When Continents Collide

Text by L. Whalen

Models by P. Prince

 

 

Model Mountain Range Bivergent Wedge

We call the Appalachian mountains home and understanding our local geology can be challenging to students. How do you explain the juxtaposition of different rock types within the orogen? What tectonic processes are responsible?

There are several distinct geologic provinces in Virginia, which result from different rock records and different events shaping their development.

physall

Image: James Madison University (http://csmres.jmu.edu)

In this video we see the addition of a smaller landmass, like an island arc or a micro-continent of Ordovician or Devonian age getting sandwiched between North America and Africa during the Alleghanian orogeny (of Carboniferous age). This represents the final step in the formation of the Appalachian mountains and the supercontinent of Pangea. Two previous such events occurred called the Taconic and Acadian orogenies. These occurred during the Mid to Late Ordovician (435 – 350 million years ago) and during the Mid to Late Devonian (350-370 million years ago) respectively.

In this model we have the two colliding landmasses (with the island arc caught up in-between) plowing into each other by having the rigid underlying crust underthrusting or pushing under the sediments that make up the oceanic basin in-between. The goal of this setup is to illustrate that the particular parts of the continental margin cover sequence, or sediments can still be identified in the resulting mountains.

Anticline_(PSF)-vector.svg.png

                   An anticlinorium is a large anticline with with superimposed smaller folds.

Image http://www.wikipedia.org

By using Adobe Illustrator to “erosionally” remove a significant portion of the resulting mountains, students can learn to visualize what the landmass look liked prior to erosion and can begin to think about how different rock types end up close together. In the model, rocks that made up the collided island arc or micro-continent becomes the steeply rotated and intensely deformed core of the mountain range (erosion later reduces it to today’s Appalachian Piedmont), the thicker, continental margin/deep ocean units become the metamorphosed Blue Ridge mega-anticlinorium, and the very thin sedimentary sequence overlying the underthrusting “continent” becomes the Valley and Ridge fold-thrust belt. Each of these parts, owing to its thickness and position within the wedge, has an obvious deformational history, and this history can be seen in hand specimens from the respective provinces.

Stategeo

Image: James Madison University (http://csmres.jmu.edu)

This model is an effective illustrator of basic collisional origin architecture, but it is only illustrative! MUCH more shortening, along with erosion loss and exhumation, would be necessary to increase the realism of the model. Multiple decollements, or planes where the rock glides past each other, would also be necessary throughout all parts of the model. The most obvious disclaimer when using this model with Appalachian students is that it has not been extended to produce an Atlantic Ocean basin. I just couldn’t bring myself to extend and further erode this one; it just looked so nice! Were realistic extension applied, the Piedmont and hinterland edge of the Blue Ridge would be significantly affected by normal faulting and basin development, and even with deep modern-day erosion, some of the basin fill would still be intact. This rifting process is a major control over Piedmont and Coastal Plain topography; although the Piedmont represents the frequently anatectic roots of the Appalachian wedge, extension and thinning reduce it to a very low elevation surface today. The crystalline rocks of the Piedmont are eventually covered by Tertiary Coastal Plain sediments towards the Atlantic margin as the thinning effects of Mesozoic extension become more pronounced.

hatcher scan (1)

Image from: Hatcher, R.D., Jr., 1990. Structural Geology: Principles, Concepts and Problems, 1990, Charles E. Merrill-Macmillan, publisher .

 

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