fault geometry / x-sections

[cossatot]

Need to come up with 2-D fault geometry and cross-sections for Pecube model

[eqsarah]

I think using the McGrew retrodeformable cross section is the most complete cross section available across the entire SSR. You can get this in using the plot digitizer? I've never used this software, and I would like to try my hand at it. I will let you know how it goes; hopefully it doesn't give me fits and I can send it to you rather quickly.

Also, for the surface trace of the faults the SSRD has been mapped in detail by McGrew and by a guy named Whitbread. Does this need to be taken into Arc and have the location of the faults digitized for lat-long coordinates?

[cossatot]

We shouldn't need the trace of the fault mapped in detail. Pecube only allows for planar faults (no change in strike) so we just need to get the strike and dip of the fault segment that intersects topography right so that the fault trace is in approximately the right location. It really won't affect the modeling results too much if it's a bit off, unless samples end up falling on the wrong side of the fault.

This is where it currently is; does this work?

[eqsarah]

Not exactly in the right spot, mostly because of the strike issue I think. I think you clipped the DEM arc (?) so would it be possible to drape a GeoTiff over it real quick to QC? I can't attach the file to the comment, so I'll send it along in an email.

[cossatot]

If this doesn't work give me the latitudes, longitudes and elevation of two points that define a line of strike for the fault, and the approximate dip of the fault where it intersects the surface.

[eqsarah]

Sending you the xz coordinates of the two faults via email. Let me know if something looks funky.

Also, still working on getting a reasonable orientation of the main SSRD. Seemed like a couple three point problems would have done the trick...except there is so much variation in orientation of the SSRD as mapped that I'm trying to come up with a reasonable/defendable average orientation as a planar fault.

[eqsarah]

The latitude and longitude to define the strike of the SSRD: 39.000531, -114.159784; 38.901634, -114.195774

Not sure in the end how different this will be from your locations. The strike line I defined is 015, and is "pinned" where the cross section line intersects the map trace of the fault. This strike line is calculated by assuming there is pure dip slip motion along the fault, so the slip direction of the SSRD defined by mineral elongations in the mylotinized portion of the Prospect Mountain Quartzite (n=94; McGrew, 1993) are parallel to dip, and then calculating strike from dip. I tried to define an average strike by doing some stereonetting/structure contours of corrugations in the fault mapped by McGrew 1993, but it did not really agree very well with the way I calculated above. I think the way above seems more defendable than the second since it is probably less prone to person errors.

Defining the strike line of the fault in the western portion of the range was less straightforward since it doesn't have any published information about slip indicators, nor does it show up in detail any of the actual geologic maps (just on the cross section of McGrew 1993--his map doesn't extend that far west and it's on figure 1 DEM large scale map of Miller et al 1999). I used the trace shown on that figure in Miller et al 1999 and approximated its location that way.

Location: 38.975418, -114.37265; 38.906318, -114.367701

I've done some more thinking about the timing of movement along the western fault, and I do think it needs to be younger than the SSRD. This is based on the idea that the breakaway for the Snake Range metamorphic core complex is hypothesized to be in the Schell Creek Range (the next range to the west) based on COCORP seismic interpretation (see Gans 1987); this interpretation is for the NSR specifically, but I think that it makes sense to extrapolate the breakaway of the SSRD to the Schell Creek Range too (also the next range west of the SSR). So if the western fault of the SSR was active at the same time as the SSRD then the breakaway would be separated from the active part of the detachment which may make sense for more recent extension but not when the SSRD initiated. Hopefully that explanation makes at least some sense. I don't think any of this alleviates when the western fault should be active, but I don't think they initiate at the same time. Too bad there is no (easy) way to get sediment samples in Spring Valley (the basin separating the Schell Creek Range and the SSR), I think it would provide us with a crap ton of useful information.

[cossatot]

Sarah,

The strike that you picked may be a little problematic. Basically, the fault trace does not represent the fault trace over the entire model, just a small segment of it. My guess is that you're basically measuring the southern side of a corrugation on the detachment, or that the actual brittle detachment surface cuts somewhat obliquely across the mylonite (this does happen sometimes). This is a problem with Pecube, that we have to select one cylindrical 'best-fit' fault geometry for an entire mountain range. Also note that the western fault has to have the same strike as the eastern fault (really a pain in the ass). Here is how it looks: You can see that the fault cuts through the range in the south and then extends way into the HW basin in the north. Maybe the issue is that I have a larger swath of real estate in the model than you had envisioned?

Here is an image with the fault trace as I had defined it earlier:

[eqsarah]

In terms of geometry, I really do think the first trace in your comments looks much more like the way the SSRD has been mapped (although I also understand that the mylonite sense of direction may not be the same as the brittle--and may not be in the SSR). The SSRD is thought to cut back into the the range quite a ways in the southern part of the range, and in fact the southern most portion of the range is thought to be all upper plate. As to swinging into the basin in the north it is probably a little extreme, but the trace of the NSRD is very close to the range front in the southern part of the NSR and then swings back to the west into the range in the northern part of the NSR. (see Miller et al. 1999 figure 3) We could put the trace of the fault closer to north-south, but I wouldn't put it any less than 0 azimuth since this would make it hard to envision the detachment continuing into the NSR. As to the western fault, if we are constrained by the detachment then it is what is.

Is the fault trace problematic because it puts some of the AFT/ZFT data on the wrong side of the fault (I can't see the sample locations in that one as well)? There is only one sample in the upper plate and I thought I excluded that sample but I must have missed it because I see it hanging out in the basin.

[cossatot]

Ok, if that is more accurate, good. Here is the same view (basically) with the samples made a lot bigger.

Let me know what corrections we need to make; particularly whether the samples in the north are too far from the fault.

Keep in mind also that the only thing we care about is making the spatial/structural relationship between the samples and both faults as accurate as possible (the western fault will probably affect the ages a lot). We really aren't trying to say that we have picked the most accurate location of the fault all over the model or outside its borders. So we don't really care if the fault is in the wrong place for the NSRD or in the hanging wall basin where there are no samples, because we're not trying to predict the thermal history of anywhere in the model but exactly at the sample locations.

[eqsarah]

Is there anywhere in the program now that has the distances in numbers rather than just the visual? If so, I have EMiller's distances for the AFT/ZFT data from the SSRD/my distances from the fault for rough rate calculations, so the actual distance number will be easier to compare than visually. If not no worries.

[cossatot]

no, it's just visual. But you could maybe make a little plot and figure it out.

[eqsarah]

Ok, that would work. Is there a scale bar somewhere in the image that got cut off for the x distance? Or do you know the length of the red arrow for the x axis in the left hand corner?

[cossatot]

Uhhh... I wouldn't try to base anything off of that image. Because it's 3D-ish, there's some amount of distortion present. The model is about 40 km north-south. Maybe plot the (x,y) location of the samples in excel or whatever and then project to the E however far, and find the line that way?

[eqsarah]

Um, yes you're absolutely right (haha, good job Sarah).

I already have all the x,y locations of the samples in excel, but I don't really know how to get the coordinates of the fault defined in the same x,y space (I have the fault in x,y space based on the cross section, but it is right near my samples, and so the distance to the fault will not be appreciably different from what I have already measured using the map the cross section was created from to begin with). Hmm...I'll muse about this but I am not sure that we are going to get this much better.

One of the issues I'm tossing around is the two points in the SE part of the range should both be in the lower plate. To accommodate that then the fault needs to be either stepped out into the basin and the fault needs basically parallel the range front. This would cause my sample transect to be farther in x distance from the fault than is correct. So would it make more sense in that case to not use those samples (2 AFT ages from Miller) because letting two samples control the location of 19 other samples seems nonsensical. In which case, the only thing we have to worry about are the samples at the northern end of the range. Which, the same logic could be applied to not let samples at the edges of the range control the main transect distance from the fault (which I think is expressed well by the current location of the fault). What do you think?

[cossatot]

When I said (x,y) this time I meant (lon, lat), not in cross-section coordinates, just to be clear.

If we can't fit all of the samples because the fault geometry changes too much, there are two options.

A) get rid of some of the samples. You will have to find a balance between the location, quality, and 'predictive ability' of the samples--i.e. if some of them have both zircon and apatite, they are more valuable than either zircon or apatite by themselves; or if some of them have a partial retention zone, or whatever.

B). We do two smaller models to incorporate everything. This can actually be fine because the model run time becomes much much less when we cut out a bunch of the mesh (it calculates temperature and everything else at every point in the model). It also lets us evaluate along-strike changes. But it's not worth doing if there are just two AFT samples hanging out way up north or whatever.

If we are going to cut out some samples, we can probably trim the model as well.

[eqsarah]

Oh, that does make it more clear (I blame the face pain), and I can do it in arc then to check the northern samples to see how far they are off compared to Emiller's reported distances.

To make the model smaller: I think that we could conceivably cut those two samples out in the south for sure, and the one sample way out in the basin (I will send you an updated spreadsheet). I don't think we can fix the fault down there unless we change the strike geometry (or we could do the two models).

[eqsarah]

Ok, so hopefully I don't have the dumbs going on today (finally ate some solid food, yay!), and will show that I am capable of working on this model:

(1) Chatted with our structural geologist (Wanda Taylor) about the corrugation/mineral elongations to define slip direction to get strike of fault. She had me redo the corrugation axis calc in a slightly different location (was on a weird side as you suspected). Ended up with 007 strike from corrugation (much better) which is in good agreement with the 015 strike of fault defined by the mineral elongation. Since these strikes seem reasonable, wanted to then check to see if the distance to the fault for EMiller's samples in the northern part of the range made sense (as we discussed yesterday)....

(2) Miller et al. (1999) never reports the values (just says they measured to edge of the range) for distance between sample location and the SSRD. The paper does have a plot with ages v. distance (figure 7), so I was able to take this into plot digitizer and get a rough idea of distance "east of outcrop of Snake Range Fault" for a specific data point (only identified by AFT age, not sample name unfortunately). I used arc to calculate distance of the AFT samples (by tying to the sample location through the age of the sample) to the 015 strike for SSRD from mineral lineations and 007 strike for SSRD from the corrugation (I have sent you an email with an excel spreadsheet comparing these values if you're interested). The location of the SSRD as currently defined (015 strike) and the newly calculated 007 strike consistently significantly under predicts how far the SSRD is from samples when compared to the data plotted in Miller et al. (1999). As I understand from you, it is important to make sure the samples are in the correct position relative to the fault, and given this comparison I do not think that any of the strike lines we have batted around are correct. That being said, I do not think that we will ever get it to really agree with the data shown by Miller et al. (1999) because they only defined the distance to the SSRD by measuring to the edge of the range front--that is certainly not the location of the SSRD in many locations. Based on all of that, I think the most defendable, and least wrong location is that 015 strike that I sent along the other day. Whew....that was a lot of typing to just say that I've come to the conclusion that we are as right as possible.

(3) With that in mind I sent you another thermo data input sheet with the two samples that landed in the HW of the SSRD removed, and the sample that landed in the HW of the western fault removed.

(4) Wanda pointed me to another resource about the strike of the western fault since she thought it was a very recent fault. I'm sure you know about the Quaternary fault and fold database, but I did not--glad I checked it out though, the western fault is reported with last movement in the Pleistocene, and a strike of 357 (although to be fair hasn't been studied in any great detail, attached the report to the email). Not in agreement with the strike of the SSRD (015), but not WAY off either. I think that it is prudent to go with the strike of 015 based on the SSRD rather than this western fault. It seems to be really young (Pleistocene movement at least, but can be older), and it does cut the footwall of the SSRD so it has to be at least younger to some degree (as I said Wanda also thinks it's much younger than the SSRD).

I'm glad that you maybe stuck (at least half way stuck) through that loooong explanation that basically ended in the same conclusion as yesterday.

[cossatot]

Sarah,

First, re comment length: that only took about 30 seconds to read. Writing long comments helps the writer clarify her thoughts, and they really only take the reader a couple of seconds more, so by all means, go for it.

Second: If this fault geometry is good, great. You are right in that none of the fault geometries we have suggested are 'correct'. We just need to go with 'best'. It will always be a gross approximation. One additional thing to keep in mind is that the two pictures above have the exact same model parameters except for the fault strike, and the age results (colors of topography) are still very close.

Third: The way the modeling works, the only purpose for having the samples there is to make sure Pecube predicts ages at those samples for comparison. We don't have to use them when evaluating the results. Basically we will run a ton of models and then choose the ones that fit the data the best, based on whatever criteria is most appropriate. So we lose nothing by keeping the samples in the file, so I will do that. We can throw them out after the fact.

[eqsarah]

Sounds great!

Thank you for the "pep talk" about writing things out too--It's just that I am trying to be really conscience that you have lots of other things pulling on your time, while this is my main time sink right now (I also know you know how to manage your own time). So thanks!

[cossatot]

Nothing is a worse time waster than doing a shitty job on something...

A very different style of comment about the relative timing of the faults:

In the absence of very unambiguous cross-cutting relationships (which I have not seen, but I haven't scraped the literature), I am skeptical that there is solid evidence that the western fault began much later than the SSRD. I think that Jon Spencer's model of MCCs being these exhumed antiforms of warped detachments with a breakaway one or two ranges over is extremely influential and may be correct for some detachments, but I think it is the exception, not the rule. The COCORP line as interpreted in Gans 87 makes it look really unlikely that the Schell Creek Fault is the breakaway for the N/SSRD, because even if the detachment bows back under the Spring Valley range, the Schell Creek Fault cuts the shit out of it, at high angle. It looks much more like the Spring Valley is a graben that is tilted to the west, and that's what's being picked up in the seismic. But maybe the relationships is more clear in the original seismic data, I'm just looking at Gans 87.

Maybe the mylonitic shear zone is really cut by the Wheeler Peak fault, which would be about as good an indication as we're likely to get. Maybe this is something to check in the field... Even so, there is still a third possiblity, one promoted by Wernicke (I think) back in the way back, where the mylonites represent this subhorizontal orogen-scale shear zone that gets exhumed by the detachment. So in this case, both the detachment and the Wheeler Peak fault would post-date the mylonites. While I have a hard time with this model, it has been promoted... It also brings up a good point that you may not want to over-rely on ductile deformation (mineral elongation direction etc) to really pin down the strike of the detachment, because it may be from a different phase of deformation.

If you go and look at active low-angle normal faults, and you should (download the KML while you're at it), you will find that most don't really fit this model. Maybe Death Valley (?). But many of them have an active moderate to high angle normal fault on the backside of the horst/footwall. Oceanic MCCs also do not seem to have detachments that daylight and drop back down again, they seem to be back-tilted horst blocks.

That being said, the biggest difference between active LANFs an inactive LANFs is that the inactive ones aren't active... so if extension is ongoing, that means that these faults got locked up and then maybe got diced up by the high angle faults. So this in and of itself means that there is a much higher likelihood that a recently-active fault is younger than, and may even cross-cut, a fault that stopped slipping 10 million years earlier...

Basically, my point is that we should be skeptical of the previous ideas about the relative ages of faulting, and many other things. What we really want to do is make those ideas into hypotheses and test them.

The biggest problem here is that if the N/SSRD is just one of Nessie's humps, then there could be SO MUCH extension on the system that we would really have to test a bazillion models. It might also be difficult to do in Pecube. I will look more at it.

[eqsarah]

You're absolutely right. I really do need to be more skeptical about these geometries, because I definitely (and it's not just me, but still) think about MCC in the Spencer model.

Field Relationships? Although I have not spent a great deal of time on the western side of the range (only one canyon to collect samples), I am not sure if it is likely that the mylonitic fabric is expressed in that portion of the range. The papers I have read about the SSR mention fabric only on the east side, and do not mention fabric on the west side. The west side of the range is also more of a bitch to walk around on...it's a combination of vegetation and topography....so it's possible it just hasn't been as extensively studied. In Williams Canyon (where I collected samples on the west side) there is Prospect Mountain Quartzite (Cpm) exposed, and I walked over a lot of it. In that particular canyon the unit is not heavily deformed, and in fact maintains primary sedimentary structures. I used to have this awesome picture of asymmetrical ripples preserved in the quartzite just like they formed yesterday and not around 540 m.y.a. Anyways, if there are mylonites over there, I have not seen them yet. It would be worth it to look in the field at the locations where the Wheeler Peak Fault juxtaposes alluvium and Cpm to see if there are truncated mylonites. Wouldn't take more than a couple days to walk a fair amount of it, but it certainly can't happen for another month or so until the snow melts (although with this drought, it might not be so bad this year). Want to come out and look? ;)

All of your other comments are very insightful. I have to muse about them and go back and re-read some literature regarding the SSR/NSR with a more open mind. I glanced at the Gans 1987 interpreted line, and you're totally right.....but I know I didn't come up with that idea on my own, I must have read somewhere someone called the Schell Creek Range the breakaway. Hmm......

[cossatot]

Here's the original data: http://www.geo.cornell.edu/COCORP/ELLIPSE/COCORP/Publications/Reprint%20110.pdf