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Late Pleistocene to Holocene transtension in the northern Cascadia forearc: Evidence from surface ruptures along the Beaufort Range fault
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  • Emerson M Lynch,
  • Christine A. Regalla,
  • Kristin Diane Morell,
  • Nicolas Harrichhausen,
  • Lucinda Jane Leonard
Emerson M Lynch
Northern Arizona University

Corresponding Author:eml365@nau.edu

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Christine A. Regalla
Northern Arizona University
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Kristin Diane Morell
University of California, Santa Barbara
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Nicolas Harrichhausen
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Lucinda Jane Leonard
University of Victoria
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The active deformation field in subduction forearcs provides critical information about the stress and strain state of the upper plate and its potential for seismogenesis. However, these properties are challenging to quantify in most subduction systems, and in the northern Cascadia forearc, few faults have been identified that can be used to reconstruct the upper plate deformation field. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island. This fault was proposed to host the 1946 M7.3 Vancouver Island earthquake, but no surface rupture or evidence of Quaternary activity has been documented, and the stress and strain conditions that promoted this event are poorly understood. We provide the first evidence that the BRF is active, using newly-collected lidar to map topographic scarps along the fault system and to reconstruct slip vectors from offset geomorphic markers. Quaternary deposits and landforms that show increasing magnitude of displacement with age provide evidence for at least three M ~6.5-7.5 earthquakes since ~15 ka, with the most recent event occurring <3-4 ka. Kinematic inversions of offset geomorphic markers show that the BRF accommodates right-lateral transtension along a steeply NE-dipping fault. This fault geometry and kinematics are similar to those modeled for the 1946 earthquake, suggesting that the BRF is a candidate source fault for this event. We find that the kinematics of the BRF are consistent over decadal to millennial timescales, suggesting that this portion of the northern Cascadia forearc has accommodated transtension over multiple earthquake cycles.
06 Jun 2023Submitted to ESS Open Archive
07 Jun 2023Published in ESS Open Archive