-grained tuffa- and, in particular, to determine if there is evi- ceous sedimentary rocks. Oligocene rocks are Modern ranges in the central part of the dence for extensional faulting prior to the late overlain by ~550 m of ca. 17–16 Ma basalt northern Basin and Range province (along Miocene high-angle faulting that formed the fl ows equivalent to the Steens Basalt in south- lat 39°–40°N; Fig. 1) formed largely during a modern ranges in the region. Well-preserved ern Oregon, and basalt fl ows are capped by widespread episode of major normal fault slip geologic relationships in the Pine Forest Range a thin 16.3 Ma ignimbrite that likely is cor- ca. 19–14 Ma (Miller et al., 1999; Stockli, 1999; (Figs. 2, 3) are ideally suited for addressing relative with either the Idaho Canyon Tuff Stockli et al., 2001; Surpless et al., 2002). In these questions. Tertiary strata range from Oli- or the Tuff of Oregon Canyon. The northern many areas, these ranges cut and uplifted older gocene to middle Miocene and are well exposed Pine Forest Range is bounded to the east by extensional terranes that formed in the Eocene in a west-tilted, normal-fault-bounded crustal a major down-to-the-east normal fault that to early Miocene (e.g., Smith, 1992; Miller block (Noble et al., 1970; this study). dips ~40°E with well-developed fault stria- et al., 1999; Hudson et al., 2000; Egger et al., In this paper, we present new geologic map- tions indicative of dip-slip motion. This fault 2003; Howard, 2003), and the magnitude of ping and 40Ar/39Ar ages of these Tertiary units, initiated at an angle of ~70° and was rotated Cenozoic extension totals 50%–100% (Smith documenting a conformable 30–16 Ma volcanic ~30° during uplift of the range. A suite of 17 et al., 1991). More recent (<12 Ma) Basin and and sedimentary sequence that shows no evi- apatite fi ssion-track ages from the Pine For- Range extensional faulting has been concen- dence for syndepositional faulting or tilting. The est footwall block demonstrates that exhu- trated largely at the eastern and western margins basement rocks exposed beneath the Tertiary mation, uplift, and slip on the range-bound- of the province and has generally involved more sequence include abundant Jurassic-Cretaceous ing fault began ca. 12–11 Ma and continued limited amounts of extension (e.g., Gilbert and granitic plutons (Fig. 3) that permit the use of until at least 7 Ma, with moderate slip since Reynolds, 1973; Dilles and Gans, 1995; Arm- apatite fi ssion-track thermochronology for dat- then. The Pine Forest Range did not undergo strong et al., 2003; Stockli et al., 2003). ing the fault slip, tilting, and uplift that formed signifi cant extension before or during peak Basin and Range faulting is less well under- the range (e.g., Fitzgerald et al., 1991; Miller et stood along the northern margin of the province al., 1999; Stockli et al., 2003). Fission-track data †Present address: U.S. Geological Survey, 345 in northwestern Nevada (Fig. 2). Extensional presented here document a single episode of Middlefi eld Road, MS 901, Menlo Park, California faulting in this region appears to be distinctly major normal faulting that was ongoing from 12 94025, USA; e-mail: [email protected]. younger and of smaller magnitude than the to 7 Ma and continued into more recent times. GSA Bulletin; January/February 2006; v. 118; no. 1/2; p. 126–139; doi: 10.1130/B25681.1; 8 fi gures; 2 tables; Data Repository item 2006013. For permission to copy, contact [email protected] 126 © 2006 Geological Society of America CENOZOIC VOLCANISM AND BASIN AND RANGE EXTENSION, NORTHWESTERN NEVADA 122° W 118° 114° 110° Columbia River Basalts 44° IN Area of LA t P Figure 2 al as ER B IV R Figure 1. Map of part of western United E ns K e SNA States, showing major Cenozoic tectonic te 42° S provinces (Wernicke, 1992), extent of Steens and Columbia River Basalts (Hooper et al., 2002), outcrop area of 37–21 Ma volcanic Pine Forest rocks (dark gray shading, from Christiansen Range ? and Yeats, 1992), and approximate extent of ? ? 30–15 Ma Basin and Range extension (Dilles U A and Gans, 1995). 40° BASIN AND RANGE E T S A I Documented E-W L E P R extension 30–14 Ma Ou R tc A rop O o f 3 D NE 7– 21 A M R V a v 38° A olc O anic L D rocks A O C 100 km N Steens Mtn. Figure 2. Shaded-relief map of northwestern ueblo Mtns. 42° N Basin and Range province, showing features P McDermitt Bilk Creek M Caldera described in text. Faults from state geologic Virgin Valley maps of Jennings et al. (1977), Stewart and Caldera Carlson (1978), and Walker and MacLeod rest R. R. (1991). McDermitt Caldera from Rytuba a Warner R. Fo tns. os ne and McKee (1984); Virgin Valley Caldera Pi a R from Castor and Henry (2000). t n Sa Area of Figure 3 Black Rock R. 0 25 50 ico Hills km Cal 41° N 120° W 119° W 118° W Geological Society of America Bulletin, January/February 2006 127 COLGAN et al. Denio Junction Key to map units Quaternary deposits Tertiary sedimentary rocks 16.2–15 Ma rhyolite 41° 52' 30" ot Valley ash-flow tuffs H 17–16 Ma basalt flows Bog 30–23 Ma volcanic rocks Area of Ashdown Mine Figure 4 Jurassic–Cretaceous granitic rocks Mahogany Mtn. Paleozoic–Triassic metasedimentary rocks Cenozoic normal fault, dotted where concealed A A' 41° 45' N 00 Fission-track sample JC00-PF48 locality (numbers shown only on samples outside A Four Springs Figure 4) l d e r C r e e k t l au F Trend and plunge of fault striations k e Duffer Peak Poles to fault planes n = 25 e r n = 33 C r e 41° 37' 30" d l A Mean fault plane New York Peak 344° 39° JC00-PF44 t l km NV u a F 0 5 k x e e r C d r a 41° 30' N n eo L 118° 52' 30" 118° 45' W 118° 37' 30" 118° 30' W Figure 3. Generalized geologic map of Pine Forest Range, compiled from Smith (1973), Stewart and Carlson (1978), and Wyld (1996), with modifi cations and new mapping by the authors (this study, Fig. 4) 128 Geological Society of America Bulletin, January/February 2006 CENOZOIC VOLCANISM AND BASIN AND RANGE EXTENSION, NORTHWESTERN NEVADA Together with regional data from nearby ranges, Caldera to the west (Fig. 3) were described and a small, roughly east-west–trending paleovalley these new data indicate that Basin and Range dated by Castor and Henry (2000). that is now tilted down to the west. extension in northwestern Nevada did not begin Here, we present descriptions of the Ter- A groundmass concentrate from the base until the late Miocene, involved only a single tiary volcanic rocks in the northern Pine For- of this unit west of Alta Creek Road (Fig. 4) generation of range-bounding normal faults, est Range, together with new 40Ar/39Ar ages yielded complex 40Ar/39Ar release spectra that and accommodated moderate total amounts of and geochemical data that establish the age and do not defi ne a plateau (Fig. 5). The two low- extension (15%–20%). composition of the rocks in the study area. Field est-temperature steps are characterized by low relationships are shown on the geologic map and radiogenic yields (<50%) and young ages; PRE-TERTIARY GEOLOGY OF THE cross section (Fig.
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