The Three Structural Styles
You have now seen, at least briefly, the three structural styles that are possible: pull-apart (Death Valley, spreading ridges); push-together (Crater Lake and Mt. St. Helens subduction, University Park and the Great Smokies obduction); and slide-past (the San Andreas Fault in California). Pull-apart behavior involves stretching of rocks until they break, forming pull-apart or gravity faults (after being pulled apart, gravity pulls one block down past the other). Pull-apart action occurs at the spreading centers, probably where the convection cells deeper in the mantle spread apart. Push-together behavior occurs at subduction and obduction zones, and produces squeeze-together folds and faults, with the faults also known as thrust faults. Slide-past boundaries, also called transform faults, occur where two large blocks of rock move past each other but not toward or away from each other. Slide-past motion produces earthquakes without mountain ranges.
Video: San Andreas Fault (3:16)
Here's a picture of perhaps the most famous slide-past fault in the world, the San Andreas Fault at the Carrizo Plain in California. We're going to look at these slide-past faults and we'll also look at what happens when one of them bends. So here's a map of it. And we're going to the Carrizo Plain National Monument, here. Notice the motion of the fault that's indicated. The fault is pretty easy to see if you look at a picture of the Carrizo Plain. There is the fault in yellow. The motion is shown in orange. You can see the motion in the geology. Here, for example, is Wallace Creek, coming in from the right. You might expect it to go straight, the way it once did, but what does it do? It's been deflected by the motion of the fault and it's carried up like that, and then leaks on out the other side. So the fault is moving. It can make earthquakes. People worry about that. But now, let's go back to the map and let's go down to the San Gabriel Mountains National Monument, and the big bend in the San Andreas fault. So down here the fault is not straight, it's bent, and because of the bend, this motion leads to a pushing together across the San Gabriel Mountains, at the Big Bend. And so we're gonna now see what that does by this next picture, looking across Los Angeles towards the mountains and the National Monument, and there it is, there's Los Angeles. Behind them is the mountains. The push together across the Big Bend has led to stresses that have pushed up the mountains. There are earthquakes associated with this. This is from the 1994 Northridge Earthquake. It was on a thrust fault down in the Basin closer to Los Angeles, but it's related to the squeeze of the big bend. If we go back to our map, imagine for a moment a fault that went the other way. Now what would happen is this, this motion would be carrying this side down, this side up, and you would have pull-apart going on in here. And, in fact, there is a little bit of that going on. Over here there is a pull-apart Basin that has a salty lake in it called the Salton Sea. It is linked to this kind of horizontal motion. This one's from the interaction of a couple of faults, it's a little more complicated, but it is possible to have pull-apart with a bend one way or to have push-together with the bend the other way. And so what do we see? Slide-past faults can make earthquakes, like the San Andreas, but when they bend they may cause push-together that helps to make mountains, or they may cause pull-apart that helps to make basins.
Now, you might imagine that we have oversimplified just a little. There is no law that rocks must move directly toward each other (push-together), exactly parallel to each other (slide-past), or directly away from each other (pull-apart); sometimes you see an oblique motion with rocks approaching on a diagonal. Or the rocks may pull apart on a diagonal. And, a bend in a slide-past boundary may produce pull-apart or push-together features, depending on which way the bend goes relative to the motion, as shown in the video above. A large bend in the San Andreas Fault just north of Los Angeles gives push-together motion, with some impressive mountain ranges and dangerous earthquakes.
Mountain ranges correspond directly to the main boundary types. Fault-block mountains—the Sierra Nevada, the Wasatch Range, the flanks of the great rift valleys of Africa, and the mid-oceanic spreading ridges—form at pull-apart margins. The mountains are high because the rocks beneath them, in the mantle, have expanded vertically because they are the hot upwelling limbs of convection cells. Volcanic-arc mountain ranges form over subducting slabs, where some of the downgoing material melts and is erupted to form stratovolcanoes; smaller ranges (such as the Coast Ranges of the Pacific Northwest, including Olympic National Park) may form from the sediments scraped off the downgoing slab just above the trench. Continent-continent or continent/island-arc obduction collisions occur at push-together convergent boundaries as well, producing folded and thrust-faulted mountain ranges.