Controlled-Source Seismic Investigation of the Generation and Collapse of a Batholith Complex, Coast Mountains, Western Canada

In July 2009, a 400-km long refraction and wide-angle reflection seismic survey was acquired across the Coast Mountains of British Columbia, western Canada. The survey is the final phase of the multidisciplinary BATHOLITHS project investigating the Jurassic to Eocene subduction-zone batholith complex. The large volume of granitic batholiths created by magmatic differentiation in arcs above subduction zones makes continental crust more felsic than the original materials derived from the mantle. However, the differentiation of the granitic rock from a mafic protolith will result in a large volume of ultramafic residual that is petrologically part of the crust, not changing the bulk composition. The fate of this residue is still unknown. It may reside hidden beneath the geophysical Moho, or it may have delaminated to sink into the mantle due to its relative density. Delimitation may occur during subduction or during the collapse of the arc after subduction stops. Delimitation may coincide with a commonly observed late pulse of magmatic activity. Active-source seismic studies coupled with complementary geophysical and geological studies may be able to distinguish the processes involved and the fate of the residue. Traveltimes from the seismic survey were used to build a 2-D seismic velocity model of the crust and uppermost mantle. East of the batholith complex, the Mesozoic Nechako sedimentary basin is indicated by velocities of 3-5 km/s to 2-4 km depth. These rocks are underlain by crystalline rocks of the Stikine terrane with velocities of 5.8-6.2 km/s extending to at least 15 km depth. A ~100 km long reflector at ~22 km depth is observed below the Stikine terrane but does not extend into the arc complex to the west. The lower crustal velocity averages ~6.7 km/s. The Moho beneath Stikinia is at ~35 km depth and the underlying upper mantle has a fast velocity of ~8.1 km/s. To the west in the continental arc complex, there are thin slow sediments in deep river valleys, but otherwise velocities of 5.6-6.2 km/s indicate granitic rocks to at least 15 km depth. Beneath the western Jurassic to mid-Cretaceous arc, felsic velocities of <6.6 km/s extend to the Moho that dips east from 30 to 34 km depth. Beneath the highest mountains and youngest Cretaceous to Eocene batholiths in the eastern arc complex, a strong seismic reflector is observed at ~26 km depth. From this reflector to a small Moho root at ~37 km depth, a high velocity of 7.0 km/s indicates very mafic rocks. Similar high velocities in the lower crust are not observed in the accreted terranes that the arc intruded, nor are they observed ~200 km along strike in the arc complex. These anomalously high velocities may indicate the presence of magmatically