Microstructural and geochemical analyses on fibrous gypsum veins in a forearc environment: a study case of Pisco Basin (Peru) and San Joaquin Valley (California).

The presence of fibrous gypsum in the form of veins in sedimentary successions has been observed and studied in various parts of the world. These veins often develop into fine-grained rocks and are commonly associated with evaporitic strata (Warren, 2006). Gypsum veins may form as a result of unloading during tectonic compression (Ferrill et al., 2022), or hydraulic fracturing due to fluid overpressure (Moragas et al., 2013; Rustichelli et al., 2016). By analysing the geochemical composition and the array and the geometry of the veins in the field it is possible to understand the geological and geodynamical setting of the precipitation environment. The East Pisco Basin (Peru) and the Western sector of San Joaquin Valley (California) are two areas where gypsum veins are present despite the absence of evaporitic layers. The two areas present some similarities: (1) development in forearc basin setting affected by subduction process associated with the presence of subducting aseismic ridges, and (2) the gypsum veins are associated with sand injection complexes. Structural survey allowed us to better understand the macrostructural setting of the areas. We collected data of gypsum veins and sandstone dykes and their relationship faults. Microstructural and geochemical analyses were performed on the samples collected both areas. Through the study of thin sections with optical and SEM microscopy, it was possible to enhance understanding of the kinematic conditions during the period of gypsum growth. Using XRD analyses we identified trace minerals which can be significant to conduct further analyses on the chemical composition of the original fluid. In addition, isotopic analyses will be decisive in understanding the possible origin of the gypsum. Results arising from the comparison of the samples from the two study areas have the potential to enhance our understanding of fluid circulation within forearc basins and the role of subduction zones dynamics.