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Studying for an exam tomorrow and stumped on this question (parts d and e).

The region show in Map 2 consists of a series of sedimentary strata that have been tilted and then eroded, producing an unconformity. Lava flows covered the unconformity, and the region underwent further tilting and then faulting before erosion transformed the region in to a plain. Using the data provided by Map 2 determine

a. amount of net slip

b. rake of net slip on the fault plan

c. plunge and trend of net slip

d. relative movement along the fault e. type of fault

Map 2

The original answer for d & e was that the eastern block moved NW relative to the western block, and it was a sinistral reverse fault. Can't remember why I chose reverse and not normal.

Lab TA appears to be saying not sinistral, just 'normal'.

So my questions are, how would you determine that it's a normal fault and not reverse? Why not sinistral?

I can't see an indication of how the foot wall (western block) has moved relative to the hanging wall. I know the TA is the person to ask but it's an online course and answers were slow to come at the best of times.

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  • $\begingroup$ If the hanging wall moves up its a reverse fault, if it moves down, its normal. $\endgroup$
    – stali
    Mar 15, 2015 at 21:04
  • $\begingroup$ Thanks, but how do I determine that, @stali? $\endgroup$
    – Jim W
    Mar 16, 2015 at 0:18
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    $\begingroup$ You can figure that out by looking at the offsets in various layers. $\endgroup$
    – stali
    Mar 16, 2015 at 3:43
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    $\begingroup$ That's because the whole thing is tilted. When the fault slipped it was a reverse fault (hanging wall went up) but subsequently, i.e., after faulting the whole thing was tilted 90 degrees northwards due to some force of nature (e.g. due to folding over time). Erosion then exposed the layers and so you can now see the offsets. $\endgroup$
    – stali
    Mar 16, 2015 at 15:58
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    $\begingroup$ Doesn't the question say that the faulting post-dates both periods of tilting rather than occurring prior to the northward tilting? $\endgroup$
    – haresfur
    Mar 17, 2015 at 5:51

1 Answer 1

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So, the basic geological history is given. From this, the units named Bed 1, were deposited, then tilted. Bed 2 was deposited and tilted (along with Bed 1). The entire sequence was then faulted.

So, how can we tell what the fault did?

First of all, notice that there was movement of the eastern block to the north-west. This can be deduced because all the contacts have been shifted in that direction.

So, let us undo that shift to the north-west:

Now, we can see that the contact between Bed 2 and Bed 1 does not line up. This means that we must have had movement up or down as well. The only way to work this one out is by knowing which way the contact dips. If it dips northwards, then the eastern block moved up. If it dipped south, then it would overlie all the sediments to the south. This suggests that Bed 2 dips northwards.

Upward movement of the eastern block would then reveal Bed 1 when the area was eroded.

My conclusion therefore, is that the fault moved the eastern block north-west and up, relative to the western block.

Did I miss anything? It is late, which I will claim as an excuse. If my logic is unclear, let me know and I can try add more pictures, or video of me waving hands about, which is my usual approach.

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  • $\begingroup$ Think I got it - that's great answer, thanks. $\endgroup$
    – Jim W
    Mar 18, 2015 at 16:30
  • $\begingroup$ Glad to have helped. :) $\endgroup$
    – mtb-za
    Mar 19, 2015 at 9:21

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