GEOPHYSICAL, GEOCHEMICAL, AND GEOLOGICAL CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE COAST PLUTONIC COMPLEX, BRITISH COLUMBIA BASED ON THE BATHOLITHS CONTINENTAL DYNAMICS PROJECT
Isotopic data and geochemistry of the Cretaceous to Eocene plutonic rocks to the east of the CSZ are consistent with melting of an amphibolite source rock. The Wrangellia composite terrane west of the Coast shear zone has geophysical properties indicating a mafic lower crust. This mafic lower crustal material was thrust beneath the eastern part of the batholith by transpression across the CSZ.
This interpretation is supported by the geophysical observation of an increase in crustal thickness across the CSZ. However, the magnitude of the change in crustal thickness varies along strike. From the Alaska boarder to at least Douglas Channel 150 km to the south, crust west of the CSZ is less than 30 km in thickness, whereas crust east of the CSZ averages 32 km in thickness. At Bella Coola, 180 km south of Douglas Channel, the crustal thickness difference is much more pronounced; the Moho as determined by wide-angle refraction data is as deep as 45 km east of the CSZ.
This change in crustal thickness correlates with degree of exhumation as recorded by metamorphic host rocks. Granulite facies metamorphic rocks that host the batholith record pressures as high as 8-10 kbar north of approximately 53 o N. South of this, the degree of exhumation is much less, with most rocks in the greenschist facies. We have documented extensional shear zones that bound the eastern margin of the CPC. These shear zones have not been identified south of Kitlope lake where the metamorphic grade is lower. We infer that the region comprising thinner crust and exhumed high-grade rocks coincides with an eclogitic crustal root produced during batholith generation, which then delaminated from the lower crust. The less exhumed portion, with thicker crust, represents an undelaminated section of the batholith. The exhumed portion of the batholith is the by product of crustal over-thickening, followed by crustal delamination, and large magnitude extension. These processes record formation and stabilization of the continental crust.