Protolith age of the Swakane Gneiss, North Cascades, Washington: Evidence of rapid underthrusting of sediments beneath an arc

Abstract

The metamorphic core of the North Cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 Ma magmatism. However, the tectonic affinity of the structurally deepest terrane, the 9–12 kbar Swakane Gneiss, is distinctly different from other terranes in the core; it is not intruded by arc‐related plutons and contains abundant Precambrian zircons. New U‐Pb analyses of detrital zircons from the Swakane Gneiss yield dates from 73 Ma to 1610 Ma with a dominant Late Cretaceous population. These data indicate that the Swakane protolith was deposited as late as 72.5 ± 0.6 Ma, the 206Pb/238U date of the youngest detrital grain. Following deposition, the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near‐ to postpeak P‐T conditions. One sheet yielded a U‐Pb crystallization age of 68.36 ± 0.07 Ma, which indicates that the gneiss was deeply buried within 5 Myr of deposition. Two possible mechanisms considered for this rapid burial (∼7 mm/yr) are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low‐angle subduction. The model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the Cascades core. Rapid burial of the Swakane protolith is coincident with burial of sediments that formed the Pelona, Orocopia and Rand schists of southern California; however, differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin.

Metamorphic rocks in central Tibet: Lateral variations and implications for crustal structure

“…Low-angle southward subduction tectonically eroded the basement of the northern Qiangtang terrane and incorporated Paleo-Tethyan, Qiangtang rocks of supracrustal affi nity, and on February 11, 2011 gsabulletin.gsapubs.org Downloaded from possibly tectonically eroded Qiangtang terrane basement into the mélange. Tectonic erosion of the upper plate and underplating of sediments beneath continents during ocean-continent subduction have been extensively documented in the western Cordillera of North America (Jacobson et al, 1996;Matzel et al, 2004;Ducea et al, 2009). In addition, low-angle subduction is inferred in our model to explain the lack of an observable well-developed Middle-Late Triassic arc (e.g., Dumitru et al, 1991) despite the documentation of similar-age blueschist-facies metasedimentary rocks and the extensive northsouth exposure of the mélange within the center of the Qiangtang terrane (Kapp et al, 2003).…”

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…Some subducted continental material is recycled into the mantle (e.g., Hilde, ; Scholl & von Huene, ; von Huene & Scholl, ), but other material is also likely added to the active continental magmatic arc. This incorporation of supracrustal material may occur through (1) underthrusting or imbrication of backarc or forearc sediments into the core of the arc system (Chin et al, ; Ducea, ; Ducea & Barton, ; Matzel et al, ) (Figures a and b), (2) subduction and subsequent low‐angle underplating of sediment into the arc (Ducea et al, ; Saleeby, ; von Huene & Scholl, ) (Figure c), and/or (3) subduction and subsequent diapiric rise of unstable sediment detached from the subducting plate, leading to “relamination” at the base of the arc crust (Behn et al, ; Castro et al, ; Chapman et al, ; Hacker et al, ) (Figure d). The introduction of fertile, sedimentary material into arc systems has major consequences as it drives the crust toward a more felsic composition (e.g., Behn et al, ; Hacker et al, ), may fuel magmatic flares‐ups (DeCelles et al, ; Ducea, ; Ducea & Barton, ), and may affect the rheology of the arc crust (Hollister & Crawford, ; Miller & Paterson, ).…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths I – The Cretaceous Coastal Batholith of Peru and its Role in South American Orogenesis and Hemispheric Subduction Flip

“…Orocopia and Pelona schist (Jacobson et al 2007) is associated with this zone, which suggests that it had once been joined with the similar Swakane gneiss of the North Cascades (Matzel et al 2004;Hildebrand 2013Hildebrand , 2014. Similarly, a major swarm of Late Cretaceous-Early Tertiary post-collisional plutons -considered by Hildebrand (2013) to have been generated by slab failure -trending through the Transverse Ranges, the Mojave and Sonoran deserts, and on through western Mexico would also match with similar age plutons of the Cascades and Idaho (Figure 12).…”