Regionally continuous Miocene rhyolites beneath the eastern Snake River Plain reveal localized flexure at its western margin: Idaho National Laboratory and vicinity

The eastern Snake River Plain (ESRP) is a northeast-trending topographic basin interpreted to be the result of the time-transgressive track of the North American plate above the Yellowstone hotspot. The track is defined by the age progression of silicic volcanic rocks exposed along the margins of the ESRP. However, the bulk of these silicic rocks are buried under 1 to 3 kilometers of younger basalts. Here, silicic volcanic rocks recovered from boreholes that penetrate below the basalts, including INEL-1, WO-2 and new deep borehole USGS-142, are correlated with one another and to surface exposures to assess various models for ESRP subsidence. These correlations are established on U/Pb zircon and 40Ar/39Ar sanidine age determinations, phenocryst assemblages, major and trace element geochemistry, δ18O isotopic data from selected phenocrysts, and initial εHf values of zircon. These data suggest a correlation of: (1) the newly documented 8.1 ± 0.2 Ma rhyolite of Butte Quarry (sample 17KS03), exposed near Arco, Idaho to the upper-most Picabo volcanic field rhyolites found in borehole INEL-1; (2) the 6.73 ± 0.02 Ma East Arco Hills rhyolite (sample 16KS02) to the Blacktail Creek Tuff, which was also encountered at the bottom of borehole WO-2; and (3) the 6.42 ± 0.07 Ma rhyolite of borehole USGS-142 to the Walcott Tuff B encountered in deep borehole WO-2. These results show that rhyolites found along the western margin of the ESRP dip ~20o south-southeast toward the basin axis, and then gradually tilt less steeply in the subsurface as the axis is approached. This subsurface pattern of tilting is consistent with a previously proposed crustal flexural model of subsidence based only on surface exposures, but is inconsistent with subsidence models that require accommodation of ESRP subsidence on either a major normal fault or strike-slip fault. Regionally continuous Miocene rhyolites beneath the eastern Snake River Plain reveal localized flexure at its western margin: Idaho National Laboratory and vicinity1 KYLE L. SCHUSLER2 DAVID M. PEARSON2 MICHAEL McCURRY2 ROY C. BARTHOLOMAY3 MARK H. ANDERS4 1. Manuscript received March 2, 2020; Accepted July 7, 2020 2. Department of Geosciences, Idaho State University, Pocatello, Idaho; Corresponding Author: peardavi@isu.edu 3. U.S. Geological Survey, Idaho Falls, Idaho 4. Department of Geology, St. Lawrence University, Canton, New York 7, No. 3, p. 241-270. DOI: 10.31582/rmag.mg. 57.3. 241 THE MOUNTAIN GEOLOGIST | July 2020 242 Vol. 57, No. 3 | www.rmag. org Kyle L. Schusler, David M. Pearson, Michael Mccurry, Roy C. Bartholomay, Mark H. Anders INTRODUCTION The eastern Snake River Plain (ESRP) is a northeast-trending, subsided basin (Fig. 1) that is interpreted as the result of intense magmatism as the North American plate migrated over the stationary Yellowstone hotspot (e.g., Suppe et al., 1975; Anders et al., 1989; Pierce and Morgan, 1992; Smith and Braile, 1994; Branney et al., 2008, and references cited therein). Silicic, large-volume, “Snake River-type” rhyolites and volcaniclastic deposits are exposed along the margins of the ESRP (Branney et al., 2008); Neogene basalts at the surface are inferred to be underlain by correlatives of these rhyolites in the subsurface of the ESRP (Doherty et al., 1979). Seven existing deep boreholes (Borehole-1, 2-2A, INEL-1, Kimberly, WO-2, Sugar City, and USGS-142) within the ESRP have penetrated through basalts into underlying rhyolites (Doherty, 1979; Doherty et al., 1979; Embree et al., 1978; Anders et al., 2014; Knott et al., 2016; Twining et al., 2017). However, the lateral continuity of subsurface rhyolites and the geometry of these rocks in the subsurface near the margin of the ESRP are poorly understood because of the distance between the boreholes and the complexity of the units that they intersect (cf. Knott et al., 2016). Given that the ESRP is a northeast-trending structural basin, some prior workers suggested that its margins were controlled by northeast-striking normal (Hamilton and Myers, 1966; Sparlin et al., 1982) or strike-slip (Pratt, 1982) faults. Based upon large lateral changes in subsurface seismic velocity beneath the ESRP, Sparlin et al. (1982) hypothesized that a major southeast-dipping normal fault bounds the western margin of the western ESRP and accommodated >4 km of offset. However, along the margins of the ESRP, in addition to tilting associated with Basin and Range faulting (e.g., Anders et al., 1993), surficial deposits of rhyolites and volcaniclastic rocks associated with Yellowstone/ESRP volcanism have long been recognized as tilted toward the axis of the ESRP (Kirkham, 1927; Kirkham, 1931; called a “downwarp” by Kirkham). Furthermore, regional extension within the adjacent Basin and Range province is accommodated along northwest-southeast striking normal faults, approximately perpendicular to the ESRP. Thus, in the absence of a surficial expression of major northeast-striking normal faults adjacent to the ESRP (e.g., Rodgers et al., 2002), as well as a lack of subsurface information regarding the geometry of the margins of the ESRP, a non-fault-related origin of the northeast-trending ESRP is generally favored. Inward tilting and subsidence adjacent to the ESRP were first interpreted as due solely to thermal contraction following the passage of the hotspot (Brott et al., 1981). However, thermal contraction explains only a minor fraction of the subsidence; the bulk of subsidence is thought to be density-driven and a thick (≥10 km) mid-crustal dioritic intrusion was hypothesized to be the main factor contributing to subsidence (Sparlin et al., 1982; Anders and Sleep, 1992; McQuarrie and Rodgers, 1998; Rodgers et al., 2002). Recent efforts to constrain the timing and amount of surficial tilting along the western margin of the ESRP (Fig. 1) were focused in the Pioneer Mountains (Michalek, 2009), the Lost River and Lemhi Ranges (McQuarrie, 1997; McQuarrie and Rodgers, 1998; Rodgers et al., 2002), and the Beaverhead Mountains (McQuarrie, 1997; McQuarrie and Rodgers, 1998; Hough, 2001). This study focuses on a critical transect between the Arco Hills (Fig. 1) and the ESRP in order to test models of subsidence. Although the spatial pattern of tilting of rocks INTRODUCTION.242 GEOLOGIC BACKGROUND . 243 Eastern Snake River Plain Volcanism . 243 Flexure and Subsidence of the Eastern Snake River Plain . 245 METHODS . 246 Thin Section Analysis . 246 Zircon U/Pb Geochronology and Lu/Hf Isotopic Analysis . 247 40Ar/39Ar Geochronology . 248 δ18O Isotopic Analysis . 249 X-ray Fluorescence Analysis . 250 Correlation Methodology . 250 RESULTS . 250 Field and Petrographic Observations . 250 Geochronology and Isotope Geochemistry . 252 DISCUSSION . 254 Correlations of Rock Units . 254 Subsurface Architecture of the ESRP . 256 CONCLUSIONS . 257 ACKNOWLEDGMENTS . 258 REFERENCES . 259