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

Arianne J Petley-Ragan (1) , Oliver Plümper (2) , Benoit Ildefonse (3) , Bjørn Jamtveit (1)

1 PGP - Physics of Geological Processes [Oslo]
2 Department of Earth Sciences, Utrecht University
3 Géosciences Montpellier

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.

Domaines: Sciences de la Terre

Arianne J Petley-Ragan, Oliver Plümper, Benoit Ildefonse, Bjørn Jamtveit. Nano-scale earthquake records preserved in plagioclase microfractures from the lower continental crust. Solid Earth, 2020, ⟨10.5194/se-2020-146⟩. ⟨hal-03206942v1⟩