Fault accommodation of permanent strain in the northern Cascadia forearc

This thesis focuses on determining how forearc strain was and is accommodated by ancient and active faults in the northern forearc of the Cascadia subduction zone on southern Vancouver Island, Canada. By examining these faults, permanent forearc strain can be resolved, helping understand subduction zone processes and providing key information for regional seismic hazard models. The following studies, described in Chapters 2-4, use field data and high-resolution topography to investigate the kinematics of two major terrane-bounding faults that bisect southern Vancouver Island: the San Juan fault and the Leech River fault. Numerical modelling is also used to test how interseismic coupling and coseismic slip on the Cascadia subduction zone megathrust would induce slip on forearc structures. \t\nChapter 2 of this thesis investigates the kinematic history of the San Juan fault on southern Vancouver Island. This fault has been hypothesized to play a role in two episodes of terrane accretion in the northern Cascadia forearc, however, direct observations of its kinematics have not been documented. To test these hypotheses, this study uses detailed geologic mapping, kinematic inversion of fault-plane slickenlines, and dating of marine sediments to constrain the timing and direction of slip of the San Juan fault. P-and T-axes derived from kinematic inversions indicate that the fault predominantly accommodated left-lateral slip. Left-lateral brittle faulting cross-cuts ∼ 51 Ma magmatic intrusions and foliation, providing a maximum age of deformation. The fault zone is non-conformably overlain by an ∼ 200 m-thick package of relatively undeformed clastic marine shelf and slope sediments. Stratigraphic correlation and strontium isotope dating of foraminifer assemblages from these sediments indicate a late Eocene–early Oligocene depositional age, and bracket the timing of left-lateral slip on the San Juan fault to the Eocene. Eocene left-lateral slip is temporally and kinematically consistent with southwest–northeast compression documented in the region during the accretion of the Siletzia ocean island plateau, suggesting slip on the San Juan fault partially accommodated accretion of this terrane to the northern Cascadia forearc. Chapter 3 examines permanent deformation of the northern Cascadia forearc by constraining the active kinematics of the Leech River fault on southern Vancouver Island. The Leech River fault lies in a key region where a south-to-north reduction in northward GNSS velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca, and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward-migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary right-lateral oblique slip on east–west trending forearc faults north of the Olympic Mountains and in the southern Strait of Georgia, are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. This study presents the results from geomorphic mapping and two new paleoseismic trenches excavated across the Leech River fault that evaluate strain in the region north of the Strait of Juan de Fuca. Trench excavations reveal right-lateral oblique slip during one surface rupturing earthquake dated to 9.4 ± 3.4 ka. These active fault kinematics are consistent with slip observed on other regional east–west trending faults, and indicate that these structures do not accommodate significant north–south shortening via thrust faulting. Chapter 4 uses boundary element method (BEM) modelling to test if elastic deformation resulting from coupling along the subduction zone interface can account for observed permanent forearc deformation in northern Cascadia. As several paleoseismic studies have been completed along its length, the Leech River–Devils Mountain fault system represents an ideal structure to test the hypothesis that permanent forearc deformation is largely driven by accumulated elastic strain not recovered during megathrust earthquakes. Right-lateral oblique slip observed on the Leech River fault (Chapter 3) is similar to observations in other studies of the along-strike Devils Mountain fault in Washington State, and to slip inferred from crustal seismicity. However, 3D BEM modelling of fault slip resulting from elastic strain induced by subduction zone coupling, predicts left-lateral and reverse slip on the Leech River–Devils Mountain fault zone. Additionally, if coseismic slip, or only the strike-slip component of subduction zone coupling, are used to drive elastic strain in the modelled forearc, predicted fault slip is also inconsistent with observations of fault kinematics. Although these models are simplified and do not accurately represent the complicated rheology of a forearc setting, they contradict the prevailing hypothesis that subduction zone coupling is the primary driver of permanent forearc deformation.

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