Tuesday, May 27, 2008

Geology on the Road: Highway 50 #3

Keystone Junction, Nevada, Revisited:

The fault and fold exposures described earlier, are right at Milepost 62, on the east side of Highway 50, just south of Keystone Junction.

The geology of the region is complex, and origin of these outcrop-scale folds are not understood by myself at present. Low-angle faults in the region are often taken to be low-angle normal faults, especially in the Egan Range north and south of Highway 50, and also especially to the east in the northern Snake Range (I'm skipping the intervening Schell Creek Range, here, which also has many low-angle faults).

The regional map of the area - in NBMG Bulletin 85, Geology and mineral resources of White Pine County, Nevada, Part I: Hose, Blake, and Smith (1976) - shows all low-angle faults as "low-angle faults," not distinguishing between reverse or normal faults. A more local map of the area -USGS Quadrangle Map GQ-1085, Geologic map of the Ruth quadrangle, White Pine County, Nevada: Brokaw, A.L., Bauer, H.L., and Breitrick, R.A. (1973) - shows the faults near Keystone Junction as "thrust faults." Both maps were published prior to general acceptance that many of the low-angle faults in the region are extensional in origin or detachment-related.

In the above portion of the Ruth Geological Quadrangle Map, the bright pink "Try" is described as middle Tertiary rhyolite flow and intrusive rock , the red "Km" is described as Cretaceous intrusive rock of 103 to 123 Ma. The purple "MPe" is the Ely Limestone, of mostly Pennsylvanian age.

I don't really know about these particular faults, and I don't really know about the folds, either! The authors below describe and discuss the extension in the region and in the immediate area.

Gans, et al
Seedorf and Maher
many others

I have not found much in the accessible online literature pertaining to thrust faults in the eastern Nevada region, although the older literature refers to all low-angle faults in the area as thrust faults. Peter Misch was key in recognizing or describing many of the low-angle faults of the region - some of the history of his work is described here. This eastern Nevada area generally lies between the Central Nevada Thrust Belt (of Paleozoic age) and the Sevier Fold and Thrust Belt (of mostly late Mesozoic age). Here is some info about thrust faults, including a diagram showing the location of the Sevier Thrust Belt; another diagram here gives a more generous width to the thrust belt.

And then there's the Jurassic "Elko Orogeny"...


Kim said...

Pete DeCelles did a nice job compiling all the data (as of 2004) about the timing of thrusting from late Jurassic to Eocene in the western US.

(DeCelles, P., Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western USA: American Journal of Science, v. 304, p. 105-168.)

The work he cites on thrust faulting in Nevada is by Speed (1977, 1978, 1983), Oldow (1983, 1984, 1990, 1993), Wyld (2001, 2002), and Taylor and others (2000).

There's also work on the metamorphism of the area north of there (Ruby Mts, Pequop Mtns) by Al McGrew and Phyllis Camilleri (and others) - they've got metamorphic evidence for when the shortening/thickening vs extension occurred. And then there's a recent paper in GSA Bulletin by Wells and Hoisch about Cretaceous extension in Nevada. (I need to blog about this, because I think it's interesting. But I need to figure out how to explain the Laramide orogeny in a way that's both accurate and clear and reflects all the controversies around it, and I'm not sure I'm up to it at the moment.)

You could probably get maps that Eric Seedorf and Phil Gans made, if you e-mailed them. (I think they both did their dissertations in that area, and they would have good reasons for telling thrust faults from normal faults.)

coconino said...

Best to add these to your B&R extension cite collections (there's more, I'm sure, but these are ones I know):

Wernicke, B.P. and Axen, G.J., On the role of isostasy in the evolution of normal fault systems: Geology, v. 16, p. 848-851.

Wernicke, B.P., Axen, G.J., and Snow, J.K., Basin and Range extensional tectonics at the latitude of Las Vegas, Nevada: Geological Society of America Bulletin, v. 100, p. 1738-1757.

coconino said...

More: I think Wernicke has moved on to other things, but Axen has still done field camp near Pioche and at least some work in the B&R in more recent times.

MJC Rocks said...

I enjoyed seeing that spot again. UNR field camp mapped the intrusion and the faults in a general way as one of the first exercises back in 1984 when I was lab assistant. It is always fun to hunt for the garnets, too!

Silver Fox said...

Thanks for all the references, Kim and coconino. And Kim, a Laramide blog post would be great, although I agree, it could be difficult!

MJC, I'm glad you enjoyed revisiting the area through these posts. I'm hoping that my little posts about roadside geology (so far mostly highway 50) will be interesting to travelers as well as to us geo-types.

In my travels so far, I haven't driven off the highway to the garnet fields, but have that on my list of places to go and things to see.