28 Sep 2020

28 Sep 2020

Review status: this preprint is currently under review for the journal SE.

Nano-scale earthquake records preserved in plagioclase microfractures from the lower continental crust

Arianne J. Petley-Ragan1, Oliver Plumper2, Benoit Ildefonse3, and Bjørn Jamtveit1 Arianne J. Petley-Ragan et al.
  • 1Physics of Geological Processes, The Njord Centre, University of Oslo, Oslo, Norway
  • 2Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands
  • 3Géosciences Montpellier, CNRS, University of Montpellier, Université des Antilles, Montpellier, France

Abstract. Seismic faulting causes wall rock damage driven by both mechanical stress and thermal energy. In the lower crust, coseismic damage has important implications for wall rock permeability, the progress of subsequent fluid-driven metamorphic reactions, and rock rheology. Wall rock microstructures reveal high-stress conditions near the slip surface during lower crustal earthquakes, however, there is limited documentation on the thermal effect. Here, we present a transmission electron microscopy study of coseismic microfractures in plagioclase feldspar from lower crustal granulites from the Bergen Arcs, Western Norway. Focused ion beam foils are collected 1.25 mm and 1.8 cm from a 2 mm thick eclogite facies pseudotachylyte vein. Dislocation-free plagioclase aggregates fill the microfractures and record a history of recovery from a short-lived high stress-temperature (σ-T) state caused by seismic slip and frictional melting along the nearby fault surface. The plagioclase aggregates retain the crystallographic orientation of the host rock and shape preferred orientation relative to the fault slip surface. We propose that plagioclase partially amorphized along the microfractures at peak stress conditions followed by repolymerization to form dislocation-free grain aggregates within the timeframe of pseudotachylyte formation. The heat from the slip surface dissipated into the wall rock causing a short-lived temperature peak. Subsequent cooling led to exsolution of intermediate plagioclase compositions by spinodal decomposition within a few millimeters distance to the fault surface. Our findings provide microstructural evidence for the high σ-T conditions that are expected in the proximity of seismic faults, highlighting the importance of micro- and nanostructures for the understanding of earthquakes ruptures.

Arianne J. Petley-Ragan et al.

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Arianne J. Petley-Ragan et al.

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Nano-scale earthquake records preserved in plagioclase microfractures from the lower continental crust A. Petley-Ragan

Arianne J. Petley-Ragan et al.


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Short summary
Earthquakes cause rapid deformation that have long-term effects on the Earth's crust. We studied the most abundant mineral, feldspar, in the vicinity of an earthquake to unravel its deformation history. With microscopy we found internal nm-scale structures that indicate a history of high stress and destruction of the feldspar atomic makeup. This was quickly followed by high temperature and rapid healing of the damage. Our findings illustrate the intense conditions imposed on rocks by earthquake.