68) Stress Strain and Faults


Welcome back geology fans! If you’ve been
a faithful viewer and seen all our episodes up to this point, then I feel you
are ready to start learning how to apply all this in the real world, or what we
geologists call “the field”. Field geology is central to our science as our
principal laboratory is the worldly one. But this world changes through time,
repositioning rocks in various ways, and we geologists come up to the results of
those billions of years of changes and have to unravel what series of grand
events took place there. The main way we do this is by identifying geologic
structures in the field. Structures are the physical shapes produced after rocks
have been subjected to various stresses. Once you have the ability to recognize
these structures, you can use them to not only unravel the depositional history of
an area, but also its stress history; was the area pulled apart in the past, pushed
together, or even sheared apart? Or maybe one then the other? We begin our
discussion on structures by pointing out the difference between stress and strain.
Stress is a force, but specific to structural geology,y stress is the force
applied over a volume of rock. There are three types of stress our information
bomb rocks could be subjected to: compression (pushing together), tension
(pulling apart), and shear (sliding in opposite directions or two oppositely
directed forces across the rock). Thinking back to our episode on plate tectonics,
convergent boundaries are generally dominated by compressive stress,
divergent boundaries are dominated by tensional stress, and conservative or
transform boundaries are dominated by shear stress. Basic silly putty can show
us that we need to bring in another concept besides the magnitude of force
to fully understand rock structures; time. If we set up a column of silly putty and
subject it to the compressive stress caused by gravity, it slowly deforms and
even flows. But if I take the same lump of silly
putty and compress it again, but rapidly with a hammer blow, we can actually see
the silly putty break, fracture. Similarly, rocks can flow or fracture depending on
temperature, pressure, composition, and the rate and magnitude of the stress applied.
The way in which a rock deforms when it is subjected to a stress is called its
strain. Like stress, strain has three forms: brittle (breaking or fracturing),
ductile (flowing like plastic or thick fluid), and elastic (bending or deforming
in some way with the stress, but if the stress is released the rock will go back
to its original shape with no permanent deformation). On a diagram plotting
stress over strain, we see that a material will slightly deform, strain, as
it is stressed. But if it fails with brittle strain the stress is immediately
released. If the failure is ductile, we see the material reach its yield point,
that is, enough stress that it begins to flow, and then continues to flow with
increasing stress reaching its maximum strength level, and further stress may
cause brittle failure again. But note how the stress is maintained as the material
deforms with ductile strain. Elastic strain would deform the material but
then send it right back to its starting position if the stress is released. One
can see that most elastic strain occurs at lower stress levels, but as stress
builds either ductile or brittle strain deformation take over. Now we can combine
types of stress and strain to produce a quick list of common geologic structures.
First off we note the general patterns. Elastic strain does not produce
permanent structures as once the stress is released the rock goes back to its
original form, and does not preserve the elastic strain as a structure. Secondly,
brittle deformations ALL cause what are known as “faults”, which are defined as
breaks in the rock across which movement has occurred. Ductile strain produces
folding with compression, and boudinage with tension, and monoclonal folds with
shear stress. The remainder of this episode will cover the brittle faults,
and we start at the top left of our punnett square to see that when
compressive stress causes brittle strain in a rock, the result is reverse faults in
most cases, and in rare cases of extreme stress the lower angle thrust faults. As
faults are generally planes of weakness, hot fluids carrying valuable minerals
can intrude into them forming rich mineral veins. Miners through the ages
have dug down along faults to pull out the ore material, and finding themselves
on the fault they coined the terms foot wall, and hanging wall. The foot wall is
under their feet, and the hanging wall is hanging over their heads. Every fault
that is not perfectly vertical has a foot wall and a hanging wall, and it is
by their relative motion that we know what type of fault we are looking at. All
faults caused by compression, whether they be reverse or thrust faults, have
their hanging wall go up relative to their foot wall. The difference between
reverse and thrust faults is that thrust faults have a more shallow angle, closer
to horizontal, and are caused by more intense and rapid compression. This is
the surface expression of the Sevier thrust fault west of Las Vegas. Such
thrust faults can cause slabs of surficial rock to move kilometers over
the rocks below, and cause one of the ways to break stratigraphic
superposition by putting older and relatively undeformed rock layers on top
of younger layers. If you see such a pattern, but the internal up indicators say
nothing has been overturned, you should suspect a thrust fault at play. If you
forget which way the hanging wall goes in a reverse or thrust fault, I suggest
you use your hands to remember. Place your hands together, tilt them off
vertical, and press them together with compressive stress. You will feel your
upper hand, the hanging wall, moving up relative to your lower “foot
wall” hand. Tilt your hands in the opposite direction, and note that when
you compress them together the upper hand, now on the other side, still goes up.
It doesn’t matter which way the fault tilts, as long as you have compressive
stress the hanging wall will go up relative to the footwall. But what if you
pull your hands apart trying to keep them in contact with each other. Feel the
way your upper “hanging wall” hand now goes down relative to the lower foot
wall hand? Again, it doesn’t matter which way the hands are tilted, the upper
“hanging wall” hand always goes down with tensional stress relative to the
footwall. This opposite of reverse fault movement is called a normal fault, and
can result in a landscape known as horst and graben topography, which translates
from the German roots to nest (horst), and grave (graben) topography. If we take a
flat landscape and pull it apart with brittle strain, faults will form and
begin to oppose each other at certain distances allowing a block to slide
downward, each side being the hanging wall of its bounding faults, to form a
down-dropped graben, while the blocks remaining in the
higher position form the horsts. Standing on the edge of the Colorado
National Monument, you’re on a high standing horst looking across the graben valley to the next distant horst. As we follow the East Pacific Rift north, we
see it dive below North America and ripping Baja California away from Mexico
on the way. But that rift disappears below North America in the western
United States, and that divergent ridge causing tensional stress has fractured
Nevada, or the area known as the Basin and Range, into a series of normally
faulted horsts and grabens. So to review, reverse and thrust faults have the
hanging wall go up relative to foot wall with thrust faults at a lower angle
going farther distance, and the normal faults have the hanging wall go down
relative to the footwall. Our last stress produces our
last fault type in the brittle strain column; the strike-slip fault. Regardless
of the tilt of the fault from the surface, and many of these are nearly
vertical, there is no relative motion of hanging wall and footwall upward or
downward for the pure strike-slip fault, only side-to-side motion. Strike-slip
faults are most obvious when they express themselves at the surface, and
recall that brakes on faults can cause earthquakes by which we can tell that
some faults are blind faults, meaning they never make it to the surface and
thus we are blind to them until they cause an earthquake. Strike-slip faults
at the surface can cause a long linear scar across which offset streams get
slowly distorted. Looking at an offset stream can let one know the direction of
movement on a strike-slip fault, which comes in two flavors: right lateral where
no matter which side of the fault you stand on you see the other side move to
your right, or left lateral where the opposite side always goes to your left.
The San Andreas Fault in California is a right lateral strike-slip fault slowly
moving Los Angeles up towards San Francisco. But the San Andreas is not
perfectly straight, and where it bends we can see the land have a bit more tension
and pull apart in an isolated basin called a sag pond. Or if it bends the
other way, it can cause more compression and minor mountain building. So offset
streams, sag ponds, and minor uplift are all associated with these strike-slip
faults. Let’s pause and practice before we move on. What kind of fault is this;
reverse or normal? What kind of stress produced it, and what kind of strain? Here
are some choices for you. Feel free to pause the image and take your time, but
we will get the answer when this five-second clock counts down. (Jeopardy music) This is a normal fault, with the hanging
wall going down relative to the footwall, and it is caused by tensional stress
with brittle strain. Okay, try this one. (music) This is a reverse fault, with a hanging
wall going up relative to the footwall, and it is caused by compressional stress
with brittle strain. And this? (music) This is a right-lateral strike-slip fault, with the
other side from you always moving to your right when you face the fault, and
it’s caused by shear stress, and I hope you’re getting it drilled into your head
that all faults are the result of brittle strain.
I also hope you note that you can never get a normal fault with compression nor a
reverse fault with tension, and neither can be made by ductile nor elastic
strain as all faults are due to brittle strain. What kind of fault is this? (music) And here I’ll guess that most of you
just answered that this is a right- lateral strike-slip fault, due to shear
stress, brittle strain. But stop for a moment and think about the full
three-dimensional picture. These beds are dipping away, and though it looks like
the distant side moved to the right of the foreground, making this look like a
right lateral strike-slip fault, the same pattern could be made if the back were
lifted up relative to the front, and then eroded back down to a level plane.
If that is the case, we don’t know at all what kind of fault this is as it depends
on the direction the brake dips into the ground. It could be normal, reversed, or
maybe it really is strike-slip. This is why strike-slip faults are sometimes
called “the fault of the confused geologist”. If you think you found a
strike-slip fault, do take some time to consider if you thought out all the ways
it could have moved to form what you see before you. But not all hope is lost if
you’re not sure about past movements at first glance. We can look at these
scraped and often polished rock surfaces called slickensides. If we find them in
place, we can look at the slickensides and get a notion of the motion. First off,
looking at the lineations, we can narrow down the movement to one of two
directions. Secondly, we can look more closely at the slickensides and perhaps
see stair-step features along them to get the exact direction of movement. If
you follow these stair steps up, that is in the direction that particular rock
wall moved. In other words, as a rock face scrapes along another rock face, each
side makes downward steps in the opposite wall. if we had good
slickensides, this fault would not confuse us as it has. We might be able to
better fix it in our punnett square as a normal, reverse, or strike-slip fault. But
Nature doesn’t like to be pigeonholed, and often throws more than one type of
stress at a rock formation. Now, it’s impossible to throw both compression and
tension at a rock formation simultaneously as they are
opposite to each other, but you can combine either with shear to produce a
variety of what are known as oblique slip faults. With tension you get normal
oblique slip faults, and with compression you get reverse oblique slip faults. A
last note on faults for this episode is that we’ve been giving examples of
faults that are rather planar, and thus when they intersect another plane like a
rock face they appear linear. But many faults bend with depth, and are not
perfectly linear. And also, faults occur within wide zones rather than along
narrow planes commonly. Fault zones have seriously deformed rock in them, and
result in fault melange and fault breccia. Recall that breccia is the term for
angular rock fragments glued together, and one can imagine the formation of a
fault could form solid rock into fault breccia. When we come back next time, we
will finish off our structural chart to see what ductile strain can produce with
compression, tension, and shear, and finish off with a structure called “joints” seen
in more mildly stressed rocks. And I hope you remain only mildly stressed until
next time, here on Earth Explorations.

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Comments

  1. Studying for my PG exam after being a GIT for over TWELVE years. It’s time I do this! Great refresher videos.

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