Tectonic controls on the origin and segmentation of the Cascade Arc, USA

Abstract

The magmatic response above subducting ocean lithosphere can range from weak to vigorous and from a narrow zone to widely distributed. The small and young Cascade Arc, riding on the margin of the tectonically active North American plate, has expressed nearly this entire range of volcanic activity. This allows an unusually good examination of arc initiation and early growth. We review the tectonic controls of Cascade-related magmatism from its inception to the present, with new considerations on the influences of tectonic stress and strain on volcanic activity. The Cascade Arc was created after accretion of the Siletzia oceanic plateau at ~ 50 Ma ended a period of flat-slab subduction. This (1) initiated dipping-slab subduction beneath most of the northern arc (beneath Washington and Oregon) and (2) enabled the more southerly subducting flat slab (beneath Nevada) to roll back toward California. As the abandoned flat slab fragmented and foundered beneath Oregon and Washington, vigorous extension and volcanism ensued throughout the northwest USA; in Nevada the subducting flat slab rolled back toward California. Early signs of the Cascade Arc were evident by ~ 45 Ma and the ancestral Cascade Arc was well established by ~ 35 Ma. Thus, from ~ 55–35 Ma subduction-related magmatism evolved from nearly amagmatic to regional flare-up to a clearly established volcanic arc in two different tectonic settings. The modern Cascades structure initiated ~ 7 Ma when a change in Pacific plate motion caused partial entrainment of the Sierra Nevada/Klamath block. This block pushes north and west on the Oregon Coast Ranges block, breaking the arc into three segments: a southern extensional arc, a central transitional arc, and a northern compressional arc. Extension enhances mafic volcanism in the southern arc, promoting basalt decompression melts from depleted mantle (low-K tholeiites) that are subequal in volume to subduction fluxed calcalkaline basalts. Compression restricts volcanic activity in the north; volcanism is dominantly silicic and intra-plate-like basalts cluster close to the main arc volcanoes. The transitional central arc accommodates dextral shear deformation, resulting in a wide volcanic arc with distributed basaltic vents of diverse affinities and no clear arc axis.

The arc-scale spatial distribution of volcano erosion implies coupled magmatism and regional climate in the Cascades arc, United States

“…Patterns of volcanic migration in the Cascades are largely consistent with tectonic block rotation of the Pacific Northwest relative to stable North America, which generates an extensional regime in the south and compressional regime in the north of the arc (Wells et al, 1998). Although geologically-inferred displacement vectors generally correspond with modern GPS velocities (McCaffrey et al, 2013), differences have been attributed to variable roll-back of the Juan de Fuca slab under North America and decoupling of upper plate deformation from underlying mantle (Wells and McCaffrey, 2013;Humphreys and Grunder, 2022). Superimposed on this and other geodynamical mechanisms would be contributions from climate to influence crustal magmatism and arc migration, but evidence for such surface influence does exist.…”

Disentangling the roles of subducted volatile contributions and mantle source heterogeneity in the production of magmas beneath the Washington Cascades

“…The longitudinal extent of Quaternary volcanism in this portion of the Cascades is anomalously broad, featuring both well-developed forearc (MSH, Indian Heaven Volcanic Field) and backarc (Simcoe Volcanic Field) volcanic centers. The breadth of the arc has been attributed to pull-apart geometries due to shear deformation resulting from the change in stress state from transtensional in the south to transpressive in the north (Humphreys and Grunder, 2022). The main arc axis at this latitude (~46°N) is located at the approximate longitude of a second active stratovolcano, Mt.…”