Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

Abstract

Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U‐Pb zircon geochronology, Sm‐Nd thermochronology, and field‐based studies to document the protoliths and early burial history of amphibolite and granulite‐facies metasedimentary rocks in the Central Gneiss Complex. U‐Pb detrital zircon data from the structurally highest sample localities yielded ~190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust‐related burial and metamorphism at >25 km depths prior to ~80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

Provenance and metamorphism of the Swakane Gneiss: Implications for incorporation of sediment into the deep levels of the North Cascades continental magmatic arc, Washington

“…The structure along which the Swakane Gneiss protolith was buried has not been identified, but may be represented by the Dinkelman décollement. Underthrusting of sediments from the retroarc side of the subduction system has been interpreted in multiple regions of the North American Cordillera (e.g., Chin et al, 2013;Pearson et al, 2017). This study highlights a potential example of forearc underthrusting within a subduction system, which may be an important mechanism that introduces supracrustal material to depth within continental arcs.…”

“…(Grove et al, 2003), corresponding to a rate of 10-11 km/ m.y. Underplating has also been proposed for the deepest exposed paragneiss in the Central Gneiss Complex in British Columbia (Pearson et al, 2017) and the Condrey Mountain Schist in northern California (Saleeby and Harper, 1993).…”

Along‐Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

“…In the central batholith, significant crustal thickening took place between ca. 110 and 90 Ma (e.g., Cook & Crawford, ; Rubin et al, ; Rusmore et al, ; Pearson et al, ); reverse faulting older than ca. 90 Ma also affected the southernmost batholith (e.g., Brown et al, ; Journeay & Friedman, ).…”

Does Underthrusting Crust Feed Magmatic Flare‐Ups in Continental Arcs?

“…Previous models explaining nonsteady state magmatism focus on: (1) the convergence rate between the subducting and overriding plates regulating magma production (e.g., Pilger, 1984); (2) slab dynamics, including the opening of slab windows and changes in the subduction angle (Z. Zhang et al., 2010); (3) delamination of an eclogitic arc root (e.g., Kay & Kay, 1993; Lee & Anderson, 2015); (4) temporal filtering of mantle‐lithosphere interactions where the crust modulates mantle input (De Silva et al., 2015); (5) relamination of material off of the subducting plate and emplacement at the base of the arc crust (e.g., A. D. Chapman et al., 2013; Ducea & Chapman, 2018; Hacker et al., 2011); (6) incorporation of fore‐arc sediments into the active arc (Pearson et al., 2017; Sauer et al., 2017, 2018); (7) arc migration into the fertile retro‐arc region (J. B. Chapman & Ducea, 2019); and (8) feedback among linked tectonic processes (e.g., DeCelles et al., 2009; Ducea, 2001; Ducea & Barton, 2007).…”