Pull-Apart Faults
Fault Types
You can push things together, pull them apart, slide them past each other - or some combination of pushing or pulling while sliding. Nature does the same, giving different types of faults, which are found in different geological settings. Here’s a quick look.
Video: Fault Type Animation (3:46)
There's three major fault types that correspond to the three major plate boundary types. And we're going to look at these. They're push-together, pull-apart, and slide-past. And so right now, we're looking at a block of rock. And this block of rock is being squeezed by great tectonic stresses that are pushing on it from the sides. And this block of rock happens to be the place that you decided to build your giant, multi-million dollar McMansion that's just sitting up there on a hill.
Underneath your beautiful McMansion, there is an interesting yellow layer of rock, which you can follow. Now, we hope that you were smart enough, when you built your McMansion, that you didn't fail to account for the earthquakes that happen in the place that you built it, because otherwise, you're going to be an unhappy camper. Because when the earthquake happens, you shove one side above the other in the push-together environment, and then your house tends to fall down, and then you'd better know your insurance agent very well. And the layer of rock will be offset in the earthquake. So that is a push-together plate boundary.
We also know that there are pull-apart plate boundaries that are going to do a different motion pattern. There is a block of rock again. You have gotten the insurance settlement on your McMansion, and you have built a new one, which is sitting up here on the hill. But uh-oh, you didn't check with your geologist friends, or you didn't remember your GEOSC 10, and so now you have a fault underneath you.
But it's a slightly different fault in this case, in the sense that rather than being pushed together, now this one is being pulled apart. If you have a pull-apart fault going on, then what you will find is that sometime later, you're in a Death Valley situation. The earthquake happens. It drops the valley relative to the mountain, and you get an offset that looks something like this.
Your yellow layer of rock is still there. It has still been offset. And again, if you haven't been careful in your construction, or you haven't worried too much about things, your house has fallen down and your insurance agent would be a good phone call.
Now, there's a third possibility. Suppose that you've now gone broke because you didn't have good insurance. You're flying over the San Andreas Fault, and as you look down at the San Andreas Fault, you see the highway that's crossing the fault, and you're very pleased to see your new gas station where you're working because you have to raise money somehow. And in the middle of your highway, there are these beautiful, really fat dashed yellow lines that you see.
Now, unfortunately, if you're not careful, you may have problems yet again, because the San Andreas Fault has motion going on. And so you come back sometime later and the road has been offset, and the offset of the road is not a good thing for you. It came about because of the tendency for the west side of California to move north and the east side of California to move south.
And when it did what it was doing, the cars come driving along. And if a car comes driving along this particular highway and isn't careful what the car is doing, why, the car will run over the rubble that is left of your gas station after the earthquake. And so these are the three possibilities, the push together, the pull apart, and the slide past earthquakes corresponding to those plate boundaries.
The great depth of Death Valley was not carved by a river, glacier, or wind and is not from a cave, like Mammoth Cave, collapsing. Instead, as shown in the diagram below, labeled Pull-apart fault at Death Valley (which is more-or-less what you would see if you could take a slice across the valley from the air at the top down into the Earth at the bottom), Death Valley has been dropped along faults as the region has been pulled apart. Similar down-dropped and upraised blocks extend across much of Utah, Nevada, and neighboring areas, forming the Basin and Range region. A little bit of such faulting even extends up into Wyoming, and the great front of the Teton Range is geologically a close cousin to the west side of Death Valley.
The geologic evidence of faulting is clear—layers of rock and even recent stream deposits are offset across the faults, as you'll see in a VTrip at the end of this module. The faulting occurs in fits and starts rather than smoothly, producing earthquakes. The Basin and Range regions remain active in producing earthquakes. (More about earthquakes is coming soon.) (If it bothers you that the faults in the diagram would cross at great depth, congratulations. This bothered early geologists, too. Most commonly for really big features such as Death Valley, one of the two faults stops at greater depth, and the other continues on but becomes flatter and becomes a nearly horizontal fault. For smaller faults, both may stop before intersecting. This isn’t a course in the details of structural geology, but if you’re really interested, the USGS and Wikipedia have a lot of information on faults.)
Because of the way that the faults are angled, the dropping of blocks requires that the region is getting wider! (Look again at the diagram above, labeled Pull-apart fault at Death Valley.) It is possible to measure this widening directly using GPS or very-long-baseline-interferometry techniques. The widening is not fast—maybe an inch or two (a few centimeters) per year across the whole Basin and Range—but the widening is occurring.
Go south from Death Valley and you will splash into the Gulf of California. The geologic record shows that all of Baja California was attached to the main part of Mexico, but has been moved out to create the Gulf, and is still moving out. Eventually, the Gulf may extend up into Death Valley as the spreading continues and the west “unzips,” although the unzipping may stop before the whole west is opened up (see figure below — Pull-apart fault in the Gulf of California).
Next, if you take a boat across the Gulf of California with a depth-finder running, you will learn that there is a ridge down the middle of the Gulf. Get in your submarine and dive on that ridge, and you will find that it is a volcano. Lava leaks up along a crack in the middle of this ridge volcano, hardens, and then moves slowly away as new lava leaks up along the crack. Interestingly, lava also leaks into Death Valley along its faults, with eruptions within the last 200 to 300 years.
Baja
Clothes packed into a too-tight suitcase may leak out as the zipper is opened. Baja California is being unzipped from the mainland of Mexico, and the leak is a volcano making new seafloor. If the unzipping continues, the sea might someday extend up toward or into Death Valley. Get a good grip on the zipper pull of your luggage, and let’s go see.
Video: Baja Animation (2:08)
So, we're going to slice our way through the earth. We're gonna drive off of Baja, California, down undersea, and across. And out in the middle, we find this big bridge. And then sitting out there is Death Valley. And then we're going to come on across and back up onto the coast of Mexico.
And just to make sure it's clear, this is all underwater. So, here's the waves of Baja. If you're really keen about SCUBA diving, this is a good place to go. And there's birds flying over the top, and there's little boats that are floating in the water, so you can see what the thing is.
Now, if you were to measure the motion of this stuff, we're going to find that it's doing what we've seen elsewhere, which is, there's a motion going away from this ridge. If we could look very carefully, we would find that there is a volcano underneath this, and so material is leaking up into the cracks and making little volcanoes. And as it comes up in the cracks here, it hardens.
Looking at what's undersea, you find that there are lots of rocks that hardened in the cracks. And so the whole seafloor is made of these rocks that hardened in the cracks, and the oldest ones are farthest away from the center. So, they get older if you go out on either side.
If you were interested in fish poop and wind-blown dust, and things of that sort, you'd find there's almost none of them on the seafloor very close to the center. And then as you go away, they become more and more common. And that's because there's been more time for them to pile up, and very close to the center of the ridge, they just have not had very much time to pile up at all.
Now, you might imagine that this is related to other things. Then you remember that way down in the earth, one finds convection cells. And so you get the convection cells coming up and then spreading aside, and a little bit of leakage is coming up into the crack that it makes, making seafloor. And all of the world's sea floors are made in this way.
Dave Janesko, a geologist, was one of the students on the CAUSE trip. Here, he and Dr. Alley explore the great Sevier Fault just west of Bryce Canyon in Utah. The pull-apart action that opened Death Valley affected a lot of the West and is responsible for the Sevier Fault. The red limestone of Bryce was deposited in a lake. Much later, black lava flowed over the top and cooled. Then, the faulting occurred, and the black rocks were dropped to lie next to the red ones. Where the two meet along the nearly vertical, although slightly inclined fault, the red rocks show no sign of having been heated by the lava, so they must have been put together after the lava cooled. The red and black show near-vertical scratches formed when the rocks slid past each other to get where they are now. So, join Dave on the fault.
Video: The Fault Hunters (2:51)
OK, so we're standing on the Sevier Fault right now. This side over here is basalt. You can see the dark, black color. Over here, again, is the Eocene pink member of the Claron Formation, and it's the red rocks. So right now, I'm going to show you exactly what was going on here, what a normal fault means.
So, all right, let me figure this out. Richard, how do you think I should do this?
What I would probably do is something like this. I'd do this one's deposited in a lake, later the lava flows came out and they were deposited on top and across in various places. And then after that, the fault broke and this one has dropped down so now that it's next to it rather than being on top of it.
Sounds good.
Something like that.
All right. Maybe I should just use that.
This rock right here is 40 million years old. This is from the Eocene Claron Formation, and it's a lake deposit. This is what we saw earlier. And this right here is a piece of the basalt, which is Miocene, which is 10 million years ago.
And what this would look like in cross-section, something like this. The Claron, some more rocks, and then the basalt was deposited on top of it. Now when the fault occurred, these rocks would drop down along it like that. And now they're sitting right next to the Claron Formation, which they really didn't have any association with before.
Right now, we're on this basalt here. I'm on the south side, and if you can look in the distance there, you can see how this line of this fault moves right along into the distance. So again, the red rocks on that side, and the basalt on that side.
Richard, why does the basalt have more trees than the red side?
The soil probably?
It erodes faster on the right.
It will erode faster, and it's probably more minerals and nutrients in it.
In basalt?
Yeah.
Yep, doing great. Just be careful out there.
Then just take a pan from this.