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You have a very nice web site. Thanks you for putting it together.

I have a question about basin and range faulting. I understand about valley (or half graven) formation. Due to stretching there is space for the hanging wall to rotate down.

But I do not understand how in an extensional regime do you get the foot wall to “rise” and form the “range” mountains in a basin and range province.

As the valley block rotates down, do they rotate up?

Can you help me understand rocks “rising” in an extensional area?

Thanks in advance for your answer.
Question Date: 2018-01-25
Answer 1:

Very interesting and perceptive question, Neil. My colleague Phil Gans can answer your question more authoritatively, but here is my attempt at simplified answer.

Envision a cross-section of the upper crust in the basin and range as a single-file line of bricks, standing on end, right next to each other, like closely spaced dominoes. Now tip that line of bricks either left or right. You've created a sawtooth ridge, the top right (or left) corner of each brick standing high (range), relative to the v-shaped valleys on either side (basins). So, yes, the ranges are higher than the basins, but overall, the elevation of everything (basins and ranges) has probably dropped relative to the original surface of the un-tipped brick plateau. The tops of the ridges have simply dropped less than have the basins, creating the difference in elevation.

Earth scientists debate how high that original plateau might have been--an extreme view is that it might have been as elevated as the Tibetan plateau is today. Another complicating factor is magma. As the crust is thinned during extension, rocks in the subsurface are decompressed, promoting their melting and the generation of magma although specialists debate what came first--the melting or the extension). Nevertheless, magma is less dense (more buoyant) than its source material, contributing to uplift. Perhaps Phil can clarify whether the ranges are currently going up in absolute terms, or whether they're sinking, but less slowly than the basins.

We appreciate your curiosity and your commitment to teach your students what we think we know about the formation of the basin and range.


Answer 2:

Over long periods of time rocks can have certain characteristics of fluids; they can flow. When half-grabens form then gradually the footwall will experience less and less weight on it as the hanging wall moves away from it. That allows rocks from below the footwall to flow upwards and cause the footwall to rise. As the footwall moves away from the hanging wall more and more material will ooze up over thousands of years to fill in the opening gap.

The concept of domains of rock floating on other domains of rock is called isostasy. Isostasy can help explain, for instance, why continents lie higher than oceans. The rocks on continents are thicker, and they are also less dense, which allows them to float on denser rocks below.

Answer 3:

Good question! The answer is still controversial. Here are some of the competing hypotheses:

(1) Basin and range faulting represents the early stages of a (possibly aborted) rift system like the east African Great Rift. The basin and range province is being pushed up by mantle convection, which causes extensional faulting in the crust on top of the hot mantle plume. The mantle plume pushes the mountains up, while the valleys subside.

(2) The mountains are high because of isostatic rebound. The valleys are low due to the formation of half-grabens, and so there is less rock in the crust, which causes the mantle to rise, pushing the non-graben parts up to make mountains. This is a similar process to what causes the masts of a ship to stick up: the ship is floating on the ocean with the lowest center of gravity possible, but the ship is shaped in such a way that the masts are on top, sticking upward.

(3) What is now the basin and range province was formerly an area of compression created by the subducting Farallon plate, much as today's Altiplano, a plateau in the Andes about 3km above sea level. Then the east Pacific rise got subducted, creating the San Andreas plate boundary, which causes extension, fracturing the formerly high "Nevadaplano" plateau into the alternating old parts of the plateau (the mountains) and the valleys in-between them.

Answer 4:

I'm not familiar with most of what you're asking, but I'm wondering if continental plates are active, on the scale you're talking about? Subduction of the plate that is pushing up the mountain ranges along the California coast, for example. It sounds like you may be talking about land movements on a smaller scale, but I'll add my 'two bits worth.'

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