Monday, August 13, 2012

Sawtooth Dike

Backing up somewhat from Midas on the Tuscarora Loop, one of the first places of interest on the way to Tuscarora from the Golconda side — besides the Midas Trough itself, which the Midas Road goes through to get to the turnoff to Midas — is a spectacular rhyolitic dike, Sawtooth dike (Zoback and Thompson, 1978), named after Sawtooth Spring (MSRMaps location).
Heading eastward into the Midas Trough on the Midas Road, the Owyhee Bluffs in the distance on the left.
Sawtooth dike running up the face of the Owyhee Bluffs.

This dark-colored, rhyolite dike is a feeder for a thick section of rhyolite flows that immediately underlie the rhyolite to rhyodacite volcanic rocks of Jake Creek Mountain, the latter of which cap this portion of Owyhee Bluffs. The older rhyolite flows fed by Sawtooth dike are, in aggregate, 10 to 400 m thick (more than 30 to more than 400 feet thick).
Air photo of Sawtooth dike (from MSRMaps).
A bit of geology: dike sections in turquoise, fault in blue.

Sawtooth dike is in two main sections, offset by a small left-lateral fault, with a small portion caught up along the fault. According to Zoback and Thompson (1978), the dike is a syntectonic intrusion, syntectonic with extension producing diking in the northern Nevada rift and with extension-related strike-slip faulting (a little more about the northern Nevada rift here). The south end of the dike is reportedly cut off by the north bounding fault of the Midas Trough (Rowan and Wetlaufer, 1973, per Zoback and Thompson, 1978).
Another view of the dike, from Zoback and Thompson (1978), used here as per GSA fair use policy. (Thanks, Andrew Alden!)

View Midas Area in a larger map

By the way, the entire road from its turnoff from I-80 or old Highway 40, through Golconda and out along what is now S.R. 789 to the current road usually called "the Midas Road," past Midas and out to Tuscarora, was known as S.R. 18 prior to the 1976 renumbering program (see this 1975 state roadmap). I'll be listing all non-40, non-I-80, non-789 roadside geology (and other) posts related to the Midas Road under the tag "18" as below.

A Few References:
Rowan, L. C., and Wetlaufer, P. H., 1973, Structural geologic analysis of Nevada using ERTS-1 images: A preliminary report, in Symposium on significant results obtained from the Earth Resources Technology Satelite-1, Vol. 1, Technical representations, Sec. A: Natl. Aeronautics and Space Adm. Paper G-20, p. 413-423.

Wallace, A.R., 1990, Geologic map of the Jake Creek Mountain quadrangle, Elko County, Nevada: U.S. Geological Survey, Geologic Quadrangle Map GQ-1672, scale 1:24000.

Zoback, M.L., and Thompson, G.A., 1978, Basin and Range rifting in northern Nevada: Clues from a mid-Miocene rift and its subsequent offsets: Geology, v. 6, no. 2, p. 111-116.

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