Sunday, April 20, 2008

Farallon Plate Part 2

Next in my hall o' fame of geoscience images is the Farallon Plate - North American Plate animtion put together by Tanya Atwater (UCSB). This is one of a series of excellent earth science animations available at this site. This cartoon continues last week's theme, the fate of the Farallon plate, and follows (in map view) the evolution of the western U.S. plate boundary from 38 Million years ago until present. I have long considered this to be one of the best and most useful geoscience illustrations ever produced.

I've seen this animation in a wide variety of talks and lectures. As someone who went to graduate school in California, every discussion of the San Andreas fault would begin with this movie. But I've also seen it used when discussing general plate tectonics, basin and range extension, or this history of magmatism in California.

The movie was made primarily using plate reconstructions. Basically you "unspread" the oceans, using the time and velocity constraints provided by sea-floor spreading anomalies to step things back in time. This is augmented and checked using terrestrial records of deformation. Although it is slightly simplified, it is a fantastic view of the development and evolution of the western U.S., especially California, and the development of the San Andreas Fault.

In the movie you will see a number of things. First off, the thick red lines are the plate boundaries. Separating the Pacific Plate from the Farallon Plate is a divergent plate boundary, where sea floor spreading is making new oceanic crust. The toothed red line separating the Farallon Plate from the North American Plate indicates a subduction plate boundary, where the Farallon Plate is being subducted underneath North America. This plate configuration begins at roughly the same time as the Humphreys figure I blogged about here. The big white arrows are plate velocity vectors relative to a stable north america. The thin black lines on North America are rough outlines of the state boundaries, prior to Basin and Range extension.

So at the beginning of the movie, the western plate boundary of North America is a subduction zone. Beginning in southern California (at about the latitude of Santa Barbara) at ~30 Ma, the divergent plate boundary (or spreading ridge) separating the Pacific and Farallon Plates is subducted. This results in in the formation of a strike slip boundary, the margin we now refer to as the San Andreas Fault.

As more of the spreading ridge is subducted, the transform San Andreas fault gets longer and longer. You also see the initiation of significant extension throughout western North America. These regions are colored a salmon color in the movie. This extension roughly doubles the present width of the western U.S. This extension leads to the formation of the Basin and Range province.

As the San Andreas continues to form, you can watch parts of southern California rotate (the transverse ranges) and/or be translated northwards. Finally, the movie finishes with the modern plate set up.

As simple as the cartoon now looks, it represents a great deal of effort to put together. And, if you are looking to discuss any geologic event in the western U.S. in the past 40 Ma or so, this cartoon provides a fantastic framework.

I've used this movie every time I've TA'd or taught a course, or given lectures to audiences unfamiliar with Cordilleran geology. The figure has the rare ability to talk to all levels of earth scientists. People with no background can really see the development of a very complex plate margin.

Now a quick disclaimer. The website that hosts these movies says they can be used for free for educational purposes. My blog is non-profit and I consider it to be primarily an educational endeavor, which is why I am posting the video. If anyone connected to the video does not appreciate that, please just email me.

Enjoy! And check out the site I reference for a whole boatload of geoscience movies.


Silver Fox said...

This is sincerely cool - thanks for posting it! And good description of How It Happens. Tanya Atwater has long been one of my heroes... (heroines). [I once had an Aussie shepherd named after her - reportedly T.Atwater got down on hands and knees and said "woof woof" in response to hearing about the dog's name.]

Anonymous said...

okay, this is totally off topic, but I was hoping you'd break this story: Famous (infamous) thermochronologist T. Mark Harrison was on the Comedy Central Show "Root of All Evil". The video is here:
It is part 3 (Kindler), with about 1:15 to go.

I found it quite therapeutic.

Anonymous said...

ooh ... I love that animation!