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Subduction of an Oceanic Plateau Across Southcentral Alaska: Scattered‐Wave Imaging.

Authors :
Mann, Michael Everett
Abers, Geoffrey A.
Daly, Kiara A.
Christensen, Douglas H.
Source :
Journal of Geophysical Research. Solid Earth. Jan2022, Vol. 127 Issue 1, p1-21. 21p.
Publication Year :
2022

Abstract

An oceanic plateau, the Yakutat terrane, has entered the subduction system across southcentral Alaska. Its down‐dip fate and relationship to overlying volcanism is still debated. Broadband seismometers from the Wrangell Volcanism and Lithospheric Fate (WVLF) temporary experiment were deployed with <20 km spacing across southcentral Alaska to study this region. An array‐based deconvolution procedure is used to isolate the scattered P and S coda of teleseismic P waves for imaging discontinuity structure. This procedure is applied to WVLF and other dense seismic arrays across southcentral Alaska in a manner that accounts for near‐surface wavespeed variations. Two imaging techniques are employed: two‐dimensional migration and three‐dimensional common‐conversion‐point (CCP) stacking. Migrating the scattered phases along WVLF stations shows the ∼18 ± 4 km thick Yakutat crust subducting beneath the Wrangell Volcanic field to the NNE. It is offset from the Alaska‐Aleutian seismic zone laterally by 250 km to the southeast at 100 km depth, and dips more steeply (45°). At depths <45 km, CCP stacking reveals that the Yakutat crust is continuous for over 450 km along strike. This shallow continuity and deeper offset suggest a tear in the subducting Yakutat slab at depths >45 km, around 146°W. CCP stacking also reveals a continuous thin low‐velocity layer atop the underthrust Yakutat crust for >450 km along strike, at all depths <35 km. The uniform low‐velocity thrust zone indicates consistent properties through multiple rupture‐zone segments, showing that low‐velocity channels generally correspond with subduction megathrusts. Plain Language Summary: Typical oceanic crust that descends in subduction zones is 6–7 km thick. The subducting crust across southcentral Alaska is an >11 km thick oceanic plateau known as the Yakutat terrane. The western side of this study region has no subduction volcanism; the eastern side is home to the Wrangell Volcanic Field (WVF), one of the most voluminous continental volcanic fields in the world. Multiple seismometer arrays are used to image subsurface discontinuity structure associated with the subduction of the Yakutat terrane. We image the boundaries of the subducting Yakutat crust, and the base of the continental crust where it does not interfere with the subducting crust. The Yakutat terrane is a continuous feature for >450 km along strike at depths <45 km, but deeper it appears to be separated into two segments with >250 km of lateral offset, a 45° difference in orientation, and a 20° difference in dip angle. These differences provide strong evidence for a tear in the subducting plate that may be the cause of the extensive WVF. The results also show low seismic velocities atop the subducted Yakutat terrane at depths <35 km, which could be sediment being dragged down along the plate interface. Key Points: Yakutat oceanic plateau is subducting beneath Wrangell Volcanic Field (WVF) to at least 110‐km depthA thin low‐velocity layer delineates the megathrust atop the subducting Yakutat crust for over 450 km along‐strike, at depths <35 kmA tear in the subducting Yakutat terrane lies 100–200 km west/northwest of the WVF [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
21699313
Volume :
127
Issue :
1
Database :
Academic Search Index
Journal :
Journal of Geophysical Research. Solid Earth
Publication Type :
Academic Journal
Accession number :
154963084
Full Text :
https://doi.org/10.1029/2021JB022697