Tectonics and Structural Geology

Features from the Field: Dikes and Sills

Samuele Papeschi and Hannah Davies May 30, 2022

Dikes and sills intruding Dalradian quartzites, Donegal, Ireland. Photo © Prof. Alan Cooper (University of Otago).

Volcanoes are everywhere along plate margins and on hot spots on the planet’s surface. Just in the past 2 years, we have witnessed enormous explosive eruptions such as that of the Hunga Tonga volcano which released an amount of energy equivalent to hundreds of atomic bombs, and massive lava flows, as at the Cumbre Vieja at La Palma, which was estimated to have a total volume of 80 million cubic me ...[Read More]

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]

San Francisco: Where the Plates Meet

William Elder November 30, 2020

View on the Golden Gate Bridge from the Presidio of San Francisco, showing the serpentine bluffs on the foreground, Fort Point under the arch of the Golden Gate Bridge and the Marin Headlands on the far side. Credit: William Elder.

San Francisco has been a natural gathering place for people across the millennia. It is not a coincidence that this city, situated at the entrance to the largest estuary on the U.S. West Coast, owes its dramatic setting to active geology on the North American plate margin. The first people of the San Francisco Peninsula, the Ramaytush Ohlone, cared for the land here for thousands of years before E ...[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: Chevron Folds

Samuele Papeschi June 12, 2020

Chevron folds in the radiolarites from Batain, Oman. Photo © Simon Virgo via Imaggeo

Folds are among the most strikingly beautiful structures we can observe in rocks. There are several ways folds may form in rocks. For instance, folds in sedimentary rocks may develop by liquefaction of soft sediments, but the most common way to produce folds – and also my favorite – is by deformation. When rocks are compressed by tectonic forces, layers (or foliations) bend and warp, p ...[Read More]

Beyond Tectonics: How mountain building shaped biodiversity

Hannah Davies January 31, 2020

Humboldt's drawing of vegetation zones at mountains of the Andes, Tenerife, Himalaya, Alpes and Pyrenees, and Lapland, inspired by his travels through Europe and South America. Available via license: CC BY-NC-ND 4.0.

This edition of “Beyond Tectonics” is brought to you by Lydian Boschman. Lydian is a postdoctoral researcher at ETH Zürich. She has a background in geology and plate tectonic reconstructions, but now works with a group of biodiversity modelers of the Landscape Ecology group at ETH, bridging the gap between geology and biology. In her research, she focuses on the uplift history of the Andes, and ho ...[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]

Minds over Methods: Virtual Microscopy for Geosciences

Liene Spruženiece July 23, 2019

Information that can be extracted from a thin-section scan using the Virtual Microscope. All micrographs represent the same area within the thin section. From the upper left side: plain polarized light, crossed polarizers rotated by 0°, crossed polarizers rotated by 45°; from the lower left side: colour intensity map, colour saturation map, phase map. Credit: Liene Spruženiece

The next “Minds over Methods” blogpost is a group effort of Liene Spruženiece (left) – postdoctoral researcher at RWTH Aachen and her colleagues Joyce Schmatz, Simon Virgo and Janos L. Urai. The Virtual Microscope is a collaborative project between RWTH Aachen University and Fraunhofer Institute for Applied Information Technology (Schmatz et al., 2010; Virgo et al., 2016). In the ...[Read More]