Slip-Rates, Obliquity Estimates and Plate Boundary Localization Along the Queen Charlotte Fault Based on Submarine Tectonic Geomorphology

Date: 4/24/2019

Time: 04:15 PM

Room: Vashon

Seismic and geodetic monitoring of active fault systems does not typically extend beyond one seismic cycle, hence it is challenging to link the characteristics of individual earthquakes with long-term fault behavior. The Queen Charlotte Fault (QCF) is a dextral strike-slip fault that defines the North America-Pacific plate boundary for ~900 km offshore southeastern Alaska and western British Columbia. Although the QCF has generated seven M_w>7 earthquakes since 1900, its kinematics and long-term behavior have never been examined systematically due to an absence of marine geophysical imaging. We present new analysis of the tectonic geomorphology along the entire length of the QCF based on comprehensive multibeam bathymetry data acquired between 2010 and 2018. Along the northernmost ~315 km, the fault is a narrow, linear and continuous trace that is closely aligned with plate motion vectors. Its strike is increasingly oblique to plate motion moving south and secondary faults and folds along the continental slope become more prevalent, suggesting transpressional deformation increases. We present a catalog of 179 seabed features (e.g., gullies, escarpments, ridges, and fan aprons) that have been offset horizontally by motion on the QCF since the late Pleistocene. Offset estimates are based on a quantitative reconstruction approach and range from 680–900 m. All of the features are relic, having formed rapidly during the final stages of deglaciation (17–13 ka) by high-energy outwash-derived sediment flows that crossed the QCF. Dated sediment cores and other proxies suggest these flows shut down abruptly, but the timing varied along strike, leading to variation in offset. The horizontal slip-rate estimates are consistently between 47–56 mm/yr over a distance of >680 km. Despite the apparent increase in transpression along the southern section of the fault, the average rate of convergence on this section may only be ~6 mm/yr. These results have implications for regional tectonic models, historical earthquake behavior, and tsunami generation along the QCF.

Presenting Author: Daniel S. Brothers

Authors