Crustal structure of the Yakutat terrane and the evolution of subduction and collision in southern Alaska

Abstract

[1] We present a two-dimensional velocity model to constrain crustal thickness and composition of the Yakutat terrane in the northern Gulf of Alaska. The model was constructed using seismic reflection and refraction data along a $455 km onshore-offshore profile. Our model shows that the crystalline crust composing the Yakutat terrane is wedge-shaped, with crustal thickness increasing west to east from $15 km to $30 km. Crustal velocity and structure are continuous across the terrane, with lower crustal velocities >7 km/s, suggesting that the Yakutat terrane is an oceanic plateau across its entire offshore extent rather than a composite oceanic-continental terrane as previously proposed. The thickest Yakutat crust is entering the adjacent St. Elias orogen where elevated exhumation rates and concentrated seismicity in this vicinity are likely influenced by incipient Yakutat-North America collision. Our model includes a $8 km thick low-velocity crustal cap extending across the eastern portion of the profile where shallow basement is imaged on marine seismic reflection data. We interpret this cap as a lithified, metamorphosed remnant accretionary prism, providing evidence of a previous attempt at Yakutat subduction along its eastern margin prior to current emplacement at the southern Alaska margin.

Triggered Seismic Events along the Eastern Denali Fault in Northwest Canada Following the 2012 Mw 7.8 Haida Gwaii, 2013 Mw 7.5 Craig, and Two Mw>8.5 Teleseismic Earthquakes

“…Recent studies have detected ambient and triggered tremor in south-central mainland Alaska (Peterson and Christenson, 2009;Gomberg and Prejean, 2013). The tremor discovered in those studies lie where the Yakutat terrane is transitioning from flat-slab subduction in the west to a zone of collision in the east, resulting in a fold-and-thrust belt (Worthington et al, 2012). This region is > 300 km from where we observed triggered tremor in the southern portion of the EDF in the Yukon Territory, and thus the lowamplitude tremor signals we observed cannot be radiating from this part of the complex subduction zone.…”

Basement and Regional Structure Along Strike of the Queen Charlotte Fault in the Context of Modern and Historical Earthquake Ruptures

“…7) was generated at a ridge and is similar in age to the surrounding crust; despite this, we do not observe obvious flexure here. It is possible that the locally thicker and more buoyant crust of the Kodiak-Bowie seamounts may have resisted underthrusting and/or blocked propagation of flexure to the south (e.g., Christeson et al, 2010;Worthington et al, 2012), though more likely it is simply an overprinting of flexure due to higher topography. We are unable to effectively image and assess flexure around the seamounts with the existing data coverage, however.…”

Persistent long-term ( c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

“…We define 'collision' in this case as highly coupled flat-slab subduction at the subduction margin. The Yakutat microplate is composed of crystalline crust that is 15 -30 km thick and is inferred to have an oceanic plateau origin (Christenson et al 2010;Worthington et al 2012). Based on tomographical studies, the Yakutat microplate is currently undergoing flat-slab subduction beneath the Chugach-Saint Elias Mountains, with a northern edge 500 km inboard of the subduction zone at a depth of about 100 km (Eberhart-Phillips et al 2006).…”