TY - JOUR
T1 - Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left-Lateral Slip on the San Juan Fault
AU - Harrichhausen, Nicolas
AU - Morell, Kristin D.
AU - Regalla, Christine
AU - Lynch, Emerson M.
AU - Leonard, Lucinda J.
N1 - Funding Information:
Funding for this project is provided by National Science Foundation Earth Science (NSF EAR) Grant 1756943 to K. Morell, and NSF EAR Grant 1756834 to C. Regalla. Additionally, N. Harrichhausen was supported by a National Science and Engineering Research Council (NSERC) Post Graduate Scholarship and Geological Society of America (GSA) Graduate Student Research Grant. The authors would like to thank the Pacheedaht First Nation for generously granting access to their land and for their interest in our research; Parks Canada for providing access, allowing for sample collection, and providing logistical support for work in Pacific Rim National Park; and TimberWest Forest Corporation and Island Timberlands for providing land access and logistical support for work on their respective properties. The authors thank M. Johns for helping to collect samples, and for processing, extracting, and identifying foraminifers for Sr isotope analyses; M. Makahnouk for Sr isotope analyses; and E. Humphrey for the scanning electron microscopy images used for foraminifer identification. K. Johnson, S. Mracek, C. Anderson, S. Dyer, and C. Green all assisted in field work and provided valuable input on our research. C. Goldblatt, and S. Citerneschi and L. Syme Citerneschi generously provided accommodation for N. Harrichhausen and K. Morell during multiple field campaigns. The authors also thank E. Nissen and the University of Victoria for logistical support and engaging discussions about regional tectonics. Thoughtful reviews and comments from B. Miller, E. Schermer, and editors M. Rusmore and R. Cecil were of great help and well appreciated.
Funding Information:
Funding for this project is provided by National Science Foundation Earth Science (NSF EAR) Grant 1756943 to K. Morell, and NSF EAR Grant 1756834 to C. Regalla. Additionally, N. Harrichhausen was supported by a National Science and Engineering Research Council (NSERC) Post Graduate Scholarship and Geological Society of America (GSA) Graduate Student Research Grant. The authors would like to thank the Pacheedaht First Nation for generously granting access to their land and for their interest in our research; Parks Canada for providing access, allowing for sample collection, and providing logistical support for work in Pacific Rim National Park; and TimberWest Forest Corporation and Island Timberlands for providing land access and logistical support for work on their respective properties. The authors thank M. Johns for helping to collect samples, and for processing, extracting, and identifying foraminifers for Sr isotope analyses; M. Makahnouk for Sr isotope analyses; and E. Humphrey for the scanning electron microscopy images used for foraminifer identification. K. Johnson, S. Mracek, C. Anderson, S. Dyer, and C. Green all assisted in field work and provided valuable input on our research. C. Goldblatt, and S. Citerneschi and L. Syme Citerneschi generously provided accommodation for N. Harrichhausen and K. Morell during multiple field campaigns. The authors also thank E. Nissen and the University of Victoria for logistical support and engaging discussions about regional tectonics. Thoughtful reviews and comments from B. Miller, E. Schermer, and editors M. Rusmore and R. Cecil were of great help and well appreciated.
Publisher Copyright:
© Wiley Periodicals LLC. The Authors.
PY - 2022/10
Y1 - 2022/10
N2 - The San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic-Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion-related strain, we use geologic mapping, kinematic inversion of fault-plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P- and T-axes from kinematic inversions indicate predominantly left-lateral slip. Left-lateral brittle faulting cross-cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non-conformably overlain by a >300 m-thick sequence of clastic marine shelf and slope sediments that are not left-laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left-lateral slip to the Eocene. Eocene left-lateral slip is temporally and kinematically consistent with regional southwest-northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.
AB - The San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic-Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion-related strain, we use geologic mapping, kinematic inversion of fault-plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P- and T-axes from kinematic inversions indicate predominantly left-lateral slip. Left-lateral brittle faulting cross-cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non-conformably overlain by a >300 m-thick sequence of clastic marine shelf and slope sediments that are not left-laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left-lateral slip to the Eocene. Eocene left-lateral slip is temporally and kinematically consistent with regional southwest-northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.
KW - fault kinematics
KW - northern Cascadia
KW - slickenline inversion
KW - terrane accretion
UR - http://www.scopus.com/inward/record.url?scp=85141717765&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85141717765&partnerID=8YFLogxK
U2 - 10.1029/2022TC007317
DO - 10.1029/2022TC007317
M3 - Article
AN - SCOPUS:85141717765
SN - 0278-7407
VL - 41
JO - Tectonics
JF - Tectonics
IS - 10
M1 - e2022TC007317
ER -