Paleoseismic Trenching Reveals Late Quaternary Kinematics of the Leech River Fault: Implications for Forearc Strain Accumulation in Northern Cascadia

Abstract

New paleoseismic trenching indicates late Quaternary oblique right-lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south-to-north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), 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 oblique right-lateral slip on west-northwest-striking 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. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike-slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right-lateral slip and the published regional SHmax directions. Vertical separation and the formation of scarp-derived colluvium along one fault also indicate a dip-slip component. Radiocarbon charcoal dating within offset glacial till and scarp-derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4 ka. The oblique right-lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west-northwest-striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.

Structural and Metamorphic History of the Leech River Shear Zone, Vancouver Island, British Columbia

“…Limited observations of shear sense indicators from mylonites within the Bear Creek Shear Zone indicate left‐lateral motion (Groome, 2000). Quaternary reactivation of this structure as a wide zone of strike‐slip faults has been documented through identification of fault scarps (Morell et al., 2017), microseismicity (Li et al., 2018), and paleoseismic trenching (Harrichhausen et al., 2021; Morell et al., 2018). Here, we focus on the ancient structure that established the tectonic contact between the LRC and MIC (the “Leech River Shear Zone”), rather than the recently active “Leech River Fault,” which is locally parallel and colocated with the terrane boundary and mylonite zone of the LRSZ.…”

“…One candidate structure is the Leech River Fault (Figure 1), a terrane‐bounding fault located on southern Vancouver Island in British Columbia that juxtaposes mafic volcanic and igneous rocks of the Metchosin Igneous Complex (MIC) to the south with the predominantly metapelitic Leech River Complex (LRC). For clarity, we will term this structure the “Leech River Shear Zone” (LRSZ) to differentiate it from the recently active “Leech River Fault,” which is locally coincident (Harrichhausen et al., 2021; Li et al., 2018; Morrell et al., 2017, 2018). The MIC is part of the Siletz‐Crescent terrane, a ∼10‐ to 30‐km‐thick sequence of Eocene mafic volcanics and intrusive suites that extends from Vancouver Island to Oregon.…”

Discovery of an Active Forearc Fault in an Urban Region: Holocene Rupture on the XEOLXELEK‐Elk Lake Fault, Victoria, British Columbia, Canada

“…Permanent forearc deformation is evidenced by instrumental crustal seismicity (e.g., Balfour et al, 2011;Bostock et al, 2019;Brocher et al, 2017;Savard et al, 2018;G. Li et al, 2018), paleoseismic studies of Quaternary-active forearc faults (Barrie & Greene, 2018;Bennett et al, 2017;Blais-Stevens et al, 2011;Blakely et al, 2009;Duckworth et al, 2021;Greene & Barrie, 2022;Harrichhausen et al, 2021;Kelsey et al, 2012;Morell et al, 2017Morell et al, , 2018Nelson et al, 2017;Personius et al, 2014;Schermer et al, 2021), and paleomagnetic, geologic, and geodetic data that show counterclockwise rotation of the Cascadia forearc north of, and margin-parallel shortening south of, the Strait of JdF (Finley et al, 2019;Mazzotti et al, 2002Mazzotti et al, , 2003McCaffrey et al, 2007;Miller et al, 2001;Prothero et al, 2008;Wells & McCaffrey, 2013).…”

“…These studies initially have been focused on the Puget Lowland in Washington, but the along‐strike continuation of several faults have been recently recognized near the San Juan and Gulf Islands (Barrie & Greene, 2018; Greene & Barrie, 2022), and on southern Vancouver Island (Morell et al., 2017). In particular, the recognition of the potential of the Leech River fault (LRF) on southern Vancouver Island to produce ∼ M 7 earthquakes (Harrichhausen et al., 2021; Morell et al., 2018) has resulted in updates to seismic hazard assessments for Victoria (Goda & Sharipov, 2021; Halchuk et al., 2019; Kukovica et al., 2019), the capital city of British Columbia with a metropolitan population of ∼400,000 (Statistics Canada, 2023). A loss estimation scenario for a rupture of M w 7.3 on the LRF would entail substantial economic and human losses (∼1,000 deaths and ∼20 billion CAD, Hobbs, 2022).…”

Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left‐Lateral Slip on the San Juan Fault

“…Finally, because we do not observe strike-slip faulting of the Carmanah Group sediments, we consider the right-lateral faults mapped along the western SJF (Sites I and II) do not represent Quaternary deformation that has been recognized elsewhere on southern Vancouver Island. Paleoseismic investigations of the Leech River fault, 10-25 km south of the SJF (Figure 2), indicate that the eastern, northwest-southeast striking segment of this structure has hosted Holocene right-lateral oblique surface rupturing earthquakes (Harrichhausen et al, 2021;Morell et al, 2017Morell et al, , 2018. As the Carmanah Group sediments only overlie the SJF at the western end of the fault, it is possible that only the western SJF may be inactive (Sites I and II), and we cannot rule out active right-lateral faulting at Sites IV and V to the east.…”