Sweeping the oceans - suspect and exotic terranes

Content:

Reading: Introduction of Howell, Jones & Schermer, 1985, Tectonostratigraphic terranes of the Circum-Pacific Region, p. 1-9 (available at course Blackboard site).

Introduction

There is a significant tectonic history seen in most mountain belts that occurs before final ocean basin closure with the collision of two continents, and that can't be explained by simple subduction processes. The Appalachians are an excellent example (as captured in the two diagrams below). The final act of closing an ocean basin (as in the Wilson Cycle) if continental collision and suturing, but some important pre-closure tectonic 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 small country or state sized real estate embedded in oceanic crust that looks like it should have difficulty being subducted. 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 (it accretes). As you will see terrane behavior is much richer than this simplified description.

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.

This serial cross section diagram depicts the evolution in the New England Appalachians with the accretion of two terranes prior to the final collision with Africa. Before the recognition of the terrane accretion phenomena it was clear that the rocks were deformed by several deformation episodes. The Taconic was one of the earlier recognized such events, and it can be understood as the accretion of the Taconic terrane. A good questions to ask is - how do we 'know' this? Image source: http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm .

This serial cross section from the USGS site - http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html - shows some of the details of the earlier formation of the Taconic terrane and its accretion. Note how the accretion is basically a modification of the Wilson cycle. In this case, one can consider the idea of terrane accretion as a paradigm expansion, and not a replacement or revolution.


Terminology associated with terrane accretion

There is a rich terminology associated with the development of the terrane accretion concept, some of which is as follows:


How are suspect terranes recognized?

Rarely is one line of evidence convincing on its own, but the convergence of the types of evidence listed below help in establishing the existence of a terrane:

This is an example of a single-zircon detrital age plot from a sample of schist collected in Alaska taken from Bradley et al. 2009 (reference and link below). Since the schist had a sedimentary protolith the zircons are almost certainly detrital, and the ages reflect the time of igneous activity (and zircon crystallization) in the source material that was eroded to produce the sediment. Within a drainage-deposition basin system one would expect the age spectra (where the peaks are) to be similar, and differing terranes will likely have different spectra. One of broader results to recognize here is the total lack of PreCambrian ages, indicating that North American Precambrian basement did not serve as a source, consistent with this being unconnected to North America. The youngest ages constrain the depositional age of the schist. In a terrane with active volcanism in source areas the youngest zircon crystallization ages may overlap with the depositional age.


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 roughly 100 Ma (Triassic to Cretaceous) for the move from 2 to 12 degrees latitude (remember that the earth's radius is 6373 km). What is this motion relative to? Is this a minimum or maximum estimate of the actual velocity of terrane movement?

2) Look at Figure 3 or 5 from this paper. What can you conclude from this plot? Give it some thought.

Paleomagnetic pole database: http://www.ngu.no/geodynamics/gpmdb/.


Types of accreted terranes?

Diverse, including:

Pillow basalts from Cape Disappointment State Park coastal Washington. These extensive pillow basalt deposits are thought to be part of the Crescent terrane and are of oceanic affinity. In my limited experience these are the best pillow basalts I have ever seen, and the locality is well worth the visit.

Glassy rinds on some of the smaller pillows at Cape Disappointment.


Mechanics of terrane accretion/dispersal?

These are complex and include:

Plate tectonic diagram for Alaska. Note the Denali fault well within the interior. This is an active strike-slip fault (dextral), which is slicing through and dispersing accreted exotic terranes . Image is from the NOAA site: http://earthobservatory.nasa.gov/Features/denali/ , which has a lot more information on the Denali fault and the associated tectonics.


Examples of the terrane accretion

North American Cordillera and Alaskan terranes.

This map from the USGS web site http://pubs.usgs.gov/gip/dynamic/Pangaea.html , shows general types of terranes, some of the larger terranes, and the amount of material added to the North American continent through this terrane accretion. Note that Alaska is basically a terrane assemblage. In this view North America appears to have grown substantially in size by terrane accretion, but there is the question of the underlying lithosphere and the extent of thin-skinned tectonics.

USGS site with map of some of the West coast terranes.

Detailed map of Klamath terranes from USGS, with a good example of a tectono-stratigraphic diagram.

Terranes in the Appalachians.

Outcrop of Persimmon Fork metavolcanics along Stevens Creek in western South Carolina, which are part of the Carolina terrane.

Close-up view of the Persimmon Fork volcanic, where the feldspar phenocrysts in a still relatively aphanitic matrix are evident. Cameral lense for scale.

Images from Prince William Forest just south of Washington DC in Virginia and of Piedmont rocks in the core of the Appalachian mountains. These are highly deformed metavolcanics of the Chopawamsic terrane. These arc volcanics are of roughly Cambrian age, and are of a distinctly different affinity than those clearly part of the eastern North American continental edge at that time. In the lower image one can clearly see the pyroclastic character of these metavolcanics, with fattened volcanic clasts, possible of what was once a lahar


Are exotic terranes just microplates? Not quite - most apper to be detached from lithosphere(?), and they are not internally rigid.

Is this a process by which continental mass has grown with time, or are we just shuffling around earlier formed pieces?


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.