Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation?

Abstract

The existing paradigm for the major ca. 56-48 Ma subduction zone reorganization in the Pacific Northwest of North America is that: (1) the Siletzia large igneous province erupted offshore to the west of North America, forming an oceanic plateau; (2) Siletzia then collided with North America, clogging the Pacific Northwest segment of the Cordilleran subduction zone; and (3) the oceanic lithosphere west of Siletzia then ruptured to initiate the new Cascadia subduction zone. Oceanic lithosphere is strong and difficult to rupture, so this would represent a rare example of such a rupture initiating a new subduction zone. This paper explores an alternative hypothesis for the reorganization, a plume-induced subduction initiation (PISI) mechanism, which has previously been applied to the initiation of Caribbean plate subduction zones in the Cretaceous. In this PISI hypothesis, a newly formed, ~1200-km-diameter Yellowstone mantle plume head rose at ca. 55 Ma beneath western North America, generating Siletzia in situ on the North American margin, as well as generating the ~1700-km-long Challis-Kamloops volcanic belt ~600 km to the east of Siletzia. This destroyed the existing Cordilleran subduction zone and allowed the new Cascadia subduction zone to form by collapse of thermally weakened oceanic lithosphere over the hot western margin of the plume head. This PISI hypothesis provides an integrated framework for understanding Siletzia, the Challis-Kamloops belt, Eocene core complexes from Idaho (U.S.) to British Columbia (Canada), underplated mafic rocks beneath Oregon and Washington (U.S.), post-17 Ma manifestations of the Yellowstone plume, and geophysical characteristics of the lithosphere beneath the Pacific Northwest.

Plume‐Induced Sinking of Intracontinental Lithospheric Mantle: An Overlooked Mechanism of Subduction Initiation?

“…Mantle plume emplacement at the transition zone between oceanic and continental lithosphere might play a key role in the initiation of ocean-continent subduction (e.g., Van der Lee et al, 2008). For example, as shown in a recent study by Stern and Dumitru (2019), the Yellowstone mantle plume emplaced at ∼55 Ma below the western coast of North America has likely disrupted an older subduction zone and allowed thermal weakening and lithospheric collapse of the oceanic Farallon plate forming the new east-dipping Cascadia subduction zone. Numerical models of plume-lithosphere interaction at passive continental margins (François et al, 2018) also reveal plume-induced incipient downthrusting of oceanic crust and mantle lithosphere ( Figure 5a).…”

Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

“…Considering the wide exposure of exhumed Franciscan complex south of the triple junction of both low and high P/T metamorphic history (Ernst & McLaughlin, 2012), it has to be presumed that north of the triple junction underplating of subducted sediment and eroded debris to depths of 5-30 km continued below the Cascade forearc. Although the Cascade subduction zone formed in the early Eocene (see recent review by Stern & Dumitru, 2019), crustal basement of the sediment-charged Cascade forearc is constructed of early Eocene and older igneous rock widely exposed along the Oregon, Washington and Vancouver British Columbia coastal area (Snavely, 1987;Flueh et al 1998) (Fig. 4).…”

“…Presuming that underplating got underway at the early Eocene formation of the Cascade subduction zone (see e.g. Stern & Dumitru, 2019), then, at ∼30 km 3 Ma −1 km −1 , since 55 Ma a volume of at least ∼1700 km 3 of sediment is estimated to have been subducted beneath each km of the Cascade margin. This is slightly less than the estimated volume of the E-reflectors and underlying low-velocity zone.…”

Plume‐Induced Subduction Initiation: Single‐Slab or Multi‐Slab Subduction?

“…Boschman et al (2019), using plate kinematic reconstructions indicated that the subduction in western Caribbean was formed in an intra-oceanic environment as a result of plume-lithosphere interaction at about 100 Ma. The formation of the Cascadia subduction zone in Eocene times was recently suggested as a second example of plume-induced subduction initiation (Stern & Dumitru, 2019).…”