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Lecture 19

EPSC 201 Lecture Notes - Lecture 19: Continental Crust, Oceanic Crust, Subduction

Earth & Planetary Sciences
Course Code
EPSC 201
Anthony Williams- Jones

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EPSC 201 Lecture 19 Notes
Dip-slip faults – faults that are being compressed or extended at a fault. A normal fault has extension of
the two plates forming the fault, which causes horsts and graben to form, and for some of the plate to
move downward. Reverse dip-slip faults are a result of compression, and can cause on side of the fault
to go upwards, to accommodate the compression. If there are two dip-slip faults present, horsts and
graben can form. The graben is like a valley, with lots of scraping along the edges. A horst is formed
when the two outlying plates are extended and move
downwards, leaving the middle landmass at the original
height, now above the outlying plates.
Strike-slip faults occur when the force is 90 degrees to
the fault. All of the movement is in the horizontal plane,
and can be measured relative to true North. These faults
are either sinsistral or dextral. If you were standing on
one plate, looking at the other, and it moved to the right,
its dextral. If the opposing plate moves to the left, its sin-
So far, we’ve only covered examples where the movement is exactly in the strike or slip direction. Most
faults are a combination of both directions, and
can be called oblique-slip faults. They have
reverse/normal and sinsistral/dextral compo-
nents. A complete description would then in-
clude those terms, as well as an analysis of the
Often, layers of rock will try to fold before the
break. There will be some curves present in
the layers, but eventually there would be a
fault. This is because the rocks are too cold, or
too brittle.

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A thrust fault is a reverse fault with a very low angle. They often occur in sedimentary layers, since the
angle of the layers will be relatively horizontal. In the picture showing thrust faults, the angles are much
too steep.
Intense shortening (over 100 km) can occur, which will cause many different thrust faults.
The basement is where the faults stop. The faults are sliding on it. Think of carpet in a house. The car-
pet is lying on hard wood underneath, which doesn’t move. SO the carpet moves and is able to fold,
while the underlying hard material does not move. So the upper layers of rock here are softer, and are
able to fold. If the upper layers are more brittle, they will form faults.
Carpets represent the soft material on the top.
Underlying hard wood floor doesn’t fault. It is separated from the sedimentary rocks (carpet) by a de-
tachment fault. The sedimentary rocks have detached themselves from the rest of the system.
The sedimentary rocks are soft and ductile compared to the basement.
Here we see strike-slip motion. Here we can see how river systems have been shifted sideways. We
are looking at a dextral strike-slip fault. The red lines of the river should match up.

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The rocks here are behaving ductile. They have been folded
many times. They may have been in a hot environment when
this happened.
We need to be able to define our folds.
Below, we define a syncline. Slicing the fold in half yields the
axial plane.
The yellow lines define the dip of the rock layers. If all the yel-
low lines (slope of the dip) point inwards, it is a syncline.
An anticline
has all the yellow lines have outward dips.
Here are some different types of anticlines. Know the re-
cumbent anticline, which is horizontal.
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