Compositional Variation in the Crust of the Coast Mountains Batholith From P-S Scattered Seismic Waves

The Coast Mountains Batholith (CMB) of British Columbia provides an excellent locale to study the processes whereby accreted terranes and subduction-related melts combine to form stable continental crust of intermediate to felsic composition. Genetic and spatial relationships between adjacent geologic units, however, are largely obscured by widespread plutonism and poorly understood structural contacts at the surface. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into subsurface lithologies. By comparing the arrival times of earthquake-generated P to S scattered phases and their multiples, we calculate average Vp/Vs and crustal thickness profiles along two orogen perpendicular transects of the CMB. Along the northern transect, which crosses the CMB in the vicinity of Kitimat (54° N), we observe high Vp/Vs (1.91 ± 0.12) in the outboard Insular superterrane, a sharp decrease to 1.71 ± 0.08 in the western Yukon Tanana block, and values fluctuating around 1.8 across the central gneiss complex and western Stikine Terrane. The crust is thinnest (26 ± 3km) beneath the outboard terranes and averages ~30 km east of the Shames River detachment. The southern transect crosses the CMB near Bella Coola (52.3° N), where extension between the Coast Shear Zone and the Shames River detachment is much more limited than along the northern line and there is no surface exposure of the central gneiss complex. Here we find thin crust (27 ± 4km) and high Vp/Vs (1.86 ± 0.12) in the outboard terranes. East of the Coast Shear Zone, we image the Moho at an average depth of 32 ± 1.5 km and Vp/Vs decreases to an average value of 1.77. We combine these data with calculated phase diagrams based on known and inferred compositions at a range of crustal depths to constrain the vertical and lateral extent of major tectonostratigraphic units.