Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

Abstract

[1] Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO 2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40 Ar/ 39 Ar incremental heating ages on rhyolites, and establish that the timing of the age-progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern-most Basin and Range (NWBR) rhyolite volcanism into a single age-progressive trend. The spatial-temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment-filled basins within the more volcanically active HLP. The westnorthwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi-stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.

Reassessing obsidian field relationships at Glass Buttes, Oregon

“…It is not clear from this map how to account for the differential origins and distributions of A/alpha and B/beta obsidians, suggesting greater-than-mapped complexity of the BGB rhyolitic units (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article). Mean F/zeta obsidian; not measured in this study, derived from data in Ambroz et al (2001) East West 6.49 ± 0.05 Ma (Ford et al 2013) 5.79 ± 0.04 Ma (Ford et al 2013) Fig. 8.…”

“…Both trends exhibit an eastwest geographic trend and dates consistent with HLP age-progressive volcanism (i.e., older obsidians in the east, younger obsidians in the west) but perhaps in two different phases. The two dates from Ford et al (2013) are based on the 40 Ar/ 39 Ar radiometric technique. These two distinct trends are not apparent in the Th vs. Eu plot of Ambroz et al (2001).…”

“…The Glass Buttes volcanic complex, perhaps the best known obsidian locale in the Pacific Northwest, lies in the middle of the HLP. Generated by a series of eruptions circa 5.8 to 6.5 Ma (Godfrey-Smith et al, 1993;Ford et al, 2013), its obsidians were used to craft stone tools for millennia, from at least as early as 7000 BCE (Carlson, 1994; perhaps even during the Clovis era, Stueber and Skinner, 2011) to recent centuries (Jackson and Ericson, 1994). Glass Buttes obsidians have been recognized throughout the Pacific Northwest, from British Columbia to the north, California to the south, and Idaho to the east (Hughes, 1978(Hughes, , 1986Carlson, 1994); however, their utilization was primarily local (Musil and O'Neill, 1997;Hutchins and Simons, 1999;Cadena, 2012).…”

“…Most geological fieldwork at this complex has focused on its relationship to regional volcanism trends (i.e., age-progressive silicic volcanism of the HLP; e.g., Godfrey-Smith et al, 1993;Jordan et al, 2004;Ford et al, 2013) or economic potential (i.e., geothermal energy, mineral resources; e.g., Berri, 1982;Johnson, 1984;Johnson and Ciancanelli, 1984;Roche, 1987). Until the 1990s, it was generally thought that obsidian at Glass Buttes had a single chemical composition (e.g., Skinner, 1983Skinner, , 1995Hughes, 1986;cf.…”

The Yellowstone “hot spot” track results from migrating basin-range extension

“…Ford et al (2012) attribute the age-progressive Newberry trend to mantle upwelling in response to slab rollback. This explanation does not, however, explain why the ageprogressive volcanism should comprise a narrow zone and not a broad region, nor does it explain the decoupling between the rhyolitic and basaltic volcanism along that trend.…”