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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 graben) formation. Due to stretching there is space foe 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: 2018-01-22
Answer 1:

What a great and complicated question!

Basin and Range geology can be really confusing. Most people are able to wrap their heads around the general principles of extension that cause thinning and cracking of the crust as it is being pulled apart. The end result being the formation of large normal faults, creating uplifted mountains and down-dropped valleys. There can also be other types of faults, but the distinct pattern of linear mountain ranges and valleys comes from these normal faults.

A simplified model for how normal faults move can be pictured by the hanging wall moving down relative to the footwall along the fault plane. The steep and abrupt mountains are formed on the upthrown side of the fault, and the low valleys are formed in the down-dropped side of the fault. In the U.S. Basin and Range Region this produced about 400 mountains' blocks and valleys with varying differences in topography, the more extreme being the low elevation of Death Valley at -282 feet compared to Telescope Peak at 11,050 feet, in the Panamint Range just to the west of the low valley.

As you mentioned, in extensional tectonic regimes, the brittle upper crust fractures and rotates down as tilted fault blocks. You can imagine a series of Jenga blocks standing vertically and tilted to one side. The tilted Jenga blocks create a series of corners pointing up with empty "valleys" pointing down between. Then, the uplift occurs due to the ductile lower crust underlying these brittle upper crust blocks. As the crust stretches, the ductile lower crust ascends, resulting in uplift, cooling, and exhumation of the ductile lower crust. The exact nuances of how the uplift occurs is still surrounded by debate. Generally it is explained by processes of lowered pressure due to extension allowing the ductile lower crust to flow up under the fault complex through isostasy, and/or decompression melting producing magma that deformed the footwall and emplace material below the fault complex. The final products are valleys lower than the original crust and mountains higher than the original crust.



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