Geo 1001 - Paper Faults - Answers


MODELS 1 & 2

Models 1 and 2 both show a single fault that cuts a series of flat-lying rock layers and a vertical igneous intrusion. For each model, try to determine what type of fault motion occurred along the fault. Was it normal, reverse, right-lateral or left-lateral motion? For each of the models, what type of tectonic stress most likely created this fault?

Model 1

This fault is a Normal Fault that was caused by tensional stress. The vertical intrusion is not offset, so there has been no lateral motion, only vertical motion. The block overlying the fault plane has moved down relative to the underlying block so it is normal rather than reverse motion.

Model 2

This fault is a left-lateral strike-slip fault created by shear stress. There has been no vertical motion, only horizontal motion. If you imagine yourself standing on one fault block and looking across the fault to the other block, the other block appears to have moved to the left compared to the block you are standing on.

MODELS 3 & 4

Models 3 and 4 both show a single fault that cuts a series of tilted rock layers. Because the rock layers are tilted, there are two possible types of fault motion that could create the geometry seen in each fault model. For each model, try to determine which two types of fault motion might have been responsible for the present pattern.
 

Model 3

The fault could either be a Normal Fault or a Right-Lateral Fault. You can arrive at this geometry by either cutting a set of tilted strata and moving them up and down along the fault plane or from side to side. If it is a normal fault then it was Tensional stress that created it. If a right-lateral fault, then it formed from Shear.
 

Model 4

The fault could either be a Reverse Fault or a Left-Lateral Fault. You can arrive at this geometry by either cutting a set of tilted strata and moving them up and down along the fault plane or from side to side. If it is a reverse fault then it was Compressional stress that created it. If a right-lateral fault, then it formed from Shear. 


MODELS 5, 6, 7 & 8

Models 5, 6, 7 and 8 all show faults that cut a series of tilted rock layers and an igneous intrusion. The addition of the intrusion should allow you to identify a single type of fault motion that created this geometry.  Was the fault motion normal, reverse, right-lateral or left-lateral motion? For each model, what type of tectonic stress created this fault? 

Model 5

With the addition of a vertical igneous intrusion, you should be able to tell that this is a Right-Lateral Fault that formed as the area was sheared. Vertical movement alone along the fault plane would not offset a vertical intrusion as shown in the map  view.
 

Model 6

With the addition of a vertical igneous intrusion, you should be able to tell that this is a Reverse Fault that formed as the area was compressed. The vertical intrusion has not be horizontally offset, so only vertical motion has occurred along the fault plane.
 

Model 7

Again, the vertical intrusion has not been horizontally offset, so there can only have been vertical motion along the fault planes. Looking at the relative motion of the fault blocks, both faults are Normal Faults and formed from tension. In both cases, the block that overlies the fault plane has moved down relative to the underlying block.
 

Model 8

This one can appear to be confusing at first.

Because the intrusion is sloping, it's surface outcrop (map view) was horizontally offset by vertical motion along the fault plane. You should be able to tell this offset did not occur as a result of horizontal motion (strike slip motion), because the igneous intrustion and tilted beds are offset in opposite directions. Simple horizontal motion would move the surface outcrops of the instrusion and tilted beds in the same direction.

If you look carefully at the relative motion along the fault plane you can tell that this is a Reverse Fault that formed from compression.