Paleo-seismic and aseismic processes and the role of fluids recorded in an exhumed carbonate fault

Worldwide, fault zones in carbonates regularly host medium to large earthquakes including recent ones in the Mediterranean and Middle East. In addition to that, faults can control fluid flow by either acting as a conduit or seal for fluid pathways and should be considered in e.g., geothermal exploration. Hence, understanding the (micro-) structural evolution of these fault zones as well as fluid mediated geochemical processes involved in their dynamic deformation history allows to better address topics of societal and economic relevance ranging from seismic hazards to the exploitation of natural resources. Unfortunately, active in-situ deformation at depth is difficult to access, emphasising the need for investigations on suitable exhumed analogues.

This study focuses on the microstructural and geochemical record of a recently exposed seismogenic dextral strike-slip fault zone in the seismically active southwestern Swiss Alps. Due to excellent outcrop conditions on glacially polished rock surfaces and a wide range of preserved tectonites and associated deformation structures, this particular fault zone provides a valuable record of potential paleoseismicity in carbonates. We combined microstructural analyses with micro-chemical and isotope data in order to reconstruct the spatio-temporal evolution of high-strain domains at variable crustal levels throughout exhumation. While the microstructural record allows us to differentiate between rate-dependent brittle and viscous deformation phases, we use the geochemical fingerprint to distinguish and characterize individual fluid pulses.

Here, we present microstructural evidence of fast, possibly seismic, deformation along a principal slip zone. While injection structures containing fluidized material, suggest highest deformation rates as feasible for seismic events, repeated brittle deformation that was accompanied by the formation of cataclasites and calcite veins, hints towards fast seismic to sub-seismic rates.
We also found that newly formed calcite crystals, in veins and linkage zones, show significantly decreasing δ¹⁸O_SMOWvalues, as low as 5 ‰ δ¹⁸O_SMOW, implying an influence of meteoric water. Clumped isotope thermometry of such calcites resulted in temperatures of 65-95°C, which are approximately 100°C lower than T_max in the area. This suggests that the analyzed material did not record any potential shear heating. Moreover, the investigated tectonites have most likely formed along a retrograde exhumation path. 

In combination with detailed observations on the m- to 10er-m-scale our observations provide a dataset that allows direct comparison of different deformation processes and correlation of paleo-seismicity to fluid flow in fault zones. Further, we contribute to the longstanding discussion of differentiating microstructural evidence for seismic slip from slow or aseismic slip in carbonate hosted fault zones.