Why Study The Cascade Arc?

When viewed at the continental scale, Cascadia is a relatively short subduction zone embedded within a long right-lateral shear zone (Atwater 1970; Fig. 1). Although short, the region varies strongly in the stress regime imparted by the transtensional (i.e., combined shear and tension) tectonic setting, yielding a magmatic and geomorphological character defined by three distinct geomorphological segments: the Klamath–Sierra Nevada block pushes the Siletzia block away from the southern Cascades (Wells et al. 1998; Fig. 1). In response, extension in the backarc is encroaching on the southern part of the arc (California through the graben-faulted central Oregon Cascades) (Fig. 2). The impact of these environments can also be seen through the window of mantle geochemistry—as reflected in the composition of erupted basalts. In the extension-dominated southern segment, mafic volcanism is unusually abundant with low-K tholeiites (LKTs) subequal to the more arc-typical calc-alkaline basalts. Such LKT magmas are largely produced by decompression of upwelling mantle, which equilibrated and ascended from the mantle not far below the Moho (Till et al. 2013; Mullen et al. 2017). The extending backarc region of the southern segment is dominated by LKTs. Associated westward-migrating silicic magmatic centers, active since ~12 Ma (Fig. 2), and strong east–west mantle anisotropy are further evidence for westward encroachment of the Basin and Range extensional province into the arc itself. In stark contrast, in the northern arc, the shear setting and arched slab beneath the concave-out subduction zone drive across-arc contraction. Consequently, volcanism is largely limited to andesite- and dacite-dominated edifices with few proximal vents of mainly calc-alkaline basalts, and virtually no LKTs. Basalts with alkalic, intraplate affinities also increase northward. Where these two regions meet, in the central Cascades of southern Washington, a zone of complex deformation occurs. The axis of major andesite–dacite stratovolcanoes loses definition, and mafic vents are scattered over a 160-km-wide zone—making Cascadia at this location one of the widest volcanic arcs in the world. Here, calc-alkaline and LKT basalts occur across the arc and are joined by enriched intraplate-type basalts that are isotopically distinct from the Cascade array and increase in prominence eastward (e.g., Pitcher and Kent 2019).

The Pacific Northwest region, of which the Cascade volcanic arc constitutes a key part, hosts one of Earth’s great continental volcanic provinces. This is largely a consequence of a remarkable transition from flat-slab to normal-dip subduction that occurred in the early Eocene (Darold and Humphreys 2013). The Cascade arc is the most recent manifestation of this transition, which occurred suddenly, but also in several distinct phases. Early Cenozoic flat-slab subduction was amagmatic and contractional, as was typical of the Laramide orogeny elsewhere. This phase ended quite suddenly with the ~50 Ma accretion of the mafic and buoyant Siletzia terrane, which caused subduction to step west of Siletzia and establish early Cascadia subduction. Foundering of the abandoned flat slab initiated a major ignimbrite flareup in the western U.S., and an abrupt change to a new, tensional environment (Gurnis et al. 2004) enabled the gravitational collapse (and attendant extension) of the Pacific Northwest interior. Meanwhile, normal-dip subduction at Cascadia created the conditions for arc volcanism to restart. Renewed Cascade volcanism followed Siletzia accretion by 10–15 Ma, with the earliest arc magmatism at ~45 Ma, and a well-developed volcanic arc was active by 35 Ma (e.g., Dragovich et al. 2016). South of Siletzia, a tear between the normal-dip Cascadia subduction and flat subducting slab beneath Nevada enabled rollback of the flat slab, which established the Ancestral Cascades. In this context, the Cascade arc is the most recent phase of a long and complex volcanic legacy in the Pacific Northwest.

An integral element in the history of Cascadia and the Pacific Northwest is the activity of the Yellowstone mantle plume (Camp and Wells 2021). Yellowstone’s offshore creation of the Siletzia oceanic plateau created a buoyant structure that resisted subduction and accreted (Wells et al. 2014). Continued Farallon plate motion resulted in normal-dip subduction initiation, abandonment of the flat slab, and ultimately the Cascade arc. The mafic composition of Siletzia is the origin of its strength, which holds the forearc largely intact and transfers the stresses created by Cascadia’s shear setting (Klamath-Sierra Nevada push and subduction obliquity; Wells et al. 1998) from the southern to northern Cascades. Re-emergence of the Yellowstone plume ~17 Ma beneath southeastern Oregon led to the Columbia River flood basalts, and the residual plume-warmed mantle may well be entrained in Cascadia corner flow and encroaching on the Cascadia mantle wedge in Oregon (Draper 1991).

To wrap up, the unusual characteristics of the Cascadia subduction zone, in concert with a wealth of geophysical, geochemical, and other data, provide opportunities to better understand the initiation and development of a volcanic arc and its interactions with the greater geodynamic environment. Beyond the oft-considered slow subduction of hot oceanic lithosphere, we draw attention to the remarkable regional volcanic flareup that accompanied Cascadia subduction initiation, the importance of tectonic stress on volcanic arc deformation and magma emplacement rates, and the influence of a mantle plume on subduction development.