New Mexico Bureauof Mines & Mineral Resources Memoir 47, May 1989 91 EXCURSION 3A: Eocene through Miocene volcanism in the Great Basin of the western United States Myron G. Best', Eric H. Christiansen', AlanL. Deino", C. Sherman Gromme", 3 4 Edwin H. McKee , and Donald C. Noble 'Brigham Young University, Provo, Utah 84602; 'Berkeley Geochronology Center, Berkeley, California 94709; 'U.S. Geological Survey, Menlo Park, California 94025; 'University ofNevada, Reno, Nevada 89557 Introduction Geologists of the U.S. Geological Survey, beginning with Cenozoic magmatic rocks in the western United States the Nevada Test Site project in the early and middle 1960's, define a complex space-time-composition pattern that for mapped individual ash-flow sheets over thousands of square many years has posed numerous interpretive challenges. kilometers in the south-central and western Great Basin and One of the more intriguing tectonomagmatic provinces is identified many of their sources. These investigations dem­ 'the northwestern segment of the Basin and Range province onstrated the feasibility of systematic geologic mapping of in Nevada, western Utah, and minor adjacent parts of Or­ ash-flow terranes and the utilization of such data for rec­ egon, Idaho, and California. This segment, characterized ognizing volcanic centers and working out the history of for the most part by internal drainage, is called the Great volcanic fields. Basin (Fig. 1). This paper summarizes characteristics of The magmatic and tectonic development of the Great volcanic rocks in the Great Basin, primarily ash-flow tuff Basin during Cenozoic time was the focus of a number of and subordinate lava, which were.deposited from Eocene papers in the late 1960's and early 1970's based on the body through Miocene time. Although widespread younger vol­ of chemical, petrologic, and radiometric-age data then avail­ canic deposits exist, their volume is much less and they are able. This information was used by Armstrong et al. (1969) not considered here. Only a few general and preliminary and McKee et al. (1970) to define patterns in age and rock interpretations of our observations are presented. distribution. Models of tectonic evolution were developed The support of the National Science Foundation through by McKee (1971), Christiansen and Lipman (1972), Lipman Grant EAR-8604195 to M. O. Best is gratefully acknowl- et al. (1972), and Noble (1972). A landmark paper by At­ edged. yx' water (1970) showed that early Tertiary plate convergence Contributions of severalgeologists have provided the ba­ along the southwestern margin of North America has been sis for study of volcanic rocks in the Great Basin. Howell supplanted since sometime in the Oligocene by transform Williams in the 1950's (unpublished reconnaissance) first motion on the San Andreas system; this transform motion realized that many of the volcanic rocks are ash-flow de­ began just south of the U.S.-Mexico border and has now .. :.:!",O. posits. J. Hoover Mackin (1960) recognized the extensive migrated to just north of San Francisco (Fig. 2). Numerous nature of the ash-flow sheets, gave stratigraphic names to later publications have refined, summarized, and/or re­ some, and emphasized their usefulness as "instantaneous" viewed the information in these early papers, but have not time horizons to better understand basin and range structure. fundamentally changed the original concepts. Paul L. Williams (1960) and Earl F. Cook (1965), students Despite these past efforts, research on volcanic deposits of Mackin, used petrographic properties, especially phe­ in the Great Basin is still in its infancy, in comparison to nocryst proportions, as well as stratigraphic position, to volcanic fields such as the Sanjuan field of Colorado. This correlate extensive ash-flow sheets in the eastern Great Ba­ is in part because of the vast area and enormous volume of sin. Cook named units, recognized the southward transgres­ volcanic rock in the Great Basin (Table 1). Although the sion in time of their sources, and realized that the volcanism entire region is covered by county, state, or other geologic occurred over a brief interval of geologic time. He grappled maps at a scale of 1:250,000, generalization of rock units with the significance of the basal Cenozoic unconformity, hampers their usefulness for structural, stratigraphic, and and concluded that basin and range topography originated petrologic interpretations, such as delineation of source cal­ after deposition of the tuffs. deras and associated outflow-tuff sheets. Less than half of f' TABLE l---Comparison of major Tertiary volcanic fields in western North America that contain significant volumes of ash-flow deposits (Lipman, ·t 1975; Steven and Lipman, 1976; Steven et al., 1984; Ratte et al., 1984; Stewart and Carlson, 1976; Swanson and McDowell, 1984). See Fig. 2. ~' Intermediate lava Silicic tuff Inception of Approximate Period of Approximate Area major activity volume major activity volume Number of Volcanic field (10' krrr') (Ma) (10' krn') (Ma) (10' krrr') calderas Great Basin 360* 40 ?5? 33-20 50? 50± Marysvale 10 30 7 27-23 <1 4 San Juan 25 35 50 29-27 10 15 Mogollon-Datil 25 40 ? 29-27 10 10? Sierra Madre Occidental 300 42 ? Oligocene ?300? ?350? 92 the region is covered by larger-scale maps (1:62,500 or In the region that now forms the Great Basin and sur­ greater) that show individual formations. Relatively few rounding areas, volcanism began in the north in the Eocene detailed petrologic investigations of individual ash-flow sheets about 43 Ma (Armstrong et al., 1969) and swept southward and their magma systems have been published. along an arcuate, roughly east-west front into southern Ne­ vada (McKee, 1971; McKee and Silberman, 1975; Noble Regional geologic setting et al., 1976) by Miocene time (Fig. 2). Accompanying this Building on earlier publications, Lipman (1980), Stewart transgressive volcanism was a southward-moving wave of (1983), Elston (1984), Chadwick (1985), and Wernicke et east-west to northwest-southeast crustal extension, which, al. (1987) addressed the late Mesozoic and Cenozoic tec­ though imperfectly defined, is expressed in detachment-fault tonomagmatic evolution of the western United States. Fun­ terranes and metamorphic core complexes. Locally, hun­ damental elements of this evolution include Cretaceous (and dreds of percent extension have been documented (e.g., locally latest Jurassic) to early Eocene east-west crustal Wernicke et al., 1987; Gans et al., submitted; Taylor et al., shortening and granitic plutonism. This was followed by 1987; Wust, 1986). South of the region of southward mi­ east-west crustal extension and widespread, locally volu­ grating volcanism is an east-west amagmatic corridor where minous high-K subalkaline volcanism that shifted in many there has been sparse Phanerozoic magmatism. This corridor places to bimodal basalt-rhyolite activity since about middle effectively separates the Great Basin from the southern Basin Miocene time. and Range province where northward-sweeping volcanism FIGURE I---Composite satellite image of the Great Basin that approximately covers the northwestern part of the Basin and Range province, bounded on the west by the Sierra Nevada and on the east by the Colorado Plateaus. The state of Nevada is shown by dashed line. 93 a significant aspect of Great Basin activity since about 12 Ma, particularly along the eastern and western margins of the region but also locally in the center (Best and Hamblin, 1978; McKee and Noble, 1986). Many silicic tuffs and lavas younger than about 17 Ma are peralkaline or topaz-bearing --- (Noble and Parker, 1975; Christiansen et aI., 1986). After the middle Miocene, the general east-west orientation of magmatic zones changed to north-south (Best et aI., 1980; Stewart, 1983), probably reflecting a fundamental change in the state of stress in the lithosphere (Best, 1988a). Since the middle Miocene, crustal extension affected a o larger area (Anderson and Ekren, 1968; Armstrong et aI., § 1969; Scholz et aI., 1971; Proffett, 1977; McKee and Noble, 1986; Wernicke et aI., 1987), forming what has conven­ tionally been called the Basin and Range province ofwestern :n~ North America. The existence of early Miocene ash-flow Francisco sheets derived from sources within the Great Basin in the I ...... 8 -1~ flanking, generally topographically higher Colorado Pla­ -:-- teaus and Sierra Nevada (Figs. 1, 2) proves uplift and tec­ tonic differentiation of these two provinces did not begin until some time later (Rowley et aI., 1978; Deino, 1985). Thus, considerations of at least pre-Miocene volcanism of ~ the Great Basin should include deposits in the Marysvale 43-;;7".. field on the western Colorado Plateaus. There is, however, /08-17 no documented pre-Miocene volcanism in the Sierra Ne­ vada. Inthe Great Basin, middle Tertiary volcanic rocks were almost everywhere deposited on a clean erosion surface of miles 500 MEXICO subdued relief carved into Paleozoic and Mesozoic rocks (McKee, 1988). Only locally are pre-volcanic fluvial and kilometers 700 lacustrine deposits found above this profound unconformity (McKee et aI., 1970; McKee and Silberman, 1975; Nilsen FIGURE 2~Pattern of Eocene-Miocene volcanism in the Great Basin, and McKee, 1979). The most widespared of these is the flanking Sierra Nevada and Colorado Plateaus (delineated by fine stippling), and southern Basin and Range province. Solid lines are isochrons (in Ma) Paleocene-Eocene Sheep Pass Formation, a clastic unit that 2 showing transgressive sweep of volcanism (isochrons labeled 38, 35, and crops out over an area of only 4100 km in east-central 18 in Great Basin are from this study and indicate most southerly progress . Nevada, but is locally as thick as 1 km. Not only are Tertiary of activity at the given time; other isochrons from Cross and Pilger, 1978; sedimentary rocks highly localized beneath the volcanic de­ cf. Noble, 1972; Armstrong, 1975; Stewart and Carlson, 1976; Burke and McKee, 1979; Nilsen and McKee, 1979; Lipman, 1980).