ALBERT

Description: A polyphasic tectonic-fluid system of a fault that involves crystalline and carbonate rocks (Hospital fault, Barcelona Plain) has been inferred from regional to thin section scale observations combined with geochemical analyses. Cathodoluminescence, microprobe analyses and stable isotopy in fracture-related cements record the circulation of successive alternations of hydrothermal and low-temperature meteoric fluids linked with three main regional tectonic events. The first event corresponds to the Mesozoic extension, which had two rifting stages, and it is characterized by the independent tectonic activity of two fault segments, namely southern and northern Hospital fault segments. During the Late Permian-Middle Jurassic rifting, these segments controlled the thickness and distribution of the Triassic sediments. Also, dolomitization was produced in an early stage by Triassic seawater at shallow conditions. During increasing burial, formation of fractures and their dolomite-related cements took place. Fault activity during the Middle Jurassic–Late Cretaceous rifting was localized in the southern segment, and it was characterized by hydrothermal brines, with temperatures over 180°C, which ascended through this fault segment precipitating quartz, chlorite, and calcite. The second event corresponds to the Paleogene compression (Chattian), which produced exhumation, folding and erosion, favouring the percolation of low-temperature meteoric fluids which produced the calcitization of the dolostones and of the dolomite cements. The third event is linked with the Neogene extension, where three stages have been identified. During the syn-rift stage, the southern segment of the Hospital fault grew by tip propagation. In the relay zone, hydrothermal brines with temperature around 140°C upflowed. During the late postrift, the Hospital fault acted as a unique segment and deformation occurred at shallow conditions and under a low-temperature meteoric regime. Finally, and possibly during the Messinian compression, NW-SE strike-slip faults offset the Hospital fault to its current configuration. A polyphasic tectonic-fluid system of a fault that involves crystalline and carbonate rocks has been inferred from regional to thin section scale observations combined with geochemical analyses. During the Mesozoic, the southern and northern segments of the Hospital fault had independent tectonic activity and hydrothermal brines ascended through the southern segment. Low-temperature meteoric fluids dominated during the Paleogene compression. Finally, during the Neogene extension, fault tip propagation connected both segments and hydrothermal brines and later low-temperature meteoric fluids circulated.

Description: The upper Burdigalian Vilobí Gypsum Unit, located in the Vallès Penedès half-grabben (NE Spain), consists of a 60-m thick succession of laminated-to-banded secondary and primary gypsum affected by Neogene extension in the western part of Mediterranean Sea. This Tertiary extensional event is recorded in the evaporitic unit as six fracture sets (faults and joints), which can be linked with basin-scale deformation stages. All fractures are totally or partially infilled by four types of new gypsum precipitates showing a large variety of textures and microstructures. A structural and petrological study of the unit allows us to establish the following chronology of the fracture formation and infilling processes, from oldest to youngest: (i) S1 and S2 normal faults sets formation and precipitation of sigmoidal gypsum fibres; (ii) S3 joint set formation and growing of perpendicular fibres; (iii) S4 inverse fault development, infilled by oblique gypsum fibres and deformation of the previous fillings; and (iv) S5 and S6 joint formations and later dissolution processes, infilled by macrocrystalline gypsum cements. The fractures provided the pathway for fluid circulation in the Vilobí Unit. The oxygen, sulphur and strontium isotope compositions of the host rocks and the new precipitates in the fractures suggest clear convective recycling processes from the host-sulphates to the fracture infillings, recorded by a general enrichment trend to heavier S–O isotopes, from the oldest precipitates (sigmoidal fibres) to the youngest (macrocrystalline cements) and to the preservation of the strontium signals in the infillings. The present study evidenced the effects of early post-rift deformation stages in an evaporite unit and their linkage to basin scale events. It has been established the parental fluids of the fracture infillings of the Vilobí Gypsum Unit and their evolution during the different precipitation events. Geochemical data reveals chemical recycling processes linked to fluid flow along the fractures dissolving the closest sulphates. Thus, each successive infilling event resulted from recycling of the preexisting gypsum.