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


Pearson, David M.
MacLeod, Douglas R.
Ducea. Mihai N.. Gehrels, George E.
and Patchett, P. Jonathan

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- to 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 intra-arc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

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Figure 9. Tectonic cartoons showing: a) retroarc underthrusting of Stikinia beneath the Coast Mountains batholith; b) and c) hypothesized underthrusting of forearc rocks, possibly correlative to the Chugach accretionary complex. Trench-side underthrusting may have occurred during an episode of shallow subduction, activation of an out-of-sequence fault within the lower Chugach accretionary complex, and deep duplexing along the subduction interface.

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Tectonics doi: 10.1002/2017TC004594