Deformation bands, sandstones and thrust tectonics

Keystone Thrust near Las Vegas, Nevada.

Keystone Thrust west of Las Vegas, Nevada.

Shear-enhanced compaction bands bisected by chevron-style compaction bands, Valley of Fire State Park, NV.

Shear-enhanced compaction bands bisected by chevron-style compaction bands, Valley of Fire State Park, NV.

Porous sandstones tend to develop deformation bands during deformation rather than classical fractures found in rocks with low or no porosity. Recent fieldwork in SE Nevada allowed me to return to an area where deformation bands formed during thrusting. They formed in Jurassic Aztec Sandtone, and the thrusting is Sevier in age, with the Keystone Thrust as the main structure.

First of all, this area tells us that deformation bands form in contraction as well as extension. Second, it shows that a significant portion of bands in the Aztec sandstone are thick (cm) bands with very little shear displacement. Such bands are commonly called Shear Enhanced Compaction Bands (SECBs) and seem to require higher porosities than the more ordinary compactional shear bands. Hence they do not occur everywhere in the overthrusted sandstones. A related type of bands, compaction bands, formed at the same time. These are even more dependent on high porosity, and are scarcer.

Shear-enhanced compaction band, 3 cm thick with minute amount of shear and a compaction component of similar magnitude. Muddy Mountains, Nevada.

Shear-enhanced compaction band, 3 cm thick with minute amount of shear and a compaction component of similar magnitude. Thin cataclastic bands post-date the SECB. Muddy Mountains, Nevada.

One of the striking things about this area (Red Rock Canyon near Las Vegas, Valley of Fire and the Muddy Mountains) is how little deformation structures the sandstones express, considering that they have been overrun by thousands of tons of Cambrian and younger rocks above the Keystone and similar thrusts. Beats me. Here are a few pictures from the area that I hope you will enjoy.

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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One Response to Deformation bands, sandstones and thrust tectonics

  1. I want to start an anonymous advice blog, will it show up in google search if people try to search their problem?.

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