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

Abstract

The Queen Charlotte Fault (QCF) is a major strike-slip fault that forms the boundary between the Pacific and North American plates from 51°to 58°N. Near 53.2°N, the angle of oblique convergence predicted by the Mid-Ocean Ridge VELocity (MORVEL) interplate pole of rotation decreases from > 15°in the south to < 15°in the north. South of 53.2°N, the convergent component of plate motion results in the formation of a 40 km wide terrace on the Pacific plate west of QCF and earthquakes with thrust mechanisms (including the 2012 Haida Gwaii earthquake sequence) are observed. North of 53.2°N, in the primary rupture zone of the M 8.1 strike-slip earthquake of 1949, the linear terrace disappears, and topography of the continental slope west of the QCF is characterized by a complex pattern of ridges and basins that trend obliquely to the primary trace of the QCF. Deformation within the Pacific plate appears to occur primarily through strike-slip faulting with a minor thrust component on secondary synthetic faults. The orientations of these secondary faults, as determined from seismic reflection and bathymetric data, are consistent with the reactivation of faults originally formed as ridge-parallel normal faults and as thrust faults formed parallel to the QCF south of the bend at 53.2°N and subsequently translated to the north. We suggest that an oblique convergence angle of 15°represents a critical threshold separating distinct crustal responses to transpression. This result is consistent with theoretical and analog strain models of transpressive plate boundaries. The sharpness of this transition along the QCF, in contrast to purely continental transform boundaries, may be facilitated by the relatively simple structure of oceanic crust and the presence of pre-existing, optimally oriented faults in the young Pacific plate.

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

“…The Haida Gwaii earthquake can be explained by oblique convergence along the southern QCF where the component of convergence is 15-20 mm=yr as predicted by MORVEL (DeMets et al, 2010;Tréhu et al, 2015;see also Data and Resources). A maximum 100-120 km of convergence based on a 20 mm=yr rate must have been accommodated along the QCF over the last 5-6 Ma (Crouch et al, 1984;Hyndman and Hamilton, 1993).…”

“…Comparing the sidescan sonar and seismic reflection data allowed characterization of (Sandwell et al, 2013) with the modern coastline for reference. New Quaternary fault mapping from this study (thick white lines) and the faults proposed by Tréhu et al (2015; double white lines) are also plotted, along with a compilation of previously mapped faults in the area (Reed et al, 2005; thinner black lines). Inferred faults from Reed et al (2005) are plotted with black dashed lines and blind faults (Reed et al, 2005) with black stippled lines.…”

“…1). Based on structural analysis, Tréhu et al (2015) suggest that the change in strike of the QCF at 53.2°N significantly affects the response of the Pacific plate upper crust and overlying sediments to transpression, with pure shear dominant to the south and simple shear dominant to the north. The results of this study are consistent with this idea.…”

“…Processing was completed by M. Scheidhauer (Scheidhauer, 1997;Scheidhauer et al, 1999). This survey was important for interpretation of faults and basement structure in the area, and examples of the EW9412 profiles can be seen in Rohr et al (2000) and Tréhu et al (2015).…”

“…A study by Bostwick (1984) proposes two models for the 1949 event: a longer, bilateral rupture model based on aftershocks and a shorter, northward-propagating rupture based on surface-wave directivity (see fig. 3 from Tréhu et al, 2015, for a summary of different rupture models for the 1949 event). Recent Global Positioning System (GPS) analysis of seismicity supports the latter, shorter rupture model for the 1949 event (Ding et al, 2015).…”