Controlled-Source Shear-Wave Velocity Model of the Crust in the Coast Mountains Batholith Complex, Western Canada

Shear-wave seismic travel times were inverted to derive the crustal velocity structure of the Coast Mountains in the central coast of British Columbia. The seismic refraction and wide-angle reflection data were acquired in 2009 as part of the multi-disciplinary BATHOLITHS project investigating continental arc processes in the Jurassic to Eocene Coast Plutonic Complex. Previous P-wave modeling of the seismic data discovered a high-velocity (7.0-7.2 km/s) layer in the lower crust beneath the youngest (late Cretaceous to Eocene) portion of the arc complex.This high velocity body and an associated small crustal root are interpreted to be residue derived during the melting that produced the upper-crustal granitic batholith. In addition to P waves, the seismic data also contain strong S-wave arrivals from the upper crust and upper mantle, and strong S-wave reflections from the Moho. An S-wave velocity model of the upper crust was derived from the direct S-wave arrivals (Sg). Beneath a shallow Mesozoic basin, the Stikine accreted arc terrane east of the batholith complex has velocities of 3.4-3.5 km/s extending to ~20 km depth. The Vp/Vs ratio in Stikine upper crust slowly increases from 1.73 to 1.75, and is interpreted to represent a felsic to intermediate bulk composition. Seismic velocity in the batholith of 3.5-3.6 km/s and Vp/Vs ratio of 1.74-1.75 confirm its felsic composition. S-Wave reflections from the Moho (SmS) are consistent with the Moho depth model previously derived from the P-wave reflections (PmP). The Moho deepens from ~35 km under Stikinia to ~37 km under the batholith complex, then shallows toward the coast. Fixing the Moho depth, the S-wave velocity of the lower crust was inverted from the SmS data. The lower crust in the Stikine terrane has velocity of 3.5-3.75 km/s and a ~1.78-1.80 Vp/Vs ratio, and is interpreted to consist of amphibolite or other hydrated mafic rocks. In contrast, the lower crustal root beneath the highest mountains and youngest batholith complex has an unusually high S-wave velocity of 3.8-4.0 km/s in the same region that has high P-wave velocity. The high P- and S-wave velocities and a high Vp/Vs ratio of 1.81-1.86 in this root are interpreted to represent a bulk composition of mafic garnet granulite. This garnet granulite and large volumes of granodiorite-dominated melt were created by arc dehydration melting of amphibolite (or hydrated gabbro) in the pre-existing lower crust. Similar melt residual has not been observed under other exhumed continental arcs. Although S-wave refractions from the upper-most mantle (Sn) are weak, they indicate a mantle velocity of 4.5-4.8 km/s.