Tectonics and Structural Geology

Features from the Field: Sheath Folds

Samuele Papeschi and Hannah Davies October 4, 2021

Spectacular sheath folds in Cap de Creus (Spain), produced by deformation in a Variscan shear zone. Photo by Dr. Melanie Finch.

Shear zones are areas of intense deformation that localize the movement of one block of the crust with respect to another. In previous posts, we have seen that shear zones contain some very deformed rocks called mylonites, lineations that tell us the direction of movement, and useful kinematic indicators, such as S-C fabrics, that allow geologists to understand which way the rocks moved. However, ...[Read More]

Features from the Field: Stretching Lineations

Samuele Papeschi and Hannah Davies June 21, 2021

Measuring stretching lineations is the base to understanding the kinematics of large structures, like this protomylonitic metasandstone from a shear zone in the Glengarry Basin (Western Australia). Rocks slid past each other either up or down along this lineation. Which way? We need to look at a section oriented parallel to this lineation. Photo by Rolinator via wikimedia.org.

Deep beneath our feet, deformation of rocks at high temperature produces impressive structures such as shear zones, that localize the movement of two volumes of rock with respect to one another. Shear zones are strongly deformed bands with strongly foliated structures (i.e., with rocks that look like a pile of leaves) and kinematic indicators, such as S-C fabrics, that tell us geologists which way ...[Read More]

Features from the Field: Shear Zones and Mylonites

Samuele Papeschi, Sandra McLaren and Hannah Davies January 25, 2021

Dextral, ductile shear zones crosscut foliated schist at Cap de Creus (Catalona, Spain). Photo © Jordi Carreras

The San Andreas Fault in California, the Alpine Fault in New Zealand, or the Main Frontal Thrust in the Himalayas are some of the most famous and largest fault zones that accommodate the relative displacement between two adjacent crustal blocks. Such faults, however, represent only the shallower expression of something much bigger: a crustal shear zone. In the first 10 kilometers or so of the crus ...[Read More]

Features from the Field: Angular Unconformities

Samuele Papeschi October 21, 2020

The worldwide famous angular unconformity at Siccar Point, between vertical layers of Silurian graywacke (below) and gently dipping Devonian layers of the Old Red Sandstone Fm. (above). Photo © Dave Souza via Wikimedia.commons

An angular unconformity is an erosional surface that truncates older, tilted sedimentary layers and that is overlain by younger layers, oriented parallel to the unconformity. The discovery and interpretation of angular unconformities, like the famous Hutton’s unconformity at Siccar Point, Scotland, marked a paradigm shift in the geological theories of the 18th century. At that time, the theo ...[Read More]

Features from the field: crenulation cleavage

Samuele Papeschi April 27, 2020

Crenulation cleavage in schists from the Island of Elba (Italy). Note how the older foliation is folded and a new foliation developed. Photo © Samuele Papeschi

In one of the former episodes of the ‘Features from the field’ series we have talked about foliations, and how they develop when rocks are pushed together by the movement of tectonic plates. It is quite uncommon, however, that tectonic forces are active in the same direction for an unlimited period of time. The rule, rather than the exception, is that the orientation of tectonic forces ...[Read More]

Features from the field: Foliation

Samuele Papeschi August 30, 2019

Jurassic slate in La Grave (Hautes-Alpes, France). These rocks easily split along foliation surfaces that dip towards the left of the image. Don’t believe me? Try to hammer them! Photo © Samuele Papeschi

Have you ever walked on a mountain trail, passing past outcrops of rocks and noticed that many rocks appear to be split along a well-defined orientation? If you have, you might have seen one of the most important structures in metamorphic rocks – called foliation. The term ‘foliation’ derives from the Latin folium, meaning ‘leaf’. A rock with a foliation looks like a pile of R ...[Read More]

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

EGU Guest blogger August 16, 2019

Cathodoluminescence image of calcite vein in background, with 100 micron laser ablation spots overlain top right and U-Pb isochron overlain left. Image credit: Nick Roberts

For this edition of Minds over Methods, we have invited Nick Roberts, a research scientist at the British Geological Survey, working within the Geochronology and Tracers Facility (GTF) running a LA-ICP-MS laboratory. Nick has a background in ‘hard-rock’ geology, incorporating geochemistry, geochronology, and magmatic and metamorphic petrology across a wide range of tectonic settings, and is now in ...[Read More]

Features from the field: Boudinage

Samuele Papeschi May 19, 2019

Boudins of amphibolite layers (metamorphosed basalts) that were stretched within quartz schists (Norway). Photo credits © Haakon Fossen.

The Features from the Field series is back! In our previous posts, we have shown how rocks can deform during ductile deformation, producing folds. Folds very commonly develop in rocks when rock layers are shortened by tectonic forces in a specific direction. On the other hand, when layers are extended, we develop boudins. Boudins – the term comes from the French word for ‘sausage’ – are frag ...[Read More]

Meeting Plate Tectonics – Eric Calais

David Fernández-Blanco April 16, 2019

Credit: Grotzinger, J. and Jordan, T. (2014). Understanding Earth. [Plate boundaries by Peter Bird, UCLA]

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie! Meeting Eric Calais Eric Calais is Professor of Geophysics and Head of the Geosciences department at the Ecole Norm ...[Read More]

Minds over Methods: Mineral reactions in the lab

EGU Guest blogger March 4, 2019

CL image of a sheared quartz-muscovite gouge. Credit: André Niemeijer.

Mineral reactions in the lab André Niemeijer, Assistant Professor, Department of Earth Sciences at Utrecht University, the Netherlands In this blogpost we will go on a tour of the High Pressure and Temperature (HPT) Laboratory at Utrecht University and learn about some of the interesting science done there. André’s main interest is fault friction and all the various processes that are invol ...[Read More]