Sweeping the oceans - suspect and exotic terranes.

Reading in text: p. 198-208

There is a tectonic history seen in most mountain belts that occurs before final ocean basin closure and the collision of two continents. Subduction closes oceanic basins, but this pre-closure history can't be accounted for by simple subduction tectonics as discussed in this class so far. Something else was occurring at times during the oceanic basin closure - the accretion of terranes. There is a uniformitarian component to this idea, in that when we look at present oceanic basins there is a striking amount of country sized-real estate embedded in oceanic crust that looks like it should have difficulty going back down below during subduction. What happens to this real estate when it encounters a subduction zone? The basic idea is that it gets plastered on to the edge of the overriding plate. As you will see the behavior is much richer than this.

The idea of terrane accretion is another distinct paradigm revolution/expansion in the growth of plate tectonic theory. Sometimes this has been referred to as collage tectonics, which seems to fit pretty well to me.

Terminology:

How are suspect terranes recognized?


Exercise: Assessing the paleolatitude history of some Alaskan terranes. Take the data table and simple map from Panuska & Stone, 1985 and use it to answer the following questions.

1) Take the paleolatitude for the Wrangellia terrane and compute a relative velocity in cms/yr if the span of time is 70 Ma for the move from 2 to 15 degrees latitude. What is this motion relative to? Is this a minimum or maximum estimate of the actual velocity of terrane movement?

2) Plot the paleolatitude position versus time for the Wrangellia, Alexander and Peninsular terranes? Use the midpoints of the geologic periods given. You can use Excel to do this. What can you conclude from this plot? Give it some thought.

3) What additional information could be added to the above graph that would help understand the terrane history?


Types of lithotectonic elements and accreted terranes?

Mechanics of accretion/dispersal?


Examples of the terrane approach.


Are these just microplates? Not quite - detached from lithosphere, not internally rigid.

Is this a process by which continental mass has grown with time?


References:


Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.