Testing the correlation of rocks east and west of the Harrison Lake Shear Zone: Implications for the eastern boundary of the Insular Superterrane

Abstract

Located in southwest British Columbia the Harrison Lake Shear Zone (HLSZ) is one of a number of west-verging faults that form the Coast Belt Thrust System, which developed along the inboard margin of the Insular superterrane as it collided with the western margins of ancestral North America. Within the southern Coast Belt the inboard margin of the Insular superterrane is thought to occur along the Central Coast Belt Detachment (CCBD). The HLSZ has historically been interpreted as an intra-terrane fault and thus not considered a potential candidate for the inboard margin of the Insular superterrane, due to previous studies correlating the Peninsula and Brokenback Hill assemblages west of the HLSZ with metamorphosed Slollicum schist to the east. Point counting analysis, detrital zircon U-Pb geochronology, and calculated Maximum Depositional Ages (MDAs) of 7 samples, reported here, indicate this stratigraphic correlation may not be sound. Samples west of the HLSZ yield MDAs between 138 ± 0.6 Ma and 93 ± 0.8 Ma, and contain small populations of Paleozoic and Precambrian zircon grains. East of the HLSZ, samples from the Slollicum schist contain only Mesozoic grains yielding MDAs of 120 ± 1.1 Ma to 112 ± 0.7 Ma. Differing sandstone compositions also suggest that rocks west and east of the HLSZ derive from two separate source regions and were later juxtaposed by the HLSZ. The contention that prior correlations may not be valid is also supported by spatio-temporal variations in the timing of magmatism west and east of the HLSZ. West of the HLSZ magmatism is most active from 180 Ma to 70 Ma, with a lull in magmatism between 140 Ma and 120 Ma, whereas magmatism to the east is continuous from 120 Ma to 60 Ma. These findings suggest the HLSZ may mark the eastern boundary of the Insular Superterrane, not the CCBD. Because the HLSZ is not cross-cut by plutons until the late Paleogene, it is also proposed that by recognizing the HLSZ as a major terrane boundary the middle Cretaceous constraint on the timing of movement between the Insular and Intermontane superterranes in the southern Coast Belt is removed.