Faults and fracture networks in the brittle upper crust are important channels for magmatically-sourced fluids in volcanic areas. Indeed, in active volcanic arcs (which are found above subduction zones) faults can significantly influence the location of volcanoes, the location and frequency of crustal seismicity, and localize geothermal reservoirs. However, interactions between faults and volcanic hydrothermal systems, especially near and below the brittle-ductile transition, remain unresolved.
Rebecca Pearce from University College London, UK, and colleagues, used a combined magnetotelluric and seismic survey to constrain 3D interactions between fault system architecture and hydrothermal fluids along the Andean Southern Volcanic Zone. Major margin-parallel active thrust faults and ancient pre-Andean transverse faults were found to be hosts to magmatic hydrothermal reservoirs at 4–8 km depth, above the aseismic boundary detected at 8–10 km depth. One of the ancient transverse faults with an impermeable fault core causes compartmentalization of a magmatic hydrothermal reservoir, leading to increased pore fluid pressures and reactivation of the ancient fault zone — despite the fault being in an unfavourable orientation for reactivation relative to the regional stress field.
While only a case study, the 3D seismic and magnetotelluric observations provide important insights into interactions between fluid over-pressure and ancient fault reactivation. Further quantification of the mechanics of fault reactivation, in combination with additional observations from other volcanic locations, are required to better understand potential related seismic and volcanic hazards.
Pearce, R. K. et al. Reactivation of fault systems by compartmentalized hydrothermal fluids in the Southern Andes revealed by magnetotelluric and seismic data. Tectonics https://doi.org/10.1029/2019TC005997 (2020)