Eocene Basalt of Summit Creek: Slab breakoff magmatism in the central Washington Cascades, USA

Abstract

The early Eocene (52-44 Ma) was a time of tectonic reorganization and widespread magmatism in Washington, Oregon, and British Columbia (west coast of the United States and Canada) that culminated with establishment of the Cascade arc. Details of this tectonic transition remain enigmatic, and diverse scenarios involving ridge-trench interaction, slab breakoff, and/or plume magmatism have been proposed. This study focuses on the ca. 48 Ma Basalt of Summit Creek, a ~1500-m-thick sequence of subaerial lavas that erupted during the interval of time between accretion of the Siletzia terrane to the west and inception of the Cascade arc. The sequence dominantly consists of moderately evolved tholeiitic basalts (9.3-3.1 wt% MgO; Mg# = 0.66-0.29) and scarce rhyolites that erupted in the forearc but are chemically and isotopically similar to oceanic basalts of the Crescent Formation, found ~100 km to the west as part of the Siletzia terrane. Basalt of Summit Creek lavas lack subduction signatures (e.g., high field strength element depletions) and require a mechanism whereby asthenospheric melts were able to reach the surface having undergone little or no chemical interaction with fluids derived from the subducting slab or metasomatized mantle. We suggest that the accretion of Siletzia led to breakoff of the Farallon slab, and that Basalt of Summit Creek lavas formed by decompression melting of mantle that upwelled through the rupture. Slab breakoff typically produces a short-lived linear magmatic belt; the Basalt of Summit Creek appears to be part of such a belt, previously unrecognized, that parallels the Siletzia boundary and formed between 50 and 48 Ma.

Geodynamic conditions of formation of ore-magmatic systems of Eastern Donbass according to petrochemical data

“…При формировании Тузловской РМС ареального типа основным источников магматизма, образующего интрузивные тела миусско-керчикского комплекса, согласно теоретическим положениям Kant et al, 2018] могли являться подслэбовая астеносфера или деплетированная мантия. [Гребенников, Ханчук, 2021]…”

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

“…How magmas of these intrusive-extrusive rocks were produced in a relatively cold environment beneath the accretionary wedges close to trenches is a fundamental question. Some of the proposed models include: (1) ridge subduction and associated aesthenospheric upwelling (Hibbard & Karig, 1990;Underwood et al 1993;Llytwyn et al 2000;Ayuso et al 2009;Anma et al 2009); (2) slab break-off and aesthenospheric flow through a slab window (Thorkelson & Taylor, 1989;Kant et al 2018); (3) ridge-trench-transform fault (RTF) intersections and related triple junction migrations (Fox et al 1985;Cole & Basu, 1995); and (4) subduction of a young and hot oceanic plate (Sato et al 2002). The first three models are not mutually exclusive; they may overlap as in the case of ridge subduction (1) leading into slab window development and aesthenospheric upwelling (2) beneath the fore-arc.…”

Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction

“…50-45 Ma, when the Farallon-Kula or Farallon-Resurrection spreading ridge interacted with the North American plate (Fig. 1) (e.g., Breitsprecher et al, 2003;Haeussler et al, 2003a;Madsen et al, 2006;Eddy et al, 2016a;Kant et al, 2018). Thick oceanic crust formed along this ridge in a setting that is likely analogous to that of modern Iceland, as indicated by Siletzia, which is a large igneous province associated with the ridge (e.g., Wells et al, 2014).…”

“…Rollback and break-off of the Farallon slab as a result of Siletzia collision (Schmandt and Humphreys, 2011) induced vigorous magmatism in the northern Washington Cascades from ca. 49.3 Ma to 45 Ma (Eddy et al, 2016a;Miller et al, 2016;Kant et al, 2018). A new subduction zone formed outboard of Siletzia and initiated the N-S-trending ancestral Cascade arc at ca.…”