Nanoscale earthquake records preserved in plagioclase microfractures from the lower continental crust
<p>Seismic faulting causes wall rock damage, which is driven by both mechanical and thermal stress. In the lower crust, co-seismic damage increases wall rock permeability, permits fluid infiltration and triggers metamorphic reactions that transform rock rheology. Wall rock microstructures reve...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-04-01
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| Series: | Solid Earth |
| Online Access: | https://se.copernicus.org/articles/12/959/2021/se-12-959-2021.pdf |
| Summary: | <p>Seismic faulting causes wall rock damage, which is driven by both mechanical
and thermal stress. In the lower crust, co-seismic damage increases wall
rock permeability, permits fluid infiltration and triggers metamorphic
reactions that transform rock rheology. Wall rock microstructures reveal
high-stress conditions near earthquake faults; however, there is limited
documentation on the effects of a thermal pulse coupled with fluid
infiltration. Here, we present a transmission electron microscopy study of
co-seismic 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 1.3 mm thick eclogite facies
pseudotachylyte vein. Dislocation-free plagioclase and K-feldspar aggregates
in the microfractures record a history of fluid introduction and recovery
from a short-lived high-stress state caused by slip along the nearby fault.
The feldspar aggregates retain the crystallographic orientation of their
host and are elongated subparallel to the pseudotachylyte. We propose that
plagioclase partially amorphized along the microfractures at peak stress
conditions followed by repolymerization to form dislocation-free grain
aggregates. Repolymerization and recrystallization were enhanced by the
infiltration of fluids that transported Ca and K into the microfractures.
Subsequent cooling led to exsolution of intermediate plagioclase
compositions and the formation of the Bøggild–Huttenlocher intergrowth in
the grains from the fracture closest to the pseudotachylyte. Our findings
provide unequivocal evidence that the introduction of fluids in the
microfractures occurred within the timescale of the thermal perturbation,
prompting rapid annealing of damaged wall rock soon after earthquake
rupture.</p> |
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| ISSN: | 1869-9510 1869-9529 |