Those are the "antiforms" shown on
the cross section, like the Nicola and Vernon antiforms. These subsurface
structures are mighty high, with a covered relief of up to 25 km
and a lateral extent of 100 km.
Throughout, large strike-slip faults (like the San Andreas fault
in California), sliced up the crust and carried parts of it as far
north as Alaska. The most recent one is the Queen Charlotte fault
(mentioned earlier) but older ones sliced much of British Columbia
and the Yukon, emphasizing the north-south belts of the Cordillera
as seen on a previous slide (#53).
The long crustal cross-section shown in the brochure folder cuts
across the belts and extends westward across the subduction zone
and out to the Juan de Fuca ridge where, today, submarine outpourings
of basalt magma continue to form new lithosphere of the Pacific
plate to the west and the Juan de Fuca plate to the east. Here are
the major points again:
The cross section chronicles a voyage of discovery, from Alberta
to west of Vancouver Island, along an 1,100-km-long, deep cut through
stacked, twisted, displaced, reworked and remolded rocks and formations.
The Canadian Shield (mottled red), i.e. our old Precambrian craton,
doesn't end with the Alberta Basement. We can follow the basement
right under the mountains and on westward at least to below Okanagan
Lake.
In central British Columbia, between Kootenay Lake and the Fraser
River, the features showing the most vertical relief lie buried:
they occur as a series of huge underground arches, up to 25 km high.
Mt. Everest could easily fit inside them.
Geophysicists subdivide our lithosphere into an upper and a lower
part, crust and mantle (continental mantle is shown in purple; oceanic
mantle in grey), separated by the Mohorovicic discontinuity or Moho,
below which seismic responses differ from those above it. Observe
how steady the Moho is! Given the changing surface topography from
the Rockies to the Strait of Georgia, one also may have expected
some changes in the base of the crust, which is the Moho. Not so!
The Moho is remarkably flat. This is a fundamental discovery. It
means that mountain belts, resulting from collisions, were formed
above this flat base, perhaps as a rug wrinkles when pushed about
a flat floor.
The explanation is that the rocks below the Moho possess qualities
which differ from those above, including a higher density and great
strength. And just try to imagine the forces which did the wrinkling
above the Moho, and stacked up the Rockies!.
Under the west coast, our seismic x-rays have revealed a complex,
geological structure beneath the Coast Mountains. It is an intricate
pattern representing older shingling and wedging rock plates which
were imbricated when continental and oceanic plates collided and
also caused the ascent of magma from frictional and other heating,
forming all the granite rocks of the Coast Mountains and, of course,
volcanoes such as Mt. Garibaldi.
Beneath Vancouver Island and offshore, the collision continues
today. We can "see" from our remote images how the lithosphere
of the Juan de Fuca plate, moving eastward, is subducting beneath
the North American plate, which is moving westward. We have been
able to follow the subducted plate deep (70 km) beneath the mainland
on the east side of the Strait of Georgia. We also know from chemical
studies of the volcanic rocks that they formed from parts of this
subducted plate, melted at depths of 100 km and more.
Well, there we are. This completes our sweeping investigation of
the continent and its growth, down to its very roots and back to
its beginnings. We have travelled through longer than four billion
years in just, well how many hours?
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