The Borborema Province and its gigant shear zone networks

The NE “corner” of South America, known among geologists as the Borborema province, is penetrated by beautiful transcurrent shear zone networks, with the largest ones being several tens of kilometers wide and extending for hundreds of kilometers, now limited by the Atlantic Ocean in the east and the Phanerozoic Parnaiba Basin in the west. They continue under the Parnaiba basin to form the Transbrasiliano lineament, and to the NE into the African continent as part of a shear zone system that is thousands of kilometers long. I have been lucky enough to visit the region several times and hope to continue to study some aspects of these intriguing shear zones.

The NE corner of South America (Brazil) and its African counterpart, restored to the pre-Atlantic situation. Modified from Archanjo et al. (Tectonics 2002).

In Brazil the largest shear zones separate blocks of Precambrian basement of somewhat different ages and characteristics, but it is uncertain whether they represent sutures or just intracontinental shear zones formed during the late Neoproterozoic.

Large-scale shear zone pattern in NE Brazil. Modified from Granade de Araujo et al. (Terra Nova 2014)

Magnetic maps nicely portray the extraordinary shear zone pattern in this area:

Magnetic image of part of the Borborema province, with the Patos Shear Zone as the main element. Data from the Brazilian Geological Survey. The image is also included in Chapter 1 of the 2nd edition of my structural geology book in Portuguese (Geologia estrutural).

In outcrop, some of the shear zone system formed at high temperatures with melt generation, while other parts formed under lower temperatures. Outcrop pictures shown below.

Hot part of the Patos Shear Zone (migmatitic)

Strongly sheared rocks of the Senador Pompeu Shear Zone N of the Patos

Small-scale structures are abundant, as shown here from mylonites of the S part of the Patos Shear Zone. Click images for higher resolution.

Relatively low-strain S-C development in granite along the S margin of the Patos Shear Zone.

Delta-type (rotational) feldspar porphyroclast in the Patos Shear Zone

A huge strike-slip duplex is well developed in the western part of the Patos Shear Zone, also showing up nicely on the magnetic map:

Magnetic map, strike-slip duplex, Patos Shear Zone. Geological Survey of Brazil.

The strike-slip duplex (Corsini et al. JSG 1996)

In summary, this shear zone system is one of the largest in the world and display the anastomosing patterns of shear zone systems at scales from 1000 km down to that of a thin section. It contains many hidden secrets, and definitely deserves more attention.

Read more about shear zones in:

Fossen, H. & Cavalcante, G. C. G. 2017. Shear zones – A review. Earth-Science Reviews 171, 434-455.

 

Geologia Estrutural: New edition

The second edition of the Portuguese version of my book was printed a few months ago. Not only does it contain the additions and improvements of the 2nd edition of the English version (see earlier post on this blog), but a number of photos and figures + some boxes have been replaced with Brazilian (and some Portuguese) examples. The book is printed by and available from Oficina de Textos in São Paulo. 

Animations for PowerPoint

A large number of flash animations occur in my e-modules, but they are tricky to get into PowerPoint presentations, which most of us use for presentation and teaching purposes. I ended up making gif-animations of most of them. There is no start/stop option (they loop and play forever), but a start (or stop) functionality can be faked in ppt by copying the first (or last) image and put it on top of the animation. Then use ppt to animate that picture out to reveal the animation. Free

The files are collected in three ppt files (Chapter 1-10, 12-15, and 16-21) that can be downloaded from this website: http://folk.uib.no/nglhe/StructuralGeoBookExcel2ndEd.html

Stereonet Mobile: New Stereonet app for the iOS

Stereonet Mobile for the iOS (iPhone, iPad) (available from the iOS AppStore) was made by Rick Allmendinger for both collecting data in the field and analyzing data anywhere. Like Rick’s other software, it is a completely free app that is easy to use. It contours data, makes rose diagrams, rotates data, finds angles between planes, has a cool sighting function and works seamlessly with Rick’s desktop Stereonet program. Well worth a try! More at http://www.geo.cornell.edu/…/RWA/prog…/stereonet-mobile.html and https://itunes.apple.com/us/app/stereonet-mobile/id1194772610?mt=8

Shear zone initiation and fractures

Subhorizontal fracture (vein) with some ductile bending of the granulitic foliation. Greenish zone (20 cm wide) represents eclogitized granulite. Holsnøy near Bergen, Norwegian Caledonides. This is the early stage of shear zone formation at 50 km depth in these dry rocks. The central vein is usually erased in more advanced stages of shearing.

Ductile shear zones are the results of strain localization in the crust, mostly the middle and lower crust where plastic deformation mechanisms govern. They typically nucleate on preexisting structures such as foliations, veins and fractures, or they can initiate on a new fracture that develops into a plastic shear zone as shearing progresses. Fluid infiltration along these early fractures is important, and softens the rock around the fracture. This development has been described from the Alps, Cap de Creus (Spain), SW Norway and several other places. Here I show an example of a shear zone nucleating on fracture formed under eclogite facies conditions, and a shear zone involving a sheared central epidote vein that guided the strain localization.

Shear zone from the Caledonian Jotun Nappe (S Norway) showing a central ultramylonitic core that represents an early or preexisting vein on which the shear zone nucleated.

Geology photos

In connection with the release of the 2nd edition of Structural Geology, I am posting a series of geology pictures on the facebook page of the book. Here is one of the pictures:

Folded mylonites from the Norwegian Caledonides

Structural geology, 2nd edition

The second edition of my book is printed and will be shipped out as I write. We wanted to update and add a variety of things; boxes, replacing photos, adding a few illustrations, explaining some things better, etc. The largest single change is a new chapter on joints and veins. Hence the numbering has changed from Chapter 8 (the new chapter). Apart from that, the structure of the book remains the same. Thanks to those of you who sent comments and suggestions for improvements. Feedback is extremely valuable, so please do more of that!

The new cover is from the SW coast of Portugal, where Carboniferous turbidites were  involved in thin-skinned folding and thrusting during the Variscan orogeny. A range of beautiful structures are exposed along that coast. A change from the previous cover, which was from the Swiss Alps – another beautiful area to enjoy structures and structural geology.

Let me also mention that I spent time upgrading the online resources, such as additional photos, problem sets, and e-modules. In fact, e-modules are getting a closer link with the chapters in the book, and are recommended to all users of the book. Check resources here:

http://www.cambridge.org/gb/academic/textbooks/fossen2
http://folk.uib.no/nglhe/StructuralGeoBook2ndEd.html

And finally, two very nice endorsements:

“This new edition of Structural Geology has filled in a few gaps in the excellent first edition and the author and publishers are to be congratulated on their efforts to produce a really up-to-date text in a most attractive format.” Professor John Ramsay, ETH Zürich

“This is the best textbook in this field of the past decade. Both the book and the accompanying online resources have been extended with new topics and the animated e-modules are a fantastic extra teaching resource.” Professor Roger Soliva, Université Montpellier II