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

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 M w 7.5 strike-slip event near Craig, Alaska, and an M w 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.

An Abrupt Transition in the Mechanical Response of the Upper Crust to Transpression along the Queen Charlotte Fault

“…Pink contours are the twtt (s) to the top of mélange beneath the terrace and show a northward-thickening basin within the QCT between lines EW1250 and EW1262. The dashed oval shows the region of buried extensional faults discussed by Walton et al (2015). Bathymetric contours are at 500 m intervals.…”

“…Deformation must be younger than about 15 Ma, the age of the underlying oceanic crust. The Miocene-Pliocene unconformity (5.3 Ma), labeled U in Figures 8 and 9, can be tentatively identified west of the terrace based on an abrupt change in seismic interval velocity and the presence of truncated or bright reflectors (Snavely et al, 1981;Scheidhauer, 1997;Walton et al, 2015). This unconformity is regionally observed and may be associated with a period of erosion or nondeposition coinciding with the onset of transpression at 5-6 Ma.…”

“…2b). Walton et al (2015) describe buried, presently inactive, normal faults as far north as 54°N, suggesting that normal faulting that occurs southwest of Haida Gwaii is translated north with the Pacific plate.…”

“…From 52°to 54°N, the terrace is flanked to the southwest by a topographic depression in the deep ocean floor known as the Queen Charlotte Trough. This article focuses on the terrace, and a companion paper (Walton et al, 2015) focuses on the trough. Rohr et al (2000) discussed the broader regional context of the seismic reflection data presented here.…”

Introduction to the Special Issue on the 2012 Haida Gwaii and 2013 Craig Earthquakes at the Pacific-North America Plate Boundary (British Columbia and Alaska)

“…Walton et al (2015) examine seismic reflection profiles and map the amount of present-day flexure of the Pacific plate. Periods of active plate flexure are inferred from the ages of deformed and undeformed sedimentary strata deposited on the igneous crust.…”