REFLECTIONS ON MY 60 YEARS IN THE COAST MOUNTAINS OF BRITISH COLUMBIA

Following discovery of Early Tertiary granulite facies rocks in the core of the Coast Mts, the question was posed: was this the “bottom of the batholith”? Here’s some of what we learned:

Rapid, near isothermal decompression of metamorphic rock occurred in the area of the granulite facies occurrence. This is documented by the metamorphic mineral assemblages, and the cooling dates.

A small (<5%) amount of melting sufficiently lowers the strength of rock to affect large scale deformation, including where the rapid decompression was recorded. Relatively large packets of melt flow from the lower crust and into crustal scale “pressure shadows” during extension.

Early stages in the development of the batholith occurred at crustal depths of 20 or more km. The Al in hornblende barometer for granitic plutons was calibrated in the Coast Mts. The progression of time of emplacement of the plutons from west to east correlates with depth of emplacement, beginning from pressures in the west high enough for epidote to be stable on the liquidus.

The Coast shear zone can be traced for over 1200 km along the western boundary of Tertiary aged plutons in the Coast Mts. It is a candidate for the proposed Baja BC fault. Paleomagnetic evidence calls for over 1000 km of dextral displacement across it during the early Tertiary. However, the kinematic indicators along the shear zone show east side up, only. This is consistent with the near isothermal uplift of the metamorphic rocks east of the Coast shear zone.

We used a seismic ship to image the crust along a 500 km section perpendicular to the trend of the Coast Mts. The project was called ACCRETE. At the Moho, directly below the surface exposure of the Coast shear zone, there is an apparent offset of about 5 km, with the west side shallower than the east side.

Surprisingly, the seismic results showed that the crust under the Coast Mts. batholith east of the Cast shear zone is now only 30-32 km thick. This is much thinner than the 50 or so km expected at the core of an orogenic belt.

Another major surprise from the seismic study was that the compressional waves generated by the airguns in water produced shear waves at the sediment-bedrock interface at the bottom of the fjords. The combined Vp/Vs data gave us crude petrology of the entire crust for the 500km length of our seismic line.