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.

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.

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.

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 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.

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, 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.

Fractured granitic layers in gneisses sheared during Devonian extensional collapse in the SW Norwegian Caledonides (Bergen area).

Detailed view of fracture. Note how it extends downwards into a ductile deformation structures, like a fault propagation fold or little shear zone before dying out.

Metamorphic rocks found at the surface have travelled through the plastic-brittle transition (ductile-brittle transition), where they journey from the regime dominated by (quasi)plastic deformation mechanisms to that characterized by brittle mechanisms. Rocks that deform continuously or repeatedly through that journey contain sets of structures that record this transition, with (semi-)brittle structures overprinting slightly older ductile structures.

As I am preparing a presentation for the bi-annual Norwegian winter meeting in Oslo in January I thought I would share an example of this from near Bergen, Norway. The setting is exhumation of the subducted margin of Baltica, and the pictures show ductile gneiss structures affected by semi-brittle fractures that selectively form in the stiffest (mica-poor) layers. They can be seen to extend into little ductile shear zones or small fracture propagation folds in mica-bearing lithologies.

It is typical for this transition that some layers behave brittley while others (less stiff ones) still deform ductilely. Ar/Ar mica ages (ca. 400 Ma) and U-Pb dating (ca. 396 Ma) of sphene in the early fractures show that the transition into the brittle regime happened in the earliest-Middle Devonian, while other evidence (AFT and stratigraphic evidence) shows that the rocks reached near-surface conditions in the Late Jurassic.

Students examining and calculating strain geometry, concluding that we are dealing with constriction at this particular site.

Good structural localities near campus is a wonderful thing to have. One of several good sites in the vicinity of the U of Bergen campus is the strained conglomerates of Sandviksfjellet. You will find a short description on my website under Local stuff/Geology of the Bergen Arcs. It’s a great place to bring students and structural geologists in general, because of its nearness to the town and its spectacularly deformed and well-exposed strained conglomerates.

The strain geometry changes from flattening to constrictional and back to flattening as we move through the map-scale fold structure, and one explanation is given in publication 2 of the publication list at my website.

Cobbles stretched into cigar-shaped objects.

We (Prof. Joachim Jacobs, myself and our structure class) were up there this morning, and it is always a pleasure to see these awesome rocks.

Visit this locality if you ever get to Bergen!

The hard-core structure group near the upper hinge of the folded conglomerate.

Garnet-bearing gneiss at Leblon near E end of Ipanema.

A bonus attraction for structural geologists visiting Rio is the gneisses and sheared granitic rocks that crop out several places within and near the city.

Sheared granite with large feldspar porphyroclasts, separated by shear bands. A classical S-C protomylonite. Click to enlarge!

Nice garnetiferous (kinzigitic) gneisses crop out at the E end of Ipanema (Leblon). These rocks must have seen considerable pressures and temperatures, perhaps someone can tell me more about this.

A relatively safe place to combine the beach and structural geology is the point between Ipanema and Copacabana. I stopped at the southern part of this point and found some very pretty S-C structures in the sheared granite indicating sinistral shear. Well worth seeing!

Enclaves (xenoliths) occur locally that at least vaguely suggest the amount of strain that these rocks have enjoyed.

The granitic rocks are Neoproterozoic and, according to the information I have, pre- to syn-collisional with respect to the orogeny that took place when Gondwana formed, in this particular area some 570 Ma.

Enclaves with their long axes along foliation. Hard to tell their original shape, but strain is probably somewhere between 1:5 and 1:10 I would guess (but I don’t really know).