lithoprobe logo
home | contact | search about | transects | publications | links | classroom/media 
media > slideset > the Growth of Canada >

Subduction of the oceanic Juan de Fuca plate under Vancouver Island.

LITHOPROBE's studies in the Southern Cordillera have been taking place since 1984 when the project's Phase I provided seismic and electromagnetic images of the oceanic Juan de Fuca plate that is being subducted under Vancouver Island; and of overlying fragments of the plates that have been added to the collage of accreted lithospheric fragments in western Canada.
This slide incorporates later findings as well. But the first crustal section from Phase I, across Vancouver Island, has been interpreted to indicate that both tectonic erosion, wherein some of the older lower crust is stripped off and recycled into the mantle, and subduction underplating, wherein new and younger rocks are added to the continent from below, are important processes that must be considered in global models of continental evolution and growth.

Continuing research has brought important insights, much of which is summarized in the large cross section in our brochure folder, representing 1,100 km of crustal profiling, from the Alberta plains in the east to the Juan de Fuca ridge 200 km offshore in the Pacific Ocean.

Some of the findings in a nutshell:

Microcontinents advancing from the west collided with our continent and pushed thick sequences of rocks (which are up to 600 Ma old) several hundreds of kilometers east onto the western margin of the Alberta Basement (the North American craton), to form the Rocky Mountains. So there was no giant collision between two giant continents here, as we had seen in the creation of the Grenville Orogen. But it shows just how effective these pushes and shoves can be, even from microcontinents.

What was being shoved here had been deposited onto the western extension of the Alberta Basement over long times, including all the old and very old sedimentary sequences we find exposed in the Rocky Mountains today. How far did the shove go? Very far, perhaps, because there are indications that some of the fault planes accommodating this movement sole out by the Fraser River. The cross section shows the basement (mottled red) thinning and extending almost as far as the Fraser Fault, and the overlying deformed and metamorphosed sediments and other rocks (dark red) of pre-accretionary North America stretching to the fault itself.

The collisions, and accretions of exotic Mesozoic terranes in central British Columbia and the Coast Mountains, occurred 180 to 58 Ma ago, as we already have heard.

Then, the convergence stopped. Now, remove the lateral pressure on something as big and heavy as newly crunched-up mountains. Things will slide back to find a new equilibrium. Thus, compression gave way to tension, and large structures were a bit undone. Compressional arches were "unroofed", i.e. lost part of their covers as layers and blocks slid down on them.

previous page next image
home | about | transects | publications | links | classroom/media | search | contact