Pristine microstructures in pseudotachylytes formed in dry lower crust, Lofoten, Norway

Area of Science:

• Geology
• Petrology
• Tectonics

Background:

• Pseudotachylytes are fault rocks formed by frictional melting during earthquakes.
• Understanding their formation and preservation provides insights into seismic events and crustal processes.
• The Caledonian orogeny offers a geological record of ancient tectonic activity.

Purpose of the Study:

• To investigate the microstructures and mineralogy of feldspar-rich pseudotachylytes.
• To determine the formation conditions and cooling histories of these pseudotachylytes.
• To assess the role of fluid absence in preserving microstructures.

Main Methods:

• Petrographic analysis of pseudotachylyte samples.
• Microstructural examination of feldspar microlites and spherulites.
• Analysis of amorphous and cryptocrystalline materials in injection veins.

Main Results:

• Pseudotachylytes formed in dry granulites under lower crustal conditions.
• Slow cooling in central parts led to plagioclase and K-feldspar crystallization (microlites and spherulites).
• Rapid quenching in injection veins preserved amorphous/cryptocrystalline material.

Conclusions:

• Feldspar spherulites originated from melt crystallization, not devitrification.
• Fluid-absent conditions facilitate the preservation of original pseudotachylyte microstructures.
• This preservation offers a valuable geological record for understanding earthquakes.