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accreted to North America at 51–49 Ma depletion in Nb and enrichment in the H O- the age of bimodal eruptions along the north-
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(Wells et al., 2014, and references therein). soluble elements Rb, Ba, K, and Pb, a chemi- ern Nevada rift from 16.5 to 15.0 Ma (John et
Accretion may have been partly contempora- cal signature that requires melting of a al., 2000) and with high-volume rhyolite
neous with underplating of older parts of a hydrated mantle source. Camp et al. (2017) eruptions in SE Oregon and NW Nevada
greater Siletzia in the backarc region of SE attribute source enrichment to plume-induced from 16.5 to 15.5 Ma (Fig. 1) (Coble and
Washington and adjacent Oregon (Gao et al., uplift and heating of the slab resulting in the Mahood, 2016; Benson et al., 2017). The old-
2011; Schmandt and Humphreys, 2011). liberation of H O into the overlying mantle est rhyolites at 16.5–16.1 Ma (i.e., HRCC,
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Trench development west of the accreted ter- wedge, thus lowering solidus temperatures MVF, SR, S, HV, and J on Fig. 1) and the
rane was established at 48–45 Ma (Wells et and promoting partial melting of hydrated most voluminous rhyolite fields (~5000 km ;
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al., 2014). Subduction initiation at this time is mantle beneath the Oregon backarc from HRCC and MVF on Fig. 1) lie in a narrow
consistent with upper plate extension and 30 to 20 Ma (Fig. 1). belt adjacent and parallel to the projected
post-accretion magmatism of a regional tho- Thermomechanical erosion of the Farallon Yellowstone hotspot track (Fig. 1). In total,
leiitic sill complex in the northern Oregon slab is suggested by the ENE-trending belt of this 17–15 Ma event of flood basalt and
Coast Range (Wells et al., 2014) and with the 30-to-20 Ma adakite, thought to be derived bimodal volcanism produced a N–S system
onset of sporadic arc volcanism in the ances- from slab melting of oceanic crust (Fig. 1). of coeval dike swarms, rhyolitic calderas and
tral Cascades from ca. 45–36 Ma (DuBray The low solidus temperature of this mafic large, mid-crustal keel dikes expressed as
and John, 2011). source makes it highly susceptible to thermal linear aeromagnetic anomalies (Glen and
erosion and melt generation above the feed- Ponce, 2002) that extend from eastern
Overriding of the Yellowstone ing plume tail, where maximum tempera- Washington to central Nevada and perhaps
Hotspot by the North American tures are maintained (Campbell, 2005). The into southern Nevada (Pierce and Morgan,
Plate (ca. 42–34 Ma) adakite belt lies parallel to plate motion and 1992), the Nevada–Columbia Basin Magmatic
At ca. 42 Ma, the leading edge of the North has been described as an older extension of Belt (Fig. 2) of Camp et al. (2015).
American plate began to override the hotspot the Yellowstone–Snake River Plain hotspot Liu and Stegman (2012) attribute the
at the site of the newly formed trench track to the east (Camp et al., 2017). Nevada–Columbia Basin Magmatic Belt to
(Simpson and Cox, 1977; Wells et al., 2014). slab tearing along a N-S hinge line beneath
This event marks the beginning of regional Volcanic Hiatus in the Backarc eastern Oregon, followed by upwelling and
margin-parallel extension, widespread dike Region (ca. 20 Ma to 17 Ma) melting of subslab asthenosphere. Other
injection, and post-accretionary hotspot Calc-alkaline and adakite volcanism in the workers prefer a modification of this pro-
magmatism of enriched mid-oceanic ridge backarc region waned dramatically at 22 Ma cess with slab rupture near the eastern
basalt (EMORB) and OIB of the Tillamook and ceased at 20 Ma. Thus began a 3–5-m.y. hinge of plume-driven uplift (Coble and
episode from 42 to 34 Ma (Chan et al., 2012; volcanic hiatus that is well established in Mahood, 2012; Camp et al., 2015), tearing
Parker et al., 2010; Wells et al., 2014), exem- eastern Oregon and northernmost Nevada N-S in much the same manner as envisioned
plified in the Tillamook Volcanics, Yachats (Coble and Mahood, 2012, and references by Liu and Stegman (2012). Extension of the
basalt, and Grays River Volcanics (Fig. 1). therein). Coble and Mahood (2012) attribute Nevada–Columbia Basin Magmatic Belt
Location of the overridden hotspot at this this hiatus to continued plume uplift of the into NE Oregon and SE Washington (Fig. 2)
time therefore appears to have been in the Farallon slab leading to the cessation of cor- may have also been augmented by the
general vicinity of southern Oregon and ner flow in the overlying mantle wedge. The northward deflection of spreading plume
northern California, but perhaps slightly hiatus separates two distinct magmatic per- material against the thick cratonic bound-
north of the calculated hotspot track approxi- turbations in the backarc region: (1) the older ary (e.g., Sleep, 1996; Camp, 1995), and/or
mated in Figure 1 (black dotted line). Well- period of calc-alkaline magmatism from 30 by the long-distance lateral migration of
documented clockwise rotation and north- to 20 Ma, where mafic rocks were derived dikes from centralized magma chambers
ward migration of the forearc have since from a wet mantle source; and (2) a younger farther south (Wolff and Ramos, 2013).
moved Siletzia and the Tillamook Volcanics period of tholeiitic flood-basalt to bimodal Slab dismemberment beneath eastern
as much as 300 km to the north (Beck, 1984; magmatism from 17 to 15 Ma, where mafic Oregon is consistent with seismic studies that
Wells et al., 2014). rocks were derived from a dry mantle source. resolve a truncated subducting plate that ter-
minates at ~300 km depth, with fast anoma-
Slab Uplift and Volcanism in Slab Rupture and Tholeiitic Flood lies farther east interpreted as Farallon slab
the Oregon Backarc Region Basalt to Bimodal Volcanism remnants (e.g., Obrebski et al., 2010). The
(ca. 30–20 Ma) (ca. 17–15 Ma) intervening slab hole lies directly beneath the
Continued flux of the plume tail should The volcanic hiatus ended at ca. 17 Ma axial trend of the older (30–20 Ma) adakite
have accumulated a significant mass of hot with fissure eruptions of the voluminous hotspot track and the younger continuation of
plume material shielded beneath the Farallon main-phase lavas of the Columbia River the Yellowstone–Snake River Plain hotspot
slab from 42 to 30 Ma (e.g., Coble and Basalt Group, all of which share a plume-like track (e.g., figure 3f of Obrebski et al., 2010).
Mahood, 2012). Plume arrival in the Oregon component based on trace-element and iso-
backarc generated a broad region of high-K topic data (e.g., Wolff and Ramos, 2013). The Plume Relationship to Regional
calc-alkaline volcanism and a coeval but main phase generated 93% (194,000 km ) of Tectonics (17–15 Ma)
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more localized belt of adakite volcanism the Columbia River Basalt Group volume in The events at 17–15 Ma occurred during
from ca. 30–20 Ma (Fig. 1) (Camp et al., ca. 600,000 years (Kasbohm and Schoene, a time when western North America was
2017). Mafic rocks of this episode display 2018). This narrow age-range overlaps with evolving from a convergent to a transform

6 GSA Today | January 2021