Seismotectonics of the Explorer region

Abstract

The Explorer region offshore western Canada is a tectonically complex area surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic models for the region differ fundamentally. Proposed plate configurations range from multiple independent plate fragments to an Explorer plate now fused to North America along the continental margin and cut by Pacific–North America transform faults in the west. We present new seismological data constraining the region's current tectonics. We use three‐component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor solutions, they represent the region's largest earthquake source parameter data set obtained by robust waveform modeling techniques. In addition, we perform joint epicenter determination to relocate larger earthquakes recorded since 1918. The source parameters and improved locations provide a consistent tectonic picture. Earthquake slip vector azimuths along the Pacific plate boundary change smoothly and are significantly less northerly oriented than the Pacific‐North America plate motion direction, requiring an independent Explorer plate. The present‐day Pacific‐Explorer boundary is formed by transform faults subparallel to the Revere‐Dellwood‐Wilson fault. Plate motion vectors indicate that the Winona block is part of the Explorer plate. Current Explorer motion is more northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change, possibly coinciding with a northwestward ridge jump near Explorer plate's northern end transferring the Winona block from the Pacific to the Explorer plate. In response to these plate motion changes the region north of the western Sovanco fracture zone was assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone, characterized by broadly distributed seismicity, is composed of well‐defined sets of conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the dominant northwest‐southeast fault sets; however, the conjugate sets must be also active to fully accommodate present‐day Explorer plate motion. The SW portion of the strike‐slip Nootka fault zone, the Explorer‐Juan de Fuca plate boundary, is well defined by focused seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer–North America boundary shows sporadic low‐magnitude seismicity. Our Explorer–North America rotation pole predicts convergence varying from negligible at the boundary's northwest end to ∼2 cm/yr at the SE end. This convergence can be accommodated either by subduction or by crustal thickening extending to the North American continent. We favor subduction based on low deformation rates observed by onshore GPS sites. The present Explorer plate system configuration is a result of stepwise reorientation of the Explorer ridge system, each step successively reducing the subduction rate relative to North America.

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

“…It is clear that the highest level of seismic activity and greatest diversity of mechanisms for these historical events occur between 52°and 53°N. Both thrust and strike-slip mechanisms along this segment of the QCF have strikes similar to the local strike of the QCF (Berube et al, 1989;Rohr and Furlong, 1995;Braunmiller and Nabelek, 2002;Ristau et al, 2007;Rohr and Tryon, 2010), consistent with pure shear-dominated transpression (Fig. 1).…”

Fracture Alignments in Marine Sediments Off Vancouver Island fromPsSplitting Analysis

“…Figure 12b displays theoretically expected fracture directions (e.g., Wilcox et al, 1973;ChristieBlick and Biddle, 1985) with respect to the NFZ. We use the strike direction of the NFZ of N55°E determined from relocated epicenters (Braunmiller and Nábělek, 2002). Although our observations are not completely consistent with the theoretical directions of these shears, group 1 roughly corresponds to antithetic R ′ shears, and group 2 is close to the orientation of tension fractures.…”

“…1) intersecting the northern Cascadia subduction zone developed as a result of the fragmentation of the Juan de Fuca plate at 3-4 Ma (e.g., Riddihough, 1984;Braunmiller and Nábělek, 2002). This fault zone is currently the boundary between the Explorer microplate and Juan de Fuca plate ( Fig.…”

Radon Transform Methods and Their Applications in Mapping Mantle Reflectivity Structure

“…12) could translates to a temperature decrease of 250-350°C relative to the ambient mantle. An anomaly of such magnitude may be caused by residual subducted oceanic lithosphere both from ongoing subductions in the northwest eastern Aleutian trench and from the quartet of Kula, Farallon, Pacific and North America Plates in the past 50-55 Mya (Stock and Molnar 1988;Braunmiller and Nabelek 2002;An07). In particular, the deposition of the former Kula-Farallon plates into the mantle beneath western North America could have 'scarred' the mid-mantle (Grand et al 1997; van der Hilst and Gu et al (2003).…”