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

Controlled-Source Seismic Investigation of the Generation and Collapse of a Batholith Complex, Coast Mountains, Western Canada Preliminary seismic velocity model from the BATHOLITHS controlled-source seismic survey. Lower crust and Moho structure is still being modeled, and may change from this figure. The Stikine terrane to the east of the arc extends from model km 0 to 210. The Jurassic-Cretaceous arc lies west of the Coast Shear Zone at about model km 320. The youngest arc (Cretaceous-Eocene) and highest mountains are from model km 210 to 320. The curved surface of the model represents the spherical Earth.
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In 2009, the BATHOLITHS project acquired a 400-km long refraction and wide-angle reflection seismic survey across the Coast Mountains of British Columbia, Canada, a Jurassic to Eocene continental arc batholith complex.
</p><p>Granitic batholiths created by magmatic differentiation in arcs above subduction zones make continental crust more felsic than the original materials derived from the mantle. However, differentiation from a mafic protolith results in a large volume of ultramafic residual that is petrologically part of the crust, not changing the bulk composition. This residue may reside hidden beneath the geophysical Moho, or it may have delaminated to sink into the mantle due to its relative density. Delamination may occur during subduction or during the collapse of the arc after subduction stops, and may coincide with a commonly observed late pulse of magmatism. As part of the BATHOLITHS multi-disciplinary investigation of these processes, traveltimes from the seismic survey are being used to build a 2-D P-wave velocity model of the crust.
</p><p>East of the batholith complex, surface Mesozoic sedimentary and volcanic rocks are indicated by velocities of 4-5 km/s to 2-5 km depth. Beneath this basin, the Stikine terrane, an accreted late Paleozoic to early Mesozoic island arc, has felsic seismic velocities of 5.8-6.2 km/s to at least 15 km depth. A seismic reflector is observed at ~20 km depth beneath Stikinia but does not extend into the arc complex. Based on wide-angle reflections, the lower crust has a velocity of ~6.8 km/s under Stikinia, indicating mafic rocks. The Moho is at 35-38 km depth and the upper mantle has a fast velocity of ~8.1 km/s under Stikinia.
</p><p>To the west in the continental arc complex, velocities of 5.6-6.2 km/s indicate granitic rocks in the upper crust to at least 15 km depth. A strong seismic reflector is observed at ~27 km depth only beneath the highest mountains and youngest batholiths in the eastern part of the batholith complex.
</p><p>Wide-angle reflections indicate a velocity <6.6 km/s above the 27-km reflector and above the Moho in the western arc complex, indicating a felsic to intermediate composition. The Moho is at 30-33 km depth under the western Coast Mountains and dips eastward to maximum of 38-40 km beneath the highest mountains. The velocity between the 27-km reflector and the deep- est Moho is >6.8 km/s, but the data are currently being analyzed to better constrain this number and search for magmatic residual. The upper mantle has a slow velocity of ~7.9 km/s under the arc complex.
</p><p>Acknowledgements: The BATHOLITHS controlled-source seismic project is funded by an NSF grant from the Continental Dynamics Program and by a Canadian NSERC grant.</p>

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