p Crustal Evolution of the GreatBasin and the Sierra Nevada Edited by Mary M. Lahren and James H. Trexler, Jr., Department of Geological Sciences, University of Nevada, Reno, NV 89557 and Claude Spinosa Department of Geosciences, Boise State University, Boise, ID 83745 Field Trip Guidebook for the 1993 Joint Meeting of the Cordilleran/Rocky Mountain Sections of the Geological Society of America Reno, Nevada, May 19-21, 1993 Published by Department Geological Sciences Mackay School of Mines University of Nevada, Reno Reno, Nevada 89557 OLIGOCENE-MIOCENE CALDERA COMPLEXES, ASH-FLOW SHEETS, AND TECTONISM IN THE CENTRAL AND SOUTHEASTERN GREAT BASIN Myron G. Best Brigham Young University, Provo, Utah 84602 Robert B. Scott, Peter D. Rowley, WC Swadley, R. Ernest Anderson U.S. Geological Survey, Denver, Colorado 80225 C. Sherman Gromme U.S. Geological Survey, Menlo Park, California 94025 Anne E. Harding University of Colorado, Boulder, Colorado 80309 Alan L. Deino Geochronology Center, Institute of Human Origins Berkeley, California 94709 Eric H. Christiansen, David. G. Tingey, Kim R. Sullivan Brigham Young University, Provo, Utah 84602 ABSTRACT Regional extension was minimal during most of the ignimbrite flareup. However, local extension The Great Basin harbors at least sixty Tertiary occurred before the flareup and major extensional and calderas and inferred sources of tuff and several tens local strike-slip faulting beginning in the early of thousands of cubic kilometers of ash-flow deposits, Miocene affected many parts of the Great Basin, making it one of the greatest manifestations of including the Caliente and Kane Spring Wash caldera prolonged ash-flow volcanism in the terrestrial rock complexes where synvolcanic faults form many record. Some individual calderas are exposed east to caldera margins. west across three or four mountain ranges. Simple­ cooling-unit outflow tuff sheets cover areas of tens of INTRODUCTION thousands of square kilometers and range to as much as hundreds of meters thick. During the "ignimbrite The purpose of this brief review and field trip flareup" from about 31 to 22 Ma, when most of the roadlog is to sample the results of the waxing and ash-flows were erupted, extrusion of lava in the Great waning of the Great Basin ignimbrite flareup. For a Basin was minor, widely scattered, and did not form more detailed report on Great Basin volcanism during major edif:ces such as composite volcanoes. Typical the Tertiary, see Best and others (l989a). The support volcanic sections consist of multiple ash-flow tuff of the National Science Foundation through grants cooling units from nearby caldera sources and only EAR-8604195, -8618323, and -8904245 to M.G. Best local lava flows and pyroclastic-surge and -fall and E.H. Christiansen is gratefully acknowl- deposits. edged. We appreciate helpful reviews by C. Chapin, D.A. John, R.F. Hardyman, and E.H. McKee. Tuffs older than about 17 Ma in the Great "Basin are high-potassium calc-alkaline rhyolite, GEOLOGIC SETTING dacite, and sparse andesite in which phenocrysts of two feldspars, quartz, Fe-Ti oxides, and biotite are The pre-volcanic underpinning of the Great common. Rhyolite tuff occurs throughout the Basin i~ a terrane containing late Precambrian, Tertiary, whereas huge volumes of dacite ash flows """Phaneiozoic," d local Mesozoic sedimentary rocks were erupted about 31 to 27 Ma and high-temperature e ormed during compressional episodes in Paleozoic trachydacite magmas containing phenocrysts of and Mesozoic Eras and intruded locally by Mesozoic plagioclase and pyroxene erupted from many centers granitic plutons. After widespread erosion in late mostly about 27 to 23 Ma. After 17 Ma, alkaline Cretaceous and early Tertiary time which produced a metaluminous to mildly peralkaline magmas profound unconformity and, in some places, early containing Fe-rich pyroxene and olivine, sanidine, Tertiary sedimentation, volcanism began in the and quartz phenocrysts began to be erupted. Eocene about 43 Ma in northern Nevada and Utah and swept southward along an arcuate, roughly volcanism (Best and Christiansen, 1991). In and near east-west front, reaching southern Nevada by middle the Caliente and Kane Springs Wash caldera Miocene time. complexes (Figs. 1 and 2) the main extensional episode was in early to middle Miocene time (Scott, The inventory of Cenozoic rocks in the Great 1990; Rowley and others, 1992), concurrent with Basin by Stewart and Carlson (1976; see also Best caldera volcanism, as Great Basin ash-flow activity and Christiansen, 1991; Figs. 3 and 4) clearly shows waned. During the ignimbrite flareup, however, the products of the late Oligocene-early Miocene sparse clastic deposits and few angular discordances ignimbrite flareup; the volume of resulting ash-flow in outflow volcanic sections show that regional deposits in the Great Basin is not widely appreciated tectonic extension in the Great Basin as a whole was but is an order of magnitude larger than in the well limited. known San Juan and Mogollon-Datil fields in the eastern Cordillera. However, the Great Basin harbors CALDERAS only a fraction of the volume of ash flow tuffs in the Sierra Madre Occidental of Mexico. During the Recognition of calderas is hampered not only ignimbrite flareup in the Great Basin, the volume of by erosion and burial beneath younger deposits, but extruded lava was minor compared to ash-flow also in the Great Basin by widespread post-volcanic, deposits and was less than the volume of lava and local synvolcanic (Caliente area and Stillwater extruded before and after the flareup. Scarce Range), faulting that has dismembered the calderas Oligocene debris flows indicate a general lack of into small segments, blurring their margins and large volcanic edifices in contrast to, for example, the internal structure. Geographic centering within the San Juan volcanic field. outflow sheet may be misleading as outflow lobes are commonly not radially symmetric about the source Until the early Miocene, at about 24 Ma, calc­ caldera (e.g., Windous Butte Formation, Best and alkaline volcanism had produced a large volume of others, 1989a). Topographic margins are poorly dacite to rhyolite ash-flow tuff and subordinate known even for some of the better located calderas. high-potassium andesite and dacite and rhyolite lava; Piles of tuff as much as 2-3 km thick are an obvious basalt appears only after 22 Ma (Barr and others, indicator of a caldera (Ekren and others, 1973; Best 1992). During the next 8 m.y., explosive volcanism and others, 1989a, Figs. R29, R32-R38), but some waned and a broader compositional spectrum, but stilI demonstrable proximal outflow tuff deposits ponded dominated by rhyolite, appears in the overall volcanic in older calderas and on downthrown sides of record. Basaltic volcanism has been a significant synvolcanic extensional growth faults are also thick aspect of Great Basin activity after about 13 Ma, (Dixon and others, 1972; Best and others, 1989b, partieularly along the eastern and western margins of Figs. 5B and 5C). Dense compaction and widespread the region but also locally in the center (McKee and propylitic alteration of compound or multiple cooling Noble, 1986). Many silicic tuffs and lavas younger units comprising the intracaldera tuff make it more than about 17 Ma are peralkaline or topaz-bearing resistant to erosion relative to the caldera wall rocks (Noble and Parker, 1975; Christiansen and others, and hence causes the development of inverted 1986). topography that is a common clue to the existence of the caldera. Megabreccia and "rafts" of internally After the middle Miocene, the general east-west shattered but nonetheless stratigraphically coherent orientation of magmatic zones changed to north-south rock, locally more than 2 km across and hundreds of (Best and others, 1980; Stewart, 1983), probably meters thick, occur within a few kilometers of some reflecting a fundamental change in the state of stress caldera walls (Bonham and Garside, 1979, p. 40; in the lithosphere (Best, 1988). McKee, 1976; Best and others, 1989a, Figs. R12, R24, R25, R36, and R38). Extensional tectonism in the Great Basin during Tertiary time was episodic (e.g., Taylor and others, Caving of the unstable caldera escarpment 1989), was intense in some areas (e.g., Proffett, enlarges the perimeter of a caldera so that topographic 1977; Moores and others, 1968; Gans and others, diameters can be several kilometers greater than the 1989) and moderate in others (e.g., the southern ring-fault system (Best and others, 1989a, Fig. R32; Pancake Range, Snyder and others, 1972), and in Best and others, 1989b, Fig. 5B). Younger post­ general correlates poorly in space and time with caldera collanse denosits may completely fill and even + ,/ 1+ 1 I /•..•... ··············l zl kiloMeters 100 "'Ie < -l >- 1):>- ",.--">, ••••.•~UREKA~ 01" I' DELTA / ~ __+-'-~~~_\~-~~'~_:~~--- __<JJ.\ \\ I' PRESroN ."..#. I \ \. CURRANT ...... .... •...•.••. I \ + + 37i\1 \ 112\0/ Figure 1. Caldera margins and areal extents of some outflow tuff sheets in the southeastern Great Basin (Table 1). Caldera margins (heaviest lines) dashed where approximately located; dotted lines indicate indefinite source areas. Calderas and sources in the Central Nevada caldera complex include the Broken Back 2 (BB), source of Stone Cabin Formation (S), Williams Ridge (W), Hot Creek (H), Pancake Range (P), Kiln Canyon (KC), Big Ten Peak (BT), unnamed caldera source of tuff of Lunar Cuesta (L), Kawich (K), Goblin Knobs (G), Quinn Canyon Range