<p>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.</p>

<p>So, how can we tell what the fault did?</p>

<p>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.</p>

<p>So, let us undo that shift to the north-west:</p>

<p><img src="https://dl.dropboxusercontent.com/u/17444491/geology/x9wMl_2.png" alt=""></p>

<p>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.</p>

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

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

<p>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.</p>

CC BY-SA 3.0
2015-03-18T16:30:51.153Z
Think I got it - that's great answer, thanks.
Jim W
CC BY-SA 3.0
2015-03-19T09:21:14.943Z
Glad to have helped. :)
mtb-za
CC BY-SA 3.0
2015-03-17T23:27:22.460Z
2015-03-17T23:27:22.460Z
mtb-za
<p>Studying for an exam tomorrow and stumped on this question (parts <em>d</em> and <em>e</em>).</p>
<blockquote>
<p>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</p>
<p>a. amount of net slip</p>
<p>b. rake of net slip on the fault plan</p>
<p>c. plunge and trend of net slip</p>
<p>d. relative movement along the fault e. type of fault</p>
</blockquote>
<p><img src="https://i.sstatic.net/x9wMl.png" alt="Map 2" /></p>
<p>The original answer for <em>d</em> &amp; <em>e</em> 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.</p>
<p>Lab TA appears to be saying not sinistral, just 'normal'.</p>
<p><strong>So my questions are, how would you determine that it's a normal fault and not reverse?  Why not sinistral?</strong></p>
<p>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.</p>

CC BY-SA 3.0
2015-03-15T21:04:26.293Z
If the hanging wall moves up its a reverse fault, if it moves down, its normal.
stali
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2015-03-16T00:18:30.403Z
Thanks, but how do I determine that, @stali?
Jim W
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2015-03-16T03:05:31.643Z
Imagine yourself standing on the fault. Your foot points towards the footwall where as your head points towards the hanging wall.
stali
CC BY-SA 3.0
2015-03-16T03:32:24.567Z
@stali, thanks, but I mean - I know the footwall is the western block, but how do I know if the hangingwall went up or down?  I only have the information in the question...
Jim W
CC BY-SA 3.0
2015-03-16T03:43:58.473Z
You can figure that out by looking at the offsets in various layers.
stali
CC BY-SA 3.0
2015-03-16T15:45:03.753Z
@stali, I am sorry I must be missing this, I can see that the hanging wall shifted to the north west, if that's what you mean, but how does it relate to up/down of the HW?  Thanks I do appreciate it!
Jim W
CC BY-SA 3.0
2015-03-16T15:58:24.747Z
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.
stali
CC BY-SA 3.0
2015-03-17T05:51:21.620Z
Doesn't the question say that the faulting post-dates both periods of tilting rather than occurring prior to the northward tilting?
haresfur
CC BY-SA 3.0
2015-03-17T22:41:32.523Z
Yeah, the fault is the most recent geological event, prior to erosion to a plain. I will venture an answer.
mtb-za
<structural-geology><homework>
How to determine if this fault is normal, sinistral?
Jim W
# How to determine if this fault is normal, sinistral?

(Asked by: username_0 on 2015-03-15)

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](https://i.sstatic.net/x9wMl.png)

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.

**Tags:** structural-geology, homework

> **Comments:**
> **username_1**: If the hanging wall moves up its a reverse fault, if it moves down, its normal.  
> **username_0**: Thanks, but how do I determine that, @username_1?  
> **username_1**: Imagine yourself standing on the fault. Your foot points towards the footwall where as your head points towards the hanging wall.  
> **username_0**: @username_1, thanks, but I mean - I know the footwall is the western block, but how do I know if the hangingwall went up or down?  I only have the information in the question...  
> **username_1**: You can figure that out by looking at the offsets in various layers.  
> **username_0**: @username_1, I am sorry I must be missing this, I can see that the hanging wall shifted to the north west, if that's what you mean, but how does it relate to up/down of the HW?  Thanks I do appreciate it!  
> **username_1**: 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.  
> **username_2**: Doesn't the question say that the faulting post-dates both periods of tilting rather than occurring prior to the northward tilting?  
> **username_3**: Yeah, the fault is the most recent geological event, prior to erosion to a plain. I will venture an answer.  

---

## 1 answers

### Answer by username_3 on 2015-03-17. Score: 3 | Accepted answer

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:

![](https://dl.dropboxusercontent.com/u/17444491/geology/x9wMl_2.png)

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.

> **Comments:**
> **username_0**: Think I got it - that's great answer, thanks.  
> **username_3**: Glad to have helped. :)  

---

