Syn-tectonic gypsum veins in the Eocene-Miocene eastern Pisco forearc basin, Peru margin

The Peruvian forearc basin system (4-20°S) is an example of double forearc basins parallel to the trench (Thornburg & Kulm, 1981). The area of the present study is the eastern Pisco Basin, an elongated NW-SE depression belonging to the Peruvian forearc system. The eastern Pisco Basin has experienced substantial subsidence during Cenozoic (Viveen & Schlunegger, 2018) and its exhumation is interpreted as being related to the passage of the subducting Nazca Ridge during the Quaternary. The basin is characterised by a wide occurrence of gypsum veins in different arrays. The field study showed that gypsum veins pervade the whole sedimentary succession, despite the absence of evaporitic layers in the area (Rustichelli et al., 2016). Gypsum veins are present with different patterns and sizes from the non-conformity at the base of the sedimentary sequences (Eocene) up to the most recent formations (Miocene). Moreover, paleontological studies carried out in the area pointed out that calcareous fossils are generally replaced by gypsum (Gioncada et al., 2018). The structural geological survey was integrated with digital-based data from 3D virtual models of key outcrops generated by means of photogrammetry techniques. The relationships between vein arrays and faults were also analysed. Furthermore, we performed a detailed analysis of veins for each geological formation and investigated the orientation, size, and minerals of the veins. Mineralogical and geochemical studies (Sr isotopes) were performed to obtain information on the origin of the gypsum. Understanding the timing and the mechanism that generated the veins may provide an important and innovative contribution to the study of the geological evolution of the area.