TY - JOUR
T1 - Paleoseismic trenching reveals late quaternary kinematics of the leech river fault
T2 - Implications for forearc strain accumulation in northern cascadia
AU - Harrichhausen, Nicolas
AU - Morell, Kristin D.
AU - Regalla, Christine
AU - Bennett, Scott E.K.
AU - Leonard, Lucinda J.
AU - Lynch, Emerson M.
AU - Nissen, Edwin
N1 - Funding Information:
The authors would like to thank Kapoor Lumber and the Capital Regional District (CRD) for providing land access to our trench sites. Great appreciation is extended to Johanna Fischi, Vic Levson, Alice Telka, Steve Byrne, Matthew Sypus, Elena Savidge, and Fengzhou Tan, who all assisted in our paleoseismic trench study and provided invalu-able input to our research. Walter Langer of AllTerra Excavating Ltd. excavated our trenches and without his expertise, this study would not have been possible. The authors also would like to thank the Keck Carbon Cycle AMS laboratory for our radiocarbon analyses. The authors thank Brian Penserini for reading over our article, and the authors are grateful to Y. Klinger, S. Angster, K. Reicherter, and one anonymous reviewer for their thoughtful suggestions and reviews. Funding for this project is provided by National Science Foundation (NSF) EAR Grant Number 1756943 to K. Morell and Grant Number 1756834 to C. Regalla. This work was also supported by a National Science and Engineering Research Council of Canada Postgraduate Doctoral scholarship (NSERC PGS D) to N. Harrichhausen. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Funding Information:
The authors would like to thank Kapoor Lumber and the Capital Regional District (CRD) for providing land access to our trench sites. Great appreciation is extended to Johanna Fischi, Vic Levson, Alice Telka, Steve Byrne, Matthew Sypus, Elena Savidge, and Fengzhou Tan, who all assisted in our paleoseismic trench study and provided invaluable input to our research. Walter Langer of AllTerra Excavating Ltd. excavated our trenches and without his expertise, this study would not have been possible. The authors also would like to thank the Keck Carbon Cycle AMS laboratory for our radiocarbon analyses. The authors thank Brian Penserini for reading over our article, and the authors are grateful to Y. Klinger, S. Angster, K. Reicherter, and one anonymous reviewer for their thoughtful suggestions and reviews. Funding for this project is provided by National Science Foundation (NSF) EAR Grant Number 1756943 to K. Morell and Grant Number 1756834 to C. Regalla. This work was also supported by a National Science and Engineering Research Council of Canada Postgraduate Doctoral scholarship (NSERC PGS D) to N. Harrichhausen. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Publisher Copyright:
© Seismological Society of America.
PY - 2021/4
Y1 - 2021/4
N2 - 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 SH max 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.
AB - 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 SH max 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.
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U2 - 10.1785/0120200204
DO - 10.1785/0120200204
M3 - Article
AN - SCOPUS:85103859156
SN - 0037-1106
VL - 111
SP - 1110
EP - 1138
JO - Bulletin of the Seismological Society of America
JF - Bulletin of the Seismological Society of America
IS - 2
ER -