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

Harrichhausen, Nicolas; Morell, Kristin; Regalla, Christine; Lynch, Emerson; Leonard, Lucinda J.

doi:10.1029/2022tc007317

Cited by 4 publications

(5 citation statements)

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“…Additionally, quality surficial geologic maps are required to distinguish diagnostic Quaternary scarps from erosional bedrock scarps. Bedrock scarps resulting from preferential erosion and plucking of brittle fault zones are common in recently glaciated terrain, but are not indicative of active faulting as they may have formed in a different tectonic regime with different stress conditions (e.g., Harrichhausen et al., 2022; Personius et al., 2014). High‐resolution topography and bathymetry, such as lidar used in this study, are the best way to identify Quaternary scarps, which are typically very subtle, due to the short period of time for slip to accrue and the potential for rapid erosion and poor preservation potential after deglaciation.…”

Section: Discussionmentioning

confidence: 99%

“…This structure crosses the peninsula from Haro Strait in the southeast, to Saanich Inlet to the northwest, and continues to the northwest on Vancouver Island where it is mapped to be offset by several northeast‐striking strike‐slip faults (Figure 2a; Muller, 1983). The largest of the structures, the San Juan fault (SJF), is a major terrane boundary that accommodated left‐lateral oblique slip during Eocene terrane accretion and formation of the northwest‐southeast striking Cowichan fold and thrust belt (CFTB) (Figure 2a; England & Calon, 1991; Fairchild & Cowan, 1982; Harrichhausen et al., 2022). On Saanich Peninsula, the bedrock fault coinciding with the XELF juxtaposes intrusive rocks of the West Coast complex to the southwest against volcanic rocks to the northeast, both lithologies belonging to the Jurassic Bonanza island arc (Canil & Morris, 2023; Canil et al., 2013; DeBari et al., 1999; Muller, 1977, 1983).…”

Section: Geologic Settingmentioning

confidence: 99%

“…To the northwest, we consider the intersection of the XELF and the SJF as a structural boundary that could inhibit rupture propagation for two reasons. First, the SJF is a major terrane‐bounding structure that accrued substantial left‐lateral slip in the Eocene (Harrichhausen et al., 2022) and offset the bedrock fault the XELF may have reactivated (Figure 2a). Second, instrumental seismicity along the XELF diminishes to the northwest of the SJF.…”

Section: Analysesmentioning

confidence: 99%

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

Harrichhausen,

Finley,

Morell

et al. 2023

Tectonics

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Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments.

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Section: Discussionmentioning

confidence: 99%

Section: Geologic Settingmentioning

confidence: 99%

Section: Analysesmentioning

confidence: 99%

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

Harrichhausen,

Finley,

Morell

et al. 2023

Tectonics

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“…Phase 2 strata are more deformed than those of Phase 1 due to uplift, erosion and deformation of Nanaimo Group strata during the Eocene (e.g., Harrichhausen et al., 2022; Seyler et al., 2022). Consequently, there is greater uncertainty in the genetic stratigraphic framework developed for Phase 2.…”

Section: Georgia Basin Evolutionmentioning

confidence: 99%

“…These two outcrop areas are geographically separated by the Nanoose Uplift (also known as the Nanoose Arch), which is a small fault‐bounded block of Palaeozoic metamorphosed volcanic and sedimentary rocks that are overlain by a thin veneer of Nanaimo Group strata on its margins. Collision of the Siletzia Terrane with North America in the early Eocene led to extensive erosion and deformation of Nanaimo Group strata (Harrichhausen et al., 2022; Seyler et al., 2022) in proximity of the Nanoose Uplift and further south.…”

Section: Introductionmentioning

confidence: 99%

Stratigraphy, palaeogeography and evolution of the lower Nanaimo Group (Cretaceous), Georgia Basin, Canada

Girotto,

Dashtgard,

Huang

et al. 2023

Basin Research

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The Cretaceous lower Nanaimo Group in the Georgia Basin, Canada comprises multiple depositional phases with distinct depocentres that accumulated in a tectonically active forearc basin setting. Basal coarse‐clastic strata are preserved in paleotopographic depressions and grade upwards into coal‐bearing coastal plains and shallow‐marine deposits. Coal‐bearing and shallow‐marine strata grade laterally into and are overlain by, regionally extensive mudstones and turbidites deposited in deep water. A glauconitic sandstone bed within the deep‐water strata is interpreted as a condensed section and underlies a major disconformity that developed during a pause in the deposition of the lower Nanaimo Group. A second major coarse‐clastic succession occurs hundreds of metres above the glauconite bed in the central Georgia Basin and comprises conglomerate, sandstone, mudstone and coal deposited in continental depositional environments. The shift in sedimentation from the northern Georgia Basin to the central Georgia Basin is interpreted to record the emergence of an island (Nanoose Uplift) in the central Georgia Basin that acted as a major sediment source to the adjacent depocentres. The stratigraphic break between the coal‐bearing coarse‐clastic strata in the northern Georgia Basin and the significantly younger coal‐bearing coarse‐clastic strata in the central Georgia Basin indicates that the lower Nanaimo Group was deposited in distinct depocentres. Between the older, coarse‐clastic strata in the north and younger, coarse‐clastic strata in the central Georgia Basin, we hypothesize that a major deepwater canyon system (Qualicum Canyon) existed and transferred sediment from the semi‐restricted Georgia Basin to the Pacific Ocean to the west. Development of the Qualicum Canyon and exposure of the Nanoose Uplift during deposition of the younger, central coarse‐clastic strata suggests that syntectonic activity drove basin uplift and erosion and this occurred throughout the deposition of the lower Nanaimo Group.

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