Spatial and size distributions of garnets grown in a pseudotachylyte generated during a lower crust earthquake

Abstract

In the Bergen Arc, western Norway, rocks exhumed from the lower crust record earthquakes that formed during the Caledonian collision. These earthquakes occurred at about 30-50 km depth under granulite or amphibolite facies metamorphic conditions. Coseismic frictional heating produced pseudotachylytes in this area. We describe pseudotachylytes using field data to infer earthquake magnitude (M ≥ ~6.6), low dynamic friction during rupture propagation ( d < 0.1) and laboratory analyses to infer fast crystallization of microlites in the pseudotachylyte, within seconds of the earthquake arrest. High resolution 3D X-ray microtomography imaging reveals the microstructure of a pseudotachylyte sample, including numerous garnets and their corona of plagioclase that we infer have crystallized in the pseudotachylyte. These garnets 1) have dendritic shapes and are surrounded by plagioclase coronae almost fully depleted in iron, 2) have a log-normal volume distribution, 3) increase in volume with increasing distance away from the pseudotachylyte-host rock boundary, and 4) decrease in number with increasing distance away from the pseudotachylyte-host rock boundary. These characteristics indicate fast mineral growth, likely within seconds. We propose that these new quantitative criteria may assist in the unambiguous identification of pseudotachylytes in the field.

Dynamic earthquake rupture in the lower crust

“…4 reflects how processes operating at very different timescales leave their imprint in the rock structures and microstructures. Stress pulses lasting for microseconds cause a fragmentation process that precedes shear strain, while the subsequent frictional heating that causes melting may last seconds to tens of seconds ( 47 ). Metamorphic reactions in the pseudotachylyte, cataclasites, and wall rocks are driven by fluid infiltration in the wake of fracturing and permeability generation and most likely require weeks to years to crystallize the reaction products at the micrometer to millimeter scale.…”

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

“…The high‐P,T pseudotachylytes are characterized by sharp contacts to the wall rocks (Figure a), abundant dendritic garnet crystals (Figure b; Austrheim & Boundy, ) in a fine‐grained (<100 μm) matrix of kyanite, omphacite, plagioclase, K‐feldspar, zoisite, rutile, and sometimes minor amounts of quartz and sulfides (Bhowany et al, ). Injection veins into the wall rocks are common and an increasing number of smaller garnets occur toward the pseudotachylyte margins where the cooling rate of the frictional melt was highest (Clerc et al, ). Thermal modeling suggests that the frictional melt solidifies within a timescale of seconds to minutes at most, depending on melt thickness.…”

“…This suggests that the grains formed at a stage where thermal, stress or other gradients caused by the earthquake were still significant. If thermal gradients were the controlling factor, the new plagioclase grains must have formed within timescales of seconds to minutes after the fault slip (Clerc et al, ).…”

“…Note that they are concentrated near the transition from the original Proterozoic granulites to the areas converted to eclogite and amphibolite facies rocks in shear zones and breccias. (Clerc et al, 2018). Thermal modeling suggests that the frictional melt solidifies within a timescale of seconds to minutes at most, depending on melt thickness.…”

“…This suggests that the grains formed at a stage where thermal, stress or other gradients caused by the earthquake were still significant. If thermal gradients were the controlling factor, the new plagioclase grains must have formed within timescales of seconds to minutes after the fault slip (Clerc et al, 2018). Some of the micro (shear-) fracture in plagioclase furthermore show secondary development of tensile fractures with a characteristic angle to the host fracture ( Figure 6d).…”