Fluid properties and dynamics along the seismogenic plate interface

Abstract

Fossil structures, such as exhumed accretionary prisms, are the only direct recorders of the fluids wetting the plate interface near the base of the seismogenic zone. By studying exhumed accretionary prisms, it is thus possible to determine the physicochemical properties of fluids and the geometry and dynamics of their circulation. We considered here two transects encompassing the brittle-plastic transition (BPT) zone, in the Franco-Italian Alps and the Shimanto Belt in Japan, and compared our data with a broader set of examples from the literature. On quartz that grew synkinematically at peak burial conditions, we inferred fluid properties indirectly from quartz trace-element concentrations (using cathodoluminescence [CL] imaging) and directly from fluid-inclusion composition and P-ρ-T properties (using Raman and microthermometry). At ~250 °C, quartz grew principally through fracturing and two types of quartz, a CL-brown and a CL-blue, precipitated alternately. At ~350 °C, where plastic deformation and recrystallization is pervasive, only a single, homogeneously CL-brown quartz is present. The salinity of the fluid in the inclusions shallower than the BPT is consistently of the order or lower than seawater, while salinities are very scattered deeper than the BPT and often exceed seawater salinity. The gas dissolved in the fluid is predominantly CH4 shallower than the BPT, and either CH4 or CO2 deeper than the BPT, depending on the nature of the host rock and in particular on the proportion of carbonates. Cathodoluminescence properties, salinity, and nature of the gas all point to a closed-system behavior in rocks deeper than the BPT. In contrast, shallower than the BPT (i.e., at seismogenic depths), textures revealed by CL-imaging evidence the episodic influx of an external fluid, leading to the crystallization of CL-blue quartz. The scale of the circulation leading to the generation of the CL-blue quartz, or its relationship with the seismic cycle, is still unclear. Besides, the fluid pressure recorded in the abundant water-rich fluid inclusions is systematically much lower than the corresponding lithostatic pressure, irrespective of the depth domain considered. For inclusions trapped at large depth, the low fluid pressure recorded in the inclusions reflects post-entrapment reequilibration. For inclusions trapped at shallower conditions, typically at seismogenic depths, the low fluid pressure may as well be the result of large fluid pressure drop after earthquakes.