Behaviour of micas during seismic deformation

Earthquakes affect rocks in various ways, all of which leave traces. For example, frictional heating during an earthquake can produce a thin layer of melt on the slip surface. This melt layer solidifies quickly to a dark, fine-grained material called pseudotachylyte.

Pseudotachylytes are some of the clearest markers we have to identify fossil earthquakes in the field, but the rock next to the pseudotachylytes is often affected as well. Minerals next to the fault surface may be fragmented (by brittle and/or ductile processes) or altered. This can significantly affect the further development of the rock body, with respect to both deformation and alteration style.

Pseudotachylytes are commonly found in the Bergen Arcs, where they record seismic activity during the Caledonian orogeny. Recently, a new outcrop has been discovered, where pseudotachylytes occur in mica-rich rocks. The behaviour of micas (or sheet silicates in general) during seismic deformation is not well known and is the focus of this project.

The main objectives of the project are to:

  • understand the distribution of pseudotachylytes in the field by detailed mapping
  • characterise the host rock using polarised light microscopy and scanning electron microscopy
  • estimate the pressure-temperature conditions of deformation based on mineral compositions measured with the electron microprobe
  • analyse the deformation of micas using electron backscatter diffraction and identify which deformation mechanisms were active

With this thesis topic, you are part of the beginning of a project and will learn how to approach a new field area. You will get extensive training in modern techniques for the petrological and microstructural characterization of rocks.

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Fig. 1: Optical micrograph (crossed-polarised light) showing deformed biotite (brownish/greenish interference colours) next to a pseudotachylyte (bottom left). Click here for a bigger version.
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Fig. 2: Chemical map of pseudotachylyte (middle left, bottom right) and wall rock. The composition helps to recognise the minerals, in this case, K → mica, Na → plagioclase, Ca → epidote. Click here for a bigger version.


Tags: deformation, microstructures, petrology, earthquakes
Published Sep. 29, 2021 3:23 PM - Last modified Sep. 29, 2021 3:23 PM

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