Abstract
Rogerson Graben, USA, is critically placed at the intersection between the Yellowstone hotspot track and the southern projection of the west Snake River rift. Eleven rhyolitic members of the re-defined, ≥420-m-thick, Rogerson Formation record voluminous high-temperature explosive eruptions, emplacing extensive ashfall and rheomorphic ignimbrite sheets. Yet, each member has subtly distinct field, chemical and palaeomagnetic characteristics. New regional correlations reveal that the Brown’s View ignimbrite covers ≥3300 km2, and the Wooden Shoe ignimbrite covers ≥4400 km2 and extends into Nevada. Between 11.9 and ∼8 Ma, the average frequency of large explosive eruptions in this region was 1 per 354 ky, about twice that at Yellowstone. The chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation. This was followed by a trend towards less-evolved rhyolites that may record melting and hybridisation of a mid-crustal source region. Contemporaneous magmatism-induced crustal subsidence of the central Snake River Basin is recorded by successive ignimbrites offlapping and thinning up the N-facing limb of a regional basin-margin monocline, which developed between 10.59 and 8 Ma. The syn-volcanic basin topography contrasted significantly with the present-day elevated Yellowstone hotspot plateau. Concurrent basin-and-range extension produced the N-trending Rogerson Graben: early uplift of the Shoshone Hills (≥10.34 Ma) was followed by initiation of the Shoshone Fault and an E-sloping half-graben (∼10.3–10.1 Ma). The graben asymmetry then reversed with initiation of the Brown’s Bench Fault (≥8 Ma), which remained intermittently active until the Pliocene.
Highlights
Introduction and geological settingThe Yellowstone–Snake River Plain volcanic province (Fig. 1) is the youngest and best-preserved silicic intraplate volcanic province on Earth
It records large-scale (100’s to 1000’s km3) explosive eruptions that were unusual both physically and chemically, constituting the largest volume of low-δ18O volcanic rocks known on Earth (e.g. Boroughs et al 2012; Colón et al 2015)
The ignimbrites in the central Snake River Plain are unusually intensely welded and develop flow-folding and upper flow breccias, they can readily be distinguished from true lavas by several features, key of which are (a) the widespread absence of basal autobreccias, which are ubiquitous in rhyolitic lavas (Bonnichsen and Kauffman 1987; Henry and Wolff 1992; Branney et al 1992); and (b) the tendency of rheomorphic ignimbrites to thin gradually as they mantle topographic slopes, and as they approach their tapering distal terminations (e.g. Branney and Kokelaar 2002; Knott et al 2016), in contrast to the more abrupt, lobate, steep and breccia-fringed terminations of true blocky lavas (Bonnichsen and Kauffman 1987)
Summary
The Yellowstone–Snake River Plain volcanic province (Fig. 1) is the youngest and best-preserved silicic intraplate volcanic province on Earth It records large-scale (100’s to 1000’s km3) explosive eruptions that were unusual both physically (i.e. very high-temperature, pumice-poor rhyolitic eruptions of ‘Snake River type’; Branney et al 2008) and chemically, constituting the largest volume of low-δ18O volcanic rocks known on Earth The new stratigraphic framework robustly defines individual eruption-units and provides significantly higher resolution than the broader ‘composition and time (CAT) groups’ of recent studies (Bonnichsen et al 2008), and it reveals temporal trends in chemistry and mineralogy of Snake River Plain magmas during 11 large rhyolitic explosive eruptions between 11.9 and ∼8.0 Ma. We show that the eruptions occurred contemporaneously with continental extension Associated fallout ashes are dispersed widely across continental USA (Perkins 1998)
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