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


Harrichhausen, Nicolas et al. (2022), Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left-lateral Slip on the San Juan Fault, Dryad, Dataset,


The San Juan fault, 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 San Juan fault'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 San Juan fault 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 San Juan fault. 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 package 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 San Juan fault accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the San Juan fault directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead, it offsets the older accretionary boundary between these two terranes.


Structural data:

The following data were collected during three seasons of fieldwork between May 2017 and July 2019. Structural data were collected during outcrop mapping using a brunton compass and were documented using a field notebook or a digital tablet. Strike and dips of fault planes, bedding planes, and folation follow the right-hand rule. Structural data was compiled and analyzed using the programs Stereonet 10.1.6 and FaultKin 8.1.2, available at  UTM coordinates in BC Albers NAD 83 Zone 10 coordinate system are provided for each data point. Fault slickening striae were collected as rakes and converted to striae trend and plunge using FaultKin. T- and P-axes were calculated using FaultKin. In the fault kinematics table, the T-axes are the tensional incremental strain axes, and the P-axes are the pressure incremental strain axes.  Sites and domains are defined in the manuscript.

Sr isotope data:

Sr isotope data were obtained from analyses of foraminifer samples at the University of Waterloo by the Thermal Ionization Mass Spectrometry (TIMS) Facility. Foraminifers were extracted from ~ 2 kg samples collected during fieldwork. Specimens were hand-picked from samples under a microscope. The most abundant species with the least diagenetic alteration was chosen for Sr isotope analyses.  Sample locations are provided with reference to Fig.3 in the manuscript associated with this dataset. 

More details on data acquisition and methods can be found in the README.txt file.

Usage notes

Please refer to the README.txt file. 


National Science Foundation, Award: 1756943

Natural Sciences and Engineering Research Council of Canada, Award: PGGSD - 504197 - 2017

National Science Foundation, Award: 1756834