Korean version of Structural Geology is out

The front cover of the Korean edition

Somewhat to my surprise I received two copies of the Korean edition of my Structural Geology book this morning, so apparently it is out. This is a paper-back version, in contrast to the hard-cover English and Portuguese editions. I would perhaps have preferred a frontispiece more closely related to  structural geology, but it looks nice. The publisher of this edition is Sigma Press.

For me it is very strange not being able to understand a single word (or letter for that matter) of my own book, but I feel confident that the translation by Dr. Young-Seog Kim is good. I hope this edition will be found helpful by Korean students of structural geology.

Deformation bands, sandstones and thrust tectonics

Keystone Thrust west of Las Vegas, Nevada.

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. 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.

Plaster experiments: the formation of thrust structures

Click to see movie

Plaster is used for many purposes. In structural geology we can use it as a deformable material in a deformation box. It requires that we mix the plaster with water and do the deformation just before it solidifies.

Final result of one of the first contractional plaster experiment: the result of in-sequence (break-forward sequence) thrusting.

We have done plaster experiments in the past in Bergen (introduced by Roy Gabrielsen, now at Univ. of Oslo) and now we have started again. Some of these models are very interesting, particularly from a teaching/learning perspective (both professors and students learn from these experiments). Have a look at the first two experiments here.

Second model: Model with back-thrusts

The first model shows the development of a nice in-sequence imbricate structure, the second model shows the development of back-thrusts and a very nice master thrust that makes a ramp as the hanging wall overrides the footwall. Many useful details can be noted: the rotation of layers in the hanging wall and extension above the ramp are some.

Winter geology

Folded Caledonian protomylonites. The folding occurred during extensional reactivation of the low-angle basal décollement zone in S Norway (the basal thrust zone).

Snow is here for those of us living up north, but this shouldn’t stop us from enjoying geology and structures. I have done a lot of geology with skis on my feet, especially in the spring. This beauty of a fold is located in the Kvamskogen area east of Bergen, at a mountain called Solhellenuten. It formed when the Caledonian nappes were sliding toward the hinterland on the basal décollement during Devonian extension.

Fun around folds E of Bergen, Norway. Skier is Sigurd Fossen.

Deformation bands in Berlin

Deformation bands in the Nebra Sst at the Alte Nationalgalerie in Berlin. The bands are dipping gently to the right, but assuming bedding was horizontal suggests that they are actually very steep reverse structures.

Berlin is not exactly known for its outcrops of rocks, but has lots of impressive buildings made of rocks that may be interesting. Sandstone is well represented, and a nice sandstone caught my attention at a recent visit to the Museumsinsel in Berlin. It is the Nebraer Sandstein (Nebra Sandstone) used in Die Alte Nationalgallerie, which is full of nice fluvial(?) sedimentary structures.

The picture shows deformation bands affectingbut not discretely disrupting the lamination. Hence the bands are small shear zones, probably formed by granular (non-cataclastic) flow before the sandstone was very well lithified. Disaggregation bands is a term often used about these structures.

E-learning module in Porguguese: Introdução à geologia estrutural

The first page of the e-module. Click to open

My general structural geology e-module has been translated into Portuguese by Fabio Ramos Dias – the translator of the Portuguese version of “Structural Geology”. Available from my website or here.

Folded granitic veins

Folded granitic veins, Gilead, Maine. Photo by J. Dykstra Eusden.

Dyk Eusden (Prof. at Bates College, Maine) sent me this nice picture of buckled granitic veins. Note how the wavelength is smaller for the thin veins than for the thick vein; the good old wavelength-thickness relationship still works. The thicker vein (oriented NW-SE in this picture) seems to be the youngest and has recorded higher shortening strain than the thinner ones that run parallel to the foliation (NE-SW). So what happened?

Theory 1: All the deformation happened after the vein formation was completed. This implies shortening in two perpendicular directions, which means constriction (stretching in the direction more or less perpendicular to the picture). How to test this? Sections perpendicular to the image should show elongation parallel to hinge lines. A L>S fabric (strong lineation) may be expected, and perhaps boudins.

Theory 2: The thin veins were folded prior to the formation of the thick vein, then a second deformation folded the thick vein. The challenge is that the very strong shortening parallel to the thick vein must have affected the thin vein folds equally much. The shape of the thin vein folds looks fine as is. They don’t seem to have experienced any extreme post folding strain. Hence I personally like the first theory better. But I have no information about the third dimension, and the truth may be a much more complex deformation history.

The fold is from the early Silurian Rangeley Formation that was regionally deformed and metamorphosed during both the Silurian Salinic and early Devonian Acadian orogenies. The veins probably intruded and were then folded during the Acadian.