Timing of Tertiary Extension in the Railroad Valley Pioche Transect, Nevada: Constraints from Ar-40/ Ar-39 Ages of Volcanic Rocks Wanda J

The University of Maine DigitalCommons@UMaine Earth Science Faculty Scholarship Earth Sciences 1989 Timing of Tertiary Extension in the Railroad Valley Pioche Transect, Nevada: Constraints from Ar-40/ Ar-39 Ages of Volcanic Rocks Wanda J. Taylor John M. Bartley Daniel R. Lux University of Maine - Main, [email protected] Gary J. Axen Follow this and additional works at: https://digitalcommons.library.umaine.edu/ers_facpub Part of the Earth Sciences Commons Repository Citation Taylor, Wanda J.; Bartley, John M.; Lux, Daniel R.; and Axen, Gary J., "Timing of Tertiary Extension in the Railroad Valley Pioche Transect, Nevada: Constraints from Ar-40/Ar-39 Ages of Volcanic Rocks" (1989). Earth Science Faculty Scholarship. 115. https://digitalcommons.library.umaine.edu/ers_facpub/115 This Article is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Earth Science Faculty Scholarship by an authorized administrator of DigitalCommons@UMaine. For more information, please contact [email protected]. JOORNALOF GEOPHYSICALRESEARCH, VOL. 94, NO. •6, PAGES 7757-7774, JUNE 10, 1989 TIMING OF TERTIARY EXTENSION IN THE RAILROAD VALLEY-PIOCHE TRANSECT, NEVADA: CONSTRAINTSFROM 40Ar/39Ar AGES OF VOLCANIC ROCKS Wanda J. Taylor and John M. Bartley Department of Geology and Geophysics, University of Utah, Salt Lake City Daniel R. Lux Department of Geology, University of Maine, Orono GaryJ. Axen1 Geology Department, Northern Arizona University, Flagstaff Abstract. Time-space relations of extension of active extensional domains in the Great Basin and volcanism place critical constraints on may be greater than 100 km across strike. models of Basin and Range extensional processes. This paper addresses such relations Introduction in a 130-km-wide transect in the eastern Great Basin, bounded on the east by the Ely Springs Timing relations of deformation and magmatism Range and on the west by the Grant and Quinn are among the most powerful tests of tectonic Canyon ranges. Stratigramhic and structural models. In the Tertiary evolution of the Great data, combinedwith 40Ar/B9Arisotopic ages of Basin of the western United States, fundamental volcanic rocks, document a protracted but questions regarding timing of tectonic processes distinctly episodic extensional history. Field include whether tectonic extension and magmatism relations indicate four periods of faulting. occurred coevally, and whether deformation was Only one of these periods was synchronous with episodic or occurred gradually as a temporal nearby volcanic activity, which implies that continuum. In this study we address these volcanism and faulting need not be associated questions within a 130 km wide transect in the closely in space and time. Based on published eastern Great Basin, combining detailed dates and the analyses reported here, the stratigraphic and structural field data with periods of extension were (1) prevolcanic (pre- 40Ar/3•Arisotopic ages of Tertiary volcanic 32 Ma), (2) early synvolcanic (30 to 27 Ma), (3) rocks in critical contact relations. immediately postvolcanic (about 16 to 14 Ma), In this paper, we address the patterns of and (4) Pliocene to Quaternary. The break extension-related magmatism in part of the Great between the second and third periods is Basin by looking at volcanism, because the distinct. The minimum gap between the first two plutonic parts of the systems are rarely periods is 2 Ma, but the separation may be much exposed. We assume that the patterns of larger. Temporal separation of the last two volcanism give a reasonable first-order image of periods is only suggested by the stratigraphic major crustal magmatic events. record and cannot be rigorously demonstrated The study area, which we call the Railroad with present data. The three younger periods of Valley-Pioche transect (see Figure 1 for faulting apparently occurred across the entire location), was chosen because it offers transect. The oldest period is recognized only important stratigraphic relations in the Great at the eastern end of the transect, but appears Basin for determining time-space relations of to correlate about 150 km northward along strike Tertiary faulting and volcanism. Early work by with extension in the Northern Snake Range-Kern Cook [1965] showed that, in this transect, Mountains area. Therefore the oldest period Oligocene ash flow tuffs present mainly north of also is regional in extent, but affected a the transect are overlapped by Miocene tuffs different area than that affected by younger that are thicker south of the transect. periods. This relation suggests that distinct Therefore this transect exposes a long Tertiary extensional structures and master detachment stratigraphic record. Based on older work and faults were active at different times. The this study, this record extends from roughly 32 correlation of deformation periods of a few to 15 Ma. The small age range of the rocks of million years duration across the Railroad interest and the scatter of existing Valley-Pioche transect suggests that the scale conventional K/Ar dates indicated that more precise dates were needed to resolve the relative ages of rocks that do not occur in stratigraphiccontact in the field. The 1Nowat Departmentof Earth and Planetary 40Ar/•Ar incremental-releasemethod has rarely Sciences, Harvard University, Cambridge, been used to date young volcanic rocks, but was Massachusetts. chosen for this study because it permits better accuracy and analytical precision than the K/Ar Copyright 1989 by the American Geophysical Union. method.Data for all 40Ar/39Aranalyses are shown in Table 1. In this paper, we distinguish local Paper number 88JB04047. "episodes" of faulting, in which a particular 0148-0227/89/88JB-04047505.00 fault or group of faults is found in a specific 7757 7758 Taylor et al.' Timingof Tertiary Extension, Central Nevada figure 90"--+ NEEDLESRANGE % + + ROUP CALDERAS nge.• Railroad Valley White figure 5 HIGHI AND + + + +++++ + + + +++ + + + + + .- DE TA C HMœN T, + Quinn Canyon+ + +++ + + + . 38 + + + + Ely Spring + + + + Rang Pioche Dry Lake Valley FAUL I ß 0 I0 miles ;-- , figures :5 & 6 + CALDRON %%N • COMPLEX Fig. 1. Location map of the studied transect with features mentioned in the text. Paleozoic rocks shownby block pattern. Mesozoic stock shownby random dashes. Tertiary rocks undivided shown by plus pattern. Pliocene to Quaternary rocks shown by no pattern. BSR represents Burnt Springs Range. LMF represents the Little Meadows fault. Locations of Figures 3, 4, 5, 6, and 9 are shown. time-bracketing relation at a specific locality, to minimize the importance of Tertiary extension from regional "periods" of faulting, in which [Fryxell, 1984• Bartley et al., 1985, 1987]. extensional faults, over a period of at most a The latter has been mitigated by recent work of few million years, were active across an area Fryxell [1984, 1988], Bartley et al. [1988], and encompassing several mountain ranges (i.e., Gleason [1988], which suggests an episodic thousands of square kilometers). Each period of extensional history from Oligocene to Recent. faulting is distinguished by recognition of one Stratigraphic constraints on timing at the west or more faulting episodes that are closely end of the transect are not as tight as in the related in time. Dry Lake Valley area. Regional Geology Stratigraphy Recent Work Understanding of map-scale structures depends upon knowledge of stratigraphy. Dacitic to Recent field work in the Railroad Valley- rhyolitic ash flow tuffs are the predominant Pioche transect has focused near its ends, mid-Tertiary rocks of much of the Basin and around Dry Lake Valley on the east and in the Range Province of the western United States Quinn Canyon and southern Grant Ranges on the [Cook, 1965; Lipmanet al., 1972]. Commonly, west. Earlier work on the Tertiary of the Dry different ash flow tuffs are petrographically Lake Valley area was by Tschanz and Pampeyan quite similar, and their distinction in the [1970] and Ekren et al. [1977]. Recent detailed field requires detailed knowledgeof phenocryst mapping by Axen [1986] in the Ely Springs Range modes, distinctive phenocryst characteristics and Taylor [1989] in the North Pahroc Range has such as crystal habit or color (e.g., smoky significantly improved understanding of both quartz or amethyst), and relative sizes of stratigraphy and structure of Tertiary rocks in phenocrysts, as well as the nature of pumice this area. These results demonstrate that inclusions, type(s) of lithic fragments present, extension began before the mid-Oligocene and and vertical zonal variations. For example, continued episodically until the Quaternary large crystal-rich dacitic ash flow tuffs of [Axen, 1986; Taylor and Bartley, 1986; Taylor, Oligocene age, including the MonotonyTuff 1989]. The isotopic ages presented below place [Ekren et al., 1971] and the Needles RangeGroup constraints on the timing of these episodes. (CottonwoodWash Tuff, WahWah Springs Early work on the structure and stratigraphy Formation, and Lund Formation [Best and Keith, in the Quinn Canyon Range and southern Grant 1983]) are closely similar and widely knownto Range was by Sainsbury and Kleinhampl [1969] and be easily confused. In this regard, it is worth Cebull [1970] and was recently summarized by noting that the tripartite division of Needles Kleinhampl and Ziony [1985]. However, these Range Group dacites by Best and Keith [1983], authors concentrated on evidence for Mesozoic mainly using hand-specimen

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