Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 32-4
Presentation Time: 9:05 AM


RUSMORE, Margaret E., Department of Geology, Occidental College, Los Angeles, CA 90041, WOODSWORTH, Glenn J., Geological Survey of Canada, 101-605 Robson St, Vancouver, BC V6B 5J3, Canada, CECIL, M. Robinson, Department of Geological Sciences, California State University Northridge, 18111 Nordhoff St, Northridge, CA 91330-8266, BOLLEN, Elizabeth M., Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, STOWELL, Harold H., Department of Geological Sciences, The University of Alabama, 201 7th Ave., Bevill Building, Room 202, Tuscaloosa, AL 35487, GEHRELS, George E., Department of Geosciences, Univ of Arizona, Tucson, AZ 85721 and GROVE, Marty J., Department of Geological Sciences, Stanford University, Stanford, CA 94305

New mapping, structural analysis, metamorphic petrology, and geochronology (U-Pb zircon, Sm-Nd garnet, Ar/Ar biotite) define a major NW-trending, sub-vertical crustal discontinuity (Scar Creek shear zone; SCSZ) in the southern Coast Mountains batholith near Mt Waddington (51⁰N). The shear zone juxtaposes rocks with contrasting pre-Tertiary lithologies, structural and metamorphic histories, and inferred paleolatitudes, suggesting it has a fundamental role in translation of terranes. Southwest of the SCSZ, Jurassic - Early Cretaceous plutonic and volcanogenic rocks, including large diorite complexes (ca. 150 – 85 Ma), are interleaved with pre-112 Ma quartzofeldspathic schists along sinistral reverse faults active 114 Ma to 104 Ma. Local SW-directed thrust faults were active at 78 Ma. Previous paleomagnetic results imply ~1700 km of northward translation < ~ 100 Ma. The SCSZ truncates the diorite complex and biotite 40Ar/39Ar and K/Ar ages decrease abruptly from ~80-85 Ma to ~60-50 Ma across the shear zone. Northeast of the SCSZ, diorite complexes are absent and Late Cretaceous amphibolite-facies schists of the eastern Waddington thrust belt dominate. The schist formed during penetrative NE-vergent contractional deformation from ~87 to 77 Ma, with the oldest garnet growth dated at 82 Ma (Sm-Nd). Discrete thrust faults and folds formed until at least 72 Ma. Deformation was coincident with a 85-70 Ma magmatic flare up; metamorphism continued to~65 Ma (garnet Sm-Nd). Previous paleomagnetic results imply ~2700 km of northward translation of Late Cretaceous strata NE of the SCSZ. Differences in metamorphic grade and structural style across the SCSZ could have been caused by vertical slip < 65 Ma, but contrasts in rock types and paleolatitudes imply significant strike-slip displacement < 85 Ma. Similarities to faults along strike to the south (Owl Cr and western Harrison Lk) and north (Coast shear zone) suggest the presence of a latest Cretaceous intrabatholithic shear zone from at least 49⁰ N to 54⁰ N. Disruption of the batholith is obscured by the subparallel strikes of this shear zone and magmatic age bands. Restoration of <400 km of dextral slip between ~85 – 55 Ma removes anomalies in inferred paleolatitudes within the southern CMB and contributes to the larger displacement called for by paleomagnetic data.