The geological history of western North
America has been, and continues to be, shaped by its position on the eastern rim of the Pacific Ocean. The
modern Pacific Ocean’s basin is the successor of the original ocean which split Laurentia - our
continent’s cratonic core - away from the rest of the Precambrian supercontinent Rodinia, an ocean that
widened until in late Paleozoic time, it became Panthalassa, the World Ocean. Unlike the eastern side of
the continent, where continental collision was followed by re-opening of the Atlantic Ocean - the “Wilson
cycle” - western North America
has always faced the same active ocean basin. Its tectonic evolution has always been that of an active
margin, affected first by multi-episodic rifting, and then by plate-margin subduction and transcurrent
faulting, over a 700 million year period of time. Throughout this long interval, fluctuating regimes
determined by relative plate vectors have created the complex and varied geology and topography of the
region; its thrust belts, volcanoes, and granite canyons; its scarps, plateaus, and cordilleras.
The geological history of this area can be viewed as four distinct
plate-tectonic phases. The Rifting/Open Margin phase lasted from initial breakup at about 700 Ma (late
Proterozoic) until 400 Ma (Middle Devonian), when widespread subduction began along the margin. The Oceans
and Islands phase commenced as subduction built arcs and the continent retreated away from them, creating
a scenario like the modern southeastern Pacific ocean. This phase lasted until about 180 Ma (Early
Jurassic), when opening of the Atlantic ocean reversed the motion of North America such that it drove
strongly westward relative to the long-standing subduction zones off its west coast, creating a broad zone
of compression in the offshore arcs and ocean basins as well as its own miogeocline (Collisional/Orogenic
phase). The final, Post-Collisional phase commenced during the Early Tertiary when parts of the East
Pacific Rise subducted under the continent and turned the plate margin from pure subduction to a regime
with both transcurrent faulting and continued subduction of short, remnant segments. Industrial mineral
deposits formed during each of these tectonic phases. Combined effects of two or more phases were required
to form some of these deposits.
The Cordilleran miogeocline developed during the Rifting/Open Margin
phase, from its inception with deposition of the late Proterozoic, syn-rifting Windermere Supergroup,
through the deposition of the thick sequences of Paleozoic-early Mesozoic carbonate and terrigenous
siliciclastic strata that are now beautifully exposed in the Canadian Rocky Mountains. The Mt. Brussilof
magnesite deposit is hosted by Middle Cambrian carbonate within this continental shelf sequence.
Equivalent platformal strata are best exposed in the southwestern United States, the Grand Canyon being a
world-renowned example. The opening of Panthalassa was not a single event in western North America:
convincing Cambrian as well as late Proterozoic rift-related sequences occur, and alkalic to ocean-floor
basalts in the miogeocline range through Ordovician into Devonian age. The implied protracted nature of
this rift event, in contrast to the short-lived and efficient opening of the Atlantic Ocean, continues to
puzzle. The exact identity of the missing twin continent or continents, also provides grounds for lively
debate, with Australia, Australia/Antarctica, and Siberia attracting the most adherents.
The Oceans and Islands phase began in Devonian-Mississippian time
with the first widespread arc volcanism and plutonism along the continent margin: these rocks are
recognized from southern California to Alaska. West of the North American miogeocline, a large portion of
California, Oregon, Washington, British Columbia, and most of Alaska are made up of rocks of intra-oceanic
island arc to oceanic affinity that occur in relatively coherent packages separated from each other by
faults. These assemblages, famously termed a collage of “suspect terranes” by Peter Coney and Jim Monger,
had uncertain relationships to the North American continent during at least part of their history. Most of
them have now been shown either to contain faunas of eastern Pacific affinity (an excellent example can be
viewed at the Lafarge limestone quarry near Kamloops, B.C.), or to exhibit sedimentological, geochemical
and/or historical aspects that link them, however distally, to the continent. Some, however, are more
convincingly exotic imports: the Cache Creek Terrane of central British Columbia with its Tethyan,
Japanese-Chinese, late Permian fusulinid fauna; Wrangellia and Alexander, a linear belt on the coasts of
B.C. and southeastern Alaska, with its late Paleozoic cold-water, Baltic-affinity fauna; and fragments of
continental crust in Alaska with Precambrian ages that are unknown in North America.
At present, the Pacific Ocean is highly asymmetric, its west side festooned with
island arcs, its east side bare, bordering a continental margin made up of fragments of just such arcs and
marginal oceans. It is reasonable to suppose that both sides of the Pacific were once mirror images.
However, opening of the northern Atlantic Ocean at about 180 Ma destroyed that symmetry. Although
earlier compressional events affected the suspect terrances, their thrusting on top of the North American
miogeocline dates from the latest Early Jurassic, roughly 183 Ma - the same age as early rift basalts on
the eastern seaboard. From that time until the end of the Mesozoic, both suspect terranes and sedimentary strata of the miogeocline
were stacked into a complex but overall easterly-tapering thrust wedge. Oceanic terranes incorporated into
the wedge have provided both asbestos (Cassiar Mine) and jade deposits. During this Collisional/Orogenic
phase, successive Jurassic and Cretaceous magmatic arcs draped across the growing accretionary collage.
Numerous dimension stone quarries exploit granites from this phase. The notably voluminous mid-Cretaceous
arc was probably linked to an episode of rapid subduction around the entire northern Pacific Rim. Broad
plutonic provinces of this age occur from China, through Russia, into Alaska and British Columbia and
south into California and Mexico - a spectacular example of the global geological consequences of relative
plate motion.