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The Growing West

Map of the Canadian portion of the North American Cordillera that indicates the collage of accreted terranes.

LITHOPROBE's intensive lithospheric study of the western North American Cordillera, from the eastern edge of the Rocky Mountains to the offshore west of Vancouver Island, was the first encompassing, multidisciplinary investigation of such a mountain system. Not surprisingly then, the Canadian portion of the Cordillera has been the birthplace of many new concepts in global tectonics.
An 1,100-km long cross section of these findings is shown in the brochure folder. By the way, it is true to scale, i.e. no vertical exaggeration was used. You are looking at a scientific first here, at a summation of tectonic and structural insights which is benefiting research into other mountain systems, on this and other continents, and with regard to features formed recently or farther back in the geological past.

For example, geological and geophysical studies in southwestern Canada, particularly over the past 25 years, have established that the lithosphere of the Cordillera has evolved through episodes of rifting, sea-floor spreading, and plate separation, followed by subduction, ocean-basin closing, and plate accretion. Of course, you do remember all of these concepts, don't you? We just discussed rifting and sea-floor spreading when we heard about the Iapetus and Atlantic Oceans. Plate separation is a bit hazy, in that this can involve more than mere separation by rifting, but also movement of one plate with respect to the other along strike (of the separating feature, generally a giant fault, such as the Queen Charlotte Fault just west of the Queen Charlotte Islands; this is Canada’s equivalent of the San Andreas fault). Subduction, like that of the Juan de Fuca oceanic plate under Vancouver Island, we have got down pat. Ocean-basin closing is a clear case, and plate accretion we also have discussed, but will hear more about now.

The concept of accretion of far-travelled crustal blocks (terranes) that comprise plate fragments, island arcs, or microcontinents, was developed partly in the Canadian Cordillera. It now is used to interpret geological relationships observed in many other orogens of the world.

Again, let's not be frightened by words -- all they do is describe things and happenings. When you put two mud pies together you accrete them side by side. Depending on how hard you press them together, and what happens to the twosome thereafter, they may stick together. Island arcs? Think of the Aleutians, Indonesia or the Philippines; or, in a different setting altogether, the Hawaiian islands. You might say it's a loose, but still descriptive, term. What else have we got here? Oh, yeah, there are microcontinents; well, small continental plates, which we have discussed many times by now.

So, let's stir up these things and mix a tectonic cocktail of the real world.

Well, let's not opine too early here. It so is clearer than mud. Consider, how these various patterns and colours (for the different accreted terranes) do provide some degree of clarity. Geologists and geophysicists generally start out with a dog's breakfast, sorting things out by rock type, position, age, and so forth. In any case, it's a descriptive map of what, seemingly, was a less than orderly collage. Nature can be that way. On the other hand, consider how orderly the sorting of your beach sand is, for instance! And this collage also follows or indicates certain patterns; one just has to know the why and how.

At any rate, take in the big picture, the colours and borders. The big, dark red is the North American craton onto which (from the west) the Rocky Mountains, in light red, were shoved by plate tectonics. The borderline between the two designates the eastern edge of the thrust fault plane on which the outer mountains were carried onto the undisturbed craton. (And there are more such fault planes farther back into and throughout the Rockies, making the mountains form a stack of imbricated layers of rocks of different types and ages.)

Then, we have white and light green colours. The white is on land -- comprising the Intermontane composite terrane, the green consists of islands and part of the western edge of British Columbia, which is the Insular composite terrane. Clear as mud pies! Which refers to the adjective "composite." All this word is meant to say (here) is that these terranes comprise accreted material which originated from near the west coast as well as from, presumably, much farther away, where they formed on oceanic crust and whence they were carried to the west coast.

So, let's define our map some more. The old Precambrian craton is a clear case, so are the Rocky Mountains. Next to the west, the uncoloured portion is, like the green belt, a mix of accreted terranes. Curious scientists (who, in private, must be good at working Rube's cube) have found out which accreted terranes were formed near the west coast and marked them with a star (sorry, only on the legend, to keep the map demuddified).

Our continent gained, and grew westward, by the accretion of new terranes from near and far. There also was erosion and sedimentation of course, just as it is happening now, and there were volcanoes. The sediments also were added on as they reached the shore lines (having been carried their mostly by rivers) and were dumped in between whatever else was coming along as accreted terranes. Look at the Fraser River's sedimentary fan when you fly over it next time.

Researchers have figured out that since the Precambrian, the continent grew westward by about 500 km, adding a day's drive for land lubbers on their annual summer run to the ocean beaches. That addition works out to a little less than 1 km for each one million years. But this growth was not a dull, even-measured affair; it came in spurts and periods. Much of the accretion took place during the interval 180 Ma to 58 Ma, or from the Jurassic to the Lower Tertiary, which belong to the Mesozoic age (which comprises the Triassic, Jurassic, and Tertiary, from oldest to youngest). We remember that this accretion, and the subduction of a moving oceanic plate which underlies these accretions, is continuing today.

Indeed, an important.htmlect of the Cordillera is that it is evolving in much the same way today as it has for the last 200 Ma, thereby providing an "actualistic model" (has a nice ring to it, `like, where it's at' in the vernacular), anyway, providing an ongoing model that can be integrated directly with earlier processes and deformation.

LITHOPROBE's area of study or "transect" is marked on the map by the box. This Southern Cordillera Transect gives us an outstanding opportunity to look into the processes that were responsible for westward growth, the processes that influenced the crust and lithosphere prior to the accretion of terranes, and the processes that were important in the subsequent modification of the Cordillera. This modification involves another, or several, subsequent process(es), such as lithospheric "extension" as observed in the Intermontane composite terranes (about which more later).

The main tenor of LITHOPROBE's study of the Southern Cordillera is to carefully integrate all the various applicable earth-science disciplines and methods so that they all can advance, and check upon, each other. This integration, LITHOPROBE's multidisciplinary approach, includes seismic reflection, seismic refraction, and electromagnetic images of deep crustal and lithospheric structure. Hand in hand with this subsurface approach goes the work by geologists at the surface, understanding more about the geology and geochemistry of near-surface rocks, including the paleomagnetism. Then there are the gravity and magnetic fields which are measured over the region. We’ll learn more about all of these shortly. All of this contributes to our understanding of how the westward growth of the continent took place, in which way and by what specific events. This knowledge, in turn, then can be applied elsewhere.

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