Crosswords2 mins ago
Earth Science
A fault is observed in a road cut, but there are no obvious rocks units to correlate on either side of the fault to determine relative movement. How else might you determine or even infer whether the hanging wall has moved up or down relative to the footwall?
Answers
1 to 3 of 3
Best Answer
No best answer has yet been selected by Coocoo. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.
Let's assume, for the sake of simplicity, that you are observing a normal fault. A normal fault is one along which the hanging wall has moved down relative to the footwall. The fault plane of normal faults typically dips at an angle of 60 degrees from the horizontal. Normal faults are caused by rupture in response to tensional forces.
If the fault surface is visible, we can often see clear evidence of the movement of the hangingwall and footwall past each other on the surface itself. We call a fault surface a slickenside if it is polished. Slickensides are commonly striated or grooved, or coated by secondary mineral fibers such as quartz or calcite. These striations can be compared on the opposing faces of the hangingwall versus the footwall. Additionally, within the slickensides are slickenlines which are thought to indicate the direction of movement on the fault. (Stay with me here) Because slickenlines are lines (not vectors), they indicate one of two possible motion directions. To distinguish between these two options, it is necessary to look at the morphology of the fault surface. Steps are offsets or breaks in the fault surface that trend approximately perpendicular to the slickenlines. A very useful distinction can be made between congruous steps, which face in the same direction as the movement on the fault, and incongruous steps that face in the opposite direction. At some point in the cut you describe these comparisons can be made. (Thanks to University of Wisconsin referesher text).
This of course, applies to western U.S. geology, but the terms and descriptions are standard text book in content. (Where's brachiopod when you need him?)... Best of luck!
If the fault surface is visible, we can often see clear evidence of the movement of the hangingwall and footwall past each other on the surface itself. We call a fault surface a slickenside if it is polished. Slickensides are commonly striated or grooved, or coated by secondary mineral fibers such as quartz or calcite. These striations can be compared on the opposing faces of the hangingwall versus the footwall. Additionally, within the slickensides are slickenlines which are thought to indicate the direction of movement on the fault. (Stay with me here) Because slickenlines are lines (not vectors), they indicate one of two possible motion directions. To distinguish between these two options, it is necessary to look at the morphology of the fault surface. Steps are offsets or breaks in the fault surface that trend approximately perpendicular to the slickenlines. A very useful distinction can be made between congruous steps, which face in the same direction as the movement on the fault, and incongruous steps that face in the opposite direction. At some point in the cut you describe these comparisons can be made. (Thanks to University of Wisconsin referesher text).
This of course, applies to western U.S. geology, but the terms and descriptions are standard text book in content. (Where's brachiopod when you need him?)... Best of luck!
1 to 3 of 3
