TS
Tectonics and Structural Geology

metamorphism

Features from the Field: Sheath Folds

and October 4, 2021
Features from the Field: Sheath Folds
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]

TS Must-Read – Platt (1986): Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks

May 12, 2021
TS Must-Read – Platt (1986): Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks

Orogens are the locus of intense deformation and metamorphism, mainly caused by convergent tectonics and burial. Yet, deeply buried rocks – metamorphosed at high pressure (HP) – are customarily met at the surface, even in “recent” systems such as the Alps. The long-standing question is naturally “how are these rocks brought back to the surface?” At the time John Platt wrote his manuscr ...[Read More]

TS Must-Read – Riba (1976) Syntectonic unconformities of the Alto Cardener, Spanish Pyrenees: a genetic interpretation

December 14, 2020
TS Must-Read – Riba (1976) Syntectonic unconformities of the Alto Cardener, Spanish Pyrenees: a genetic interpretation

In 1976 Oriol Riba published his paper on “Syntectonic unconformities of the Alto Cardener, Spanish Pyrenees: a genetic interpretation”, and despite appearing as a niche topic at first glance, it was voted as one of the papers in our Must-Read series. A major reason for that is that the paper presents a general model for syntectonic unconformities  – an unconformity which illustrates both se ...[Read More]

Features from the field: crenulation cleavage

April 27, 2020
Features from the field: crenulation cleavage
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

August 30, 2019
Features from the field: Foliation
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]