The Ignimbrite Flareup, Flood Basalts and the Yellowstone Hotspot: How Subducting Slab Controlled Pacific Northwest Backarc Magmatism

Siletzia accretion ~48 m.y. ago resulted in normal-dip subduction that terminated Challis magmatism in Idaho and initiated the Cascade arc across the Pacific Northwest during the late Laramide orogeny, when subducted slab presumably was flat against North America lithosphere south of the Pacific Northwest. These different dips require a slab tear that extends east from central Oregon. Once torn, north-to-south propagation of the slab edge allowed asthenosphere to contact North America lithosphere (which was hydrated by the flat slab), providing the mantle-derived basalt needed to drive the ignimbrite flareup. Currently, a tear in the Juan de Fuca slab is imaged extending east from central Oregon to central Montana. A Yellowstone plume appears to ascend through this imaged gap 150 km NW of Yellowstone Park at 400 km depth. Negative slab buoyancy would dominate local flow dynamics. Assuming the imaged gap represents a perpetuation of the tear created by Siletzia accretion, a Yellowstone plume (stationary in a hotspot reference frame) would have approched the gap 15-20 Ma, coincident with Steens and Columbia River flood basalt eruptions in eastern Oregon. The sudden onset of large-volume magmatism north of the well-defined young hotspot track may be attributed to release of ponded plume through the slab gap. This plume blob rose and flattened beneath thin accreted North America, abuting against the thicker Precambrian lithosphere of Idaho. The volumetrically large Columbia River Basalt eruptions are attributed to plume-triggered delamination of the Wallowa pluton restite roots, which rapidly overturned the hot asthenosphere and may have activated the “eclogite engine”. The “backwards propagating” Newberry hotspot is attributed to entrainment of flattened plume in subduction corner flow, perhaps accentuated by continued asthenospheric upflow through the gap; the northern component of Newberry's velocity is driven by subduction obliquity and toroidal asthenospheric flow around the southern slab edge.