Paper No. 32-4
Presentation Time: 9:05 AM
NEWLY RECOGNIZED LATEST CRETACEOUS TRANSCURRENT FAULTING WITHIN THE COAST MOUNTAINS BATHOLITH (CMB), BRITISH COLUMBIA
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.