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
As the valley block rotates down, do they
Can you help me understand rocks “rising” in an
Thanks in advance for your answer.
|Question Date: 2018-01-25|
Very interesting and perceptive question, Neil. My
colleague Phil Gans can answer your question more
authoritatively, but here is my attempt at
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.
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
The concept of domains of rock floating on other
domains of rock is called isostasy.
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.
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
which causes extensional faulting in the crust on
top of the hot mantle plume. The mantle plume
pushes the mountains up, while the valleys
(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.
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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