JURA-CRETACEOUS SINISTRAL SHEAR ZONES ALONG THE WESTERN FLANK OF THE CENTRAL AND SOUTHERN COAST MOUNTAIN

One of the most controversial aspects of Cordilleran tectonics concerns the degree of coastwise displacement of terranes during Jurassic through early Tertiary time. There has been considerable debate about the amount of Late Cretaceous-early Tertiary dextral (northward) displacement, but less emphasis on the history and significance of Jurassic-Early Cretaceous sinistral (southward) displacement. Based on the contributions of many geologists, including our honorees for this session, there is growing evidence that large-scale sinistral motion occurred on Late Jurassic-Early Cretaceous shear zones that are located within and adjacent to the central and southern Coast Mountains. Apparent doubling of the southern Coast Mountains batholith has led to the suggestion of ~800 km of sinistral motion during Early Cretaceous time (Monger et al., 1994). Dispersal of fragments of Wrangellia (Plafker and Berg, 1994) and Alexander (Tochilin et al., 2014) terranes has been attributed to 600-1000 km of Jura-Cretaceous sinistral displacement within the Insular terrane. Previous field studies along the western flank of the central Coast Mountains have shown that shear zones of the appropriate sense and age exist (e.g., Chardon et al., 1999; Butler et al., 2009; Nelson et al., 2012). We are beginning field studies in the southern Coast Mountains to trace these shear zones southward, constrain amounts of offset, and reconstruct their role in juxtaposing disparate assemblages and controlling batholith emplacement. Evidence so far suggests that wide mylonite zones in the Cape Caution area record sinistral offset and are preferentially developed within and adjacent to plutons that yield 142-107 Ma U-Pb ages. As described in the accompanying abstract by Rusmore et al., sinistral-oblique shear zones (potentially with >100 km of displacement) have also been recognized in Bute Inlet. We anticipate that reconstructing the nature, age, and amount of offset on these shear zones will better define the pre-Late Cretaceous paleogeography of the Cordilleran margin, and provide a more robust basis for reconstructing southward and northward translations.