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Lecture 19

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

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- sistral. 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 geology. 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. 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. 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. Recumbent = to lie down So a recumbent anticline is just a anticline that is lying down, or hori- zontal. Monocline – just has dips in one direction If the anticline or syncline is on an angle, we take the angle with re- spect to the horizontal, and call it a plunging fold or nonplunging fold. The strike (direction relative to north) is mea- sured by taking the direction of the hinge line. The fold on the left hand side will continue into the earth. The fold will eventually intersect the horizontal, and there will be a U (syncline) or N (anticline – depicted here) shown here. The pictures here will clarify. The horizontal intersection on the right shows two plunging anti- clines and their N shaped layers on the surface. Syncline direction is measured away from the nose of the fold. The nose of the fold is the peak. Anticline direction is measured towards the tip of the nose. There are often synclines and anticlines close to each other. Its just because folding usually occurs in the same area, so if you fold something down, then something else must go up to compensate. Anticlines and synclines can come on large and small scales. They can be a few meters, up to tens or hundreds of kilometers. Here is an anticline on the left plunging to- wards the North. Often the prof will ask to draw a matching syncline and anticline plunging in some di- rection. Professor says he might ask this. How do you cre
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