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
As the valley block rotates down,do they
Can you help me understand rocks “rising” in an
Thanks in advance for your answer.
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
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
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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