Friday, May 23, 2008

Friday Fault Photos

Click photos to enlarge.

This fault can be easily accessed from Highway 50 less than 1 mile east of Keystone Junction (the turnoff to the small town of Ruth) and about 6 miles west of Ely, Nevada. Just park on the northeast side of the highway at Milepost 62 and look up (or walk up). The fault is a low-angle fault, striking about NS and dipping 30 degrees to the east, with N70E slickensides. The fault surface appears to be undulating in its overall geometry. It has Ely Limestone in the hangingwall, and possibly has Ely Limestone in the footwall - although the thin-bedded, cherty limestone in the footwall could conceivably be a different formation. As far as I know, this fault is not mapped on any major maps; I'm not sure about any thesis maps done in the area.

The first photo shows the general setting of the fault, looking to the northeast. The second photo shows a closeup of the fault, looking southward.

As seen in the third photo, the fault often has folds in the footwall. It was a little difficult to get any measurements on fold axes due to the steep nature of the cliffs. One that I did measure trended about N60W, with overturned beds dipping steeply to the northeast on the northeast limb, and beds apparently dipping rather shallowly southward on the southwest limb of the fold - but that part of the fold didn't outcrop well, and I may have been measuring the regional strike and dip of beds and not anything related to the fold itself.

Also, there may be other faults in the hangingwall, and hints of these possible structures can be seen in photos 1 and 3.

A major fault is exposed at the base of the cliff in a few places, below the fault described above. That fault shows up on local and regional maps as a thrust fault, and may be of greater offset than the one I've shown pictures of above. (The amount of offset has not been documented on either fault, as far as I know.) Exposures of that lower fault are harder to reach at the base of the limestone cliffs, and are most visible above steep, roadcut faces that can't be climbed. Standing above these steep faces requires nerves of steel. The intrusive rock that forms the footwall of the lower fault can be seen most easily by walking carefully along the highway to the north of the fault exposure seen above in photo one (that is, just off the left side of photo one).

One question I have: can the folds below the fault, as shown above, form by extension, or are they more likely to have formed by compression?

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Kim said...

Recumbent, tight-to-isoclinal folds beneath a low-angle normal fault? They would most likely be extensional. If there are any irregularities in layering in rocks within a shear zone, the layers can get folded. You would expect the folds to maybe have a sheath fold geometry, or to have their hinges rotated into parallelism with the slip direction.

It's also theoretically possible to form recumbent folds by shortening rocks vertically (that is, by horizontal extension), if the layering was steep before the extension occurred. That would imply more pure shear than simple shear, though, and my guess is that a low-angle normal fault that doesn't have a lot of stratigraphic separation on it was probably dominated by simple shear. (My grad advisor would disown me for saying that, but I don't think her models for the Basin and Range are correct.)

Silver Fox said...

Thanks, Kim. My overall impression in the area is that the low-angle faults are probably extensional, although they haven't tilted the beds right along the highway. Instead, they places shallow-dipping younger rocks on shallow-dipping older rocks.

I would think, like you said, that the fold axes should more or less parallel the slip direction - would need more measurements on faults and folds to see how close they really are!

Silver Fox said...

Thinking about it, the one fold axis that I measured might be about half way between the slip direction of the upper fault and that of the lower fault. I'll have to look into this a little more.

andrew said...

Probably you two have left me far behind, but what evidence contradicts the mapped interpretation that this is a thrust? Fox, you say in your comment (but not in your post) that young rocks are placed above older rocks. What's the age difference?

Silver Fox said...

The smaller(?) fault that I photographed places the Penn-Perm Ely Limestone on the Penn-Perm Ely Limestone (regional maps) -- and the lower, larger fault places the PP Ely Limestone on a Cretaceous intrusive rock -- so that fault is younger over older.

Evidence for low-angle normal faulting in the region to the north and south is widely accepted; in the area right along the highway, many accept that the low-angle faults are extensional. Most of the published literature refers to extension in the area:

Gans, et al


Seedorf and Maher

many others

The regional geologic map of the are shows all low-angle faults as "low-angle faults" - not attempting to distinguish between thrusts and low-angle normal faults, even in the Snake Range core complex - NBMG Bulletin 85 Geology and mineral resources of White Pine County, Nevada: Hose, Blake, and Smith (1976).

My "overall impression" in the comments above is mostly based on regional considerations.

Silver Fox said...

The Ely Limestone is now considered to be Mississippian to Pennsylvanian in age (not Penn-Perm or "PP" as in my earlier comment.