#All
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The folded mountain is rare in my experience.
Most mountains appear to be sections of crust that toppled over.
Much like a stack of dominos that were spread out and toppled over.
They are very organized in places. In parallel rows of toppled crust. One side
of the mountain is the top soil side or the top of the original uundisturbed
crust and then the other side of the mountain is a crossection of time and
exposes all the layers of the crust. The saw tooth pattern repeats alternating
between the top of the crust and the edge of the crust. This could only occur
if the sections were broken apart and then the crust spread some and the
sections fell over.
There are other types of mountains but this type is difficult to explain by
any sequence except crustal spreading.
Kamron
Hi, Kamron–
>> This could only occur if the sections were broken apart and then the crust
spread some and the sections fell over.
This is by no means the _only_ way to form thrust-fault-block mountains.
This can happen from stretching – it is called a graben. The crust thins as it
is stretched, growing depressed in the region affected. Blocks fall into the
depression and tilt. The falling would be in the region of fastest thinning,
which would not be the center of action. Expect a very broad valley with
shallow stepped walls, and a shallow crust/mantle boundary.
But let a magma flow push up the surface layers, and then let the pressure
fade; the blocks of crust may slide back into place tilted in a manner similar
to a graben. But I would not expect a depression to result in many such cases;
there may be no valley walls for the block bounds to concentrate at, and the
peaks may tend to be similar in altitude instead of stepping down as in a
graben. These would tend to be linear features if the original volcanic
activity was driven by a subduction at a nearby coast (I believe the Cascade
Mts. are such a volcanic arc, which may develop into the center of a region of
thrust-fault-block mountains with time).
Or let an igneous intrusion cool into a granite batholith underground; since it
is less dense than some rocks, it may rise after cooling. Depending on its
symmetry, it may tilt up one block – I have read that the Sierra Nevada Mts. of
Calif. may have formed that way.
There are doubtless other ways to get fault-block mountains.
–Doug
I liked the descriptions of ways the bock or the Rhine Graben. Looks like a
twin set of faults shearing and slipping. Or a wide crack that was filled in
but the layers taper you say?
The puzzle to me is the straight lines of the layers. Usually straight seldom
folded.
Its the lack of folded mountain ranges. Have you seen more than rolling hills
that appeared folded?
Kamron
Hi, Kamron–
To a first approximation, fold mountains are found where continents have
collided, while fault-block mountains are more common where subduction has
occurred near a continental margin.
Thus the mountains of the western Americas are largely fault-block, while the
Appalachians, Urals, and the Himalayas-Carpathians-Alps are fold mountains. Of
these, I have seen only a bit of the Appalachians ("You call that a mountain?")
– the western fault-block mountains and volcanic cones are the familiar ones to
me. The Ozarks seemed more like the Appalachians to me, though that is at
least in part because they are small (to a Californian) and covered with
vegetation.
–Doug
Even what little I remember of the Appalachians is they are tilted crust and
not folded crust as expected in the old mountains.
I wonder if anybody else has studied them and could confirm this.
Kamron
Kamron,
>>Even what little I remember of the Appalachians is they are tilted crust and
not folded crust as expected in the old mountains.
The Appalachians are fold mountains, you just need to learn to think on a
larger scale. The <tilted> blocks you see are just one side of the very large
folds. The other side can be many miles away. The arched top has been removed
by erosion. There are some block mountains (or at least faults) on the east
coast. The faulting occurred in the Triassic, and is related to the rifting
associated with the openning of the proto-Atlantic.
Jim
So you have been there looking at that.
I have not seen that and was wondering where the folded mountains were.
Are you sure that all the tops are facing away from each other in matching
pairs?
When I have looked at the blocks closely I usually see them lined up all
facing the same way.
Kamron