Basement and regional structure along strike of the Queen Charlotte Fault in the context of modern and historical earthquake ruptures

Bulletin of the Seismological Society of America
By: , and 

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Abstract

The Queen Charlotte fault (QCF) is a dextral transform system located offshore of southeastern Alaska and western Canada, accommodating ∼4.4  cm/yr of relative motion between the Pacific and North American plates. Oblique convergence along the fault increases southward, and how this convergence is accommodated is still debated. Using seismic reflection data, we interpret offshore basement structure, faulting, and stratigraphy to provide a geological context for two recent earthquakes, an Mw 7.5 strike‐slip event near Craig, Alaska, and an Mw 7.8 thrust event near Haida Gwaii, Canada. We map downwarped Pacific oceanic crust near 54° N, between the two rupture zones. Observed downwarping decreases north and south of 54° N, parallel to the strike of the QCF. Bending of the Pacific plate here may have initiated with increased convergence rates due to a plate motion change at ∼6  Ma. Tectonic reconstruction implies convergence‐driven Pacific plate flexure, beginning at 6 Ma south of a 10° bend the QCF (which is currently at 53.2° N) and lasting until the plate translated past the bend by ∼2  Ma. Normal‐faulted approximately late Miocene sediment above the deep flexural depression at 54° N, topped by relatively undeformed Pleistocene and younger sediment, supports this model. Aftershocks of the Haida Gwaii event indicate a normal‐faulting stress regime, suggesting present‐day plate flexure and underthrusting, which is also consistent with reconstruction of past conditions. We thus favor a Pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading. In addition, mapped structures indicate two possible fault segment boundaries along the QCF at 53.2° N and at 56° N.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Basement and regional structure along strike of the Queen Charlotte Fault in the context of modern and historical earthquake ruptures
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120140174
Volume 105
Issue 28
Year Published 2015
Language English
Publisher Seismological Society of Amercia
Contributing office(s) Alaska Science Center Geology Minerals
Description 16 p.
First page 1090
Last page 1105
Country United States
State Alaska
Other Geospatial Queen Charlotte Fault
Online Only (Y/N) N
Additional Online Files (Y/N) N