EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Pyroxene low-temperature plasticity and fragmentation as a record of seismic stress evolution in the lower crust

Lucy Campbell1 and Luca Menegon1,2
Lucy Campbell and Luca Menegon
  • 1School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth, UK
  • 2The Njord Centre, University of Oslo, Oslo, Norway

Seismic rupture of the lower continental crust requires a high failure stress, given large lithostatic stresses and potentially strong rheologies. Several mechanisms have been proposed to generate high stresses at depth, including local amplification of stress heterogeneities driven by the geometry and rheological contrast within a shear zone network. High dynamic stresses are additionally associated with the subsequent slip event, driven by propagation of the rupture tips. In the brittle upper crust, fracturing of the damage zone is the typical response to high stress, but in the lower crust, the evolution of combined crystal plastic and brittle deformation may be used to constrain in more detail the stress history of rupture, as well as  additonal parameters of the deformation environment. It is crucial to understand these deep crustal seismic deformation mechanisms both along the fault and in the wall rock, as coseismic damage is an important (and sometimes the only) method of significantly weakening anhydrous and metastable lower crust, whether by grain size reduction or by fluid redistribution.

A detailed study of pyroxene microstructures are used here to characterise the short-term evolution of high stress deformation experienced on the initiation of lower crustal earthquake rupture. These pyroxenes are sampled from the pseudotachylyte-bearing fault planes and damage zones of lower crustal earthquakes linked to local stress amplifications within a viscous shear zone network, recorded in an exhumed granulite-facies section in Lofoten, northern Norway. In orthopyroxene, initial low-temperature plasticity is overtaken by pulverisation-style fragmentation, generating potential pathways for hydration and reaction. In clinopyroxene, low-temperature plasticity remains dominant throughout but the microstructural style changes rapidly through the pre- and co-seismic periods from twinning to undulose extinction and finally the formation of low angle boundaries. We present here an important record of lower crustal short-term stress evolution along seismogenic faults.

How to cite: Campbell, L. and Menegon, L.: Pyroxene low-temperature plasticity and fragmentation as a record of seismic stress evolution in the lower crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7380,, 2020

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Display material version 1 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-7380, Rüdiger Kilian, 07 May 2020

    Hi Lucy,

    I have two questions:

    1) In your slide 7, do the "clustered low-angle" boundaries somehow have a relation to (100)<001> ? Could they repesent something like recovery features, tilt walls?

    2) In slide 8, I guess those are misorientation axes for boundaries with 10-60° misorientation angle: How do you explain that they are all close to <001>. Just because of the clast orientation and rigid body rotation?



    • AC1: Reply to CC1, Lucy Campbell, 07 May 2020

      Hi Rüdiger,

      Good questions,

      For slide 7, I don’t think the  ‘clustered’ boundaries are formed via tilt walls, I’ve tried some boundary trace analysis over various orientation boundaries within them and there is no clear slip system or even an obvious rotation axis for those that I tried. It could however be a recovery feature, my suggestion is that the clast edges, coming into contact with the pseudotachylyte melt, are subject to higher temperatures than the clast's centre, potentially facilitating the local activation of multiple slip systems.

      For slide 8, this (rigid body rotation) would indeed be my explanation based on the orientation of <001> relative to the elongation of the clast parallel to X - although it may be a bit of a coincidence that it was sectioned this way as it is not easy to identify the true transport/slip direction with pseudotachylytes...