The Growth of Continents

This month's Accretionary Wedge topic is "What recent geological advance has been most important to your geological work?" I'm paraphrasing, but that's basically my understanding. I had been thinking about writing on paleomagnetism, but the roots of that sub-discipline go back to (if I remember correctly) Cox in 1956- please don't quote me on that: I'm trying to dredge up memories from 20 years ago. (Late Note: See this article excerpt- I have a little more confidence now. Note particularly, "Paleomagnetic results for the Mesozoic and early Tertiary might be explained more plausibly by a relatively rapidly changing magnetic field, with or without wandering of the rotational pole, than by large-scale continental drift.) So the early work in that area predates my birth. Furthermore, the results that have been most important and, yes, inspiring, to me all focus on a particular topic: accretionary tectonics, or as we used to call it when I was an undergrad, "flake tectonics."

Oregon is composed of a fairly large number of discrete terranes, or "flakes" that have distinct and different histories up to a certain point in time. After that point, the geologic history of a particular terrane and its neighbors are congruent and coherent.

As a brief aside, note that "terrane" refers to a particular tectonostratigraphic unit, while the word "terrain" refers to landscape.

So to choose an imaginary but typical example, picture an island arc volcanic system. In the above cartoon, subduction is under the arc to the left, and is consuming oceanic plate between the arc and the continent to the right. Sediments (in tan/greenish?) are accumulating in the basin between the continent and the arc.

The volcanic arc continues to approach the continent, and the basin accumulates more sediment.

As the arc approaches the continent, the sediments continue to accumulate; thrusting and folding start to thicken the sequence. Ultimately, when the oceanic crust is consumed, subduction ceases.
The overlying sediments are severely deformed; while not of the same magnitude as the India/Asia continental collision, the processes are similar. The now accreted island arc and the continent to which it is attached will henceforth share the same sedimentary and tectonic history. In a practical sense, that means events and sediments can be correlated between the blocks. And in reference to my first idea for a topic, they will share similar (basically identical) paleomagnetic histories. Often, subduction will resume on the outboard of the newly accreted block in an opposite direction. This has been called a "polarity reversal," but with a very different meaning from a magnetic polarity reversal.

Geologists will immediately see that I have grossly simplified the diagrams above. For example, the magma rising in the subduction zones that does not reach the surface will crystallize to granitic rocks. Much of the sediment and volcanic rock will be metamorphosed. Faulting and folding will jumble things terribly. A further complication is that an accreted terrane may itself be composed of two or more terranes with independent histories up to a certain point, but share discernible spans of history prior to continental accretion. This is the case in terranes of the Klamath Mountains of southwestern Oregon and northern California. Back arc spreading following a polarity reversal in a subduction system (as happens between the second and fourth cartoons above) can emplace ophiolites- again, this can be seen in the Klamaths.The above diagram shows schematically the kinds of terranes that have formed much of western North America. The article in which I found it has a larger version, and discusses some of these ideas with respect to the Klamaths.

I had the privilege to take a field trip to the Snake River Canyon area with Tracy Vallier toward the end of my undergrad years; the geology there is somewhat less complex than the Klamaths (at least it seems that way to me), but is still best described as "a mess." This article focuses primarily on Idaho, but provides plenty of insight into northwestern Oregon as well.

The accreted terrain with which I am most familiar was referred to as "Siletzia." I say "was" because I haven't really followed the professional literature; I'm not certain that it still is. Actually, quick check, it does look as if it still is- see here and here. This block extends from approximately Coos Bay, Oregon northward to the southern end of Vancouver Island, British Columbia. The eastern margin, as far as I know, has not been tightly constrained, but probably lies along the eastern edge of the Cascade forearc basin, or along the western edge of the Cascade arc; in either case it is buried in basin sediments and older Cascade volcanics. At a first level of approximation, it consists of oceanic crust, apparently thickened by hot-spot volcanism, overlain by a thick sequence of turbidites (the Tyee Formation, correlates, and similar units of slightly different ages), then a shallowing sequence of marine sediments. During the time I was getting my degree, extensive research into the provenance of these sediments, along with paleomagnetic analysis of units of varying ages, showed that the history of this block was far more complex than the essentially simple sequence of rocks would suggest. Paleomag showed that the oldest rocks were oriented about 90 degrees counterclockwise from their modern orientations- that is, this block, approximately 150-200 km by 700-800 km, originally had its long axis in an east-west orientation. It now has its long axis in a north-south orientation. Progressively younger rock units show progressively less deviation from the modern orientation. Provenance analyses of the sediments suggest that the bulk of the Tyee turbidites were derived from the Idaho Batholith or chemically and isotopically similar granitic rocks, since hidden, further south. Given the age of the Tyee (Eocene), and hypothesized drainages in the northwest, it has been suggested that the drainage from the Green River Basin (home of the marvelous fossil fishes, and an enormous amount of petroleum reserves in the form of oil shale) was the stream that carried these sediments to the head of the Tyee fan.

When I was doing some volunteer teaching for OSU's experimental college, the way I explained this to (non-science) students was that the rock sequence is easy to describe and understand, but if you want to describe where the rocks came from and how they got where they are, it gets very messy very fast.

So even though I don't consider myself a professional geologist, and can't claim that there's any particular breakthrough or advance that has had great implact on my career, my avocation for the last few decades has been to try to make sense out of Oregon geology. When I started my education, tectonics was used primarily to describe the cause of the Cascade arc. By the time I finished my education, accretionary tectonics was considered fundamental to making any sense at all out of Oregon's geologic history.