Abstract The combination of structural, geochemical and palaeotopographic data proves to be an efficient tool to understand fluid transfers in the crust. This study discriminates shallow and deep fluid reservoirs on both sides of the brittle–ductile transition under an extensional regime and points out the role of major transcurrent fault activity in this palaeohydrogeological setting. Palaeofluids trapped in quartz and siderite–barite veins record the transfer of fluids and metal solute species during the Neogene exhumation of the Sierra Almagrera metamorphic belt. Ductile then brittle–ductile extensional quartz veins formed from a deep fluid reservoir, trapping metamorphic secondary brines containing low‐density volatile phases derived from the dissolution of Triassic evaporites. During exhumation, low‐salinity fluids percolated within the brittle domain, as shown by transgranular fluid inclusion planes affecting previous veins. These observations indicate the opening of the system during Serravalian to early Tortonian times and provide evidence for the penetration of surficial fluids of meteoric or basinal origin into the upper part of the brittle–ductile transition. During exhumation, synsedimentary transcurrent tectonic processes occurred from late Tortonian times onwards, while marine conditions prevailed at the Earth's surface. At depth in the brittle domain, quartz veins associated with haematite record a return to high‐salinity fluid circulation suggesting an upward transfer fed from a lower reservoir. During the Messinian, ongoing activity of the trans‐Alboran tectono‐volcanic trend led to the formation of ore deposits. Reducing fluids caused the formation of siderite and pyrite ores. The subsequent formation of galena and barite may be related to an increase of temperature. The high salinity and Cl/Br ratio of the fluids suggest another source of secondary brine derived from dissolved Messinian evaporites, as corroborated by the δ 34 S signature of barite. These evaporites preceded the main sea‐level drop related to the peak of the salinity crisis (5.60–5.46 Ma).
L'utilisation privilegiee de l'approche tectonosedimentaire a permis de definir des calendriers de paleocontraintes, tant en domaine de faible deformation (tectonique permo-cenozoique en grande-bretagne) qu'en domaine plisse (tectonique cretacee dans l'arc de castellane, s. E. France). En grande-bretagne, il a ete ainsi definit une extension nnw-sse datee du permien au trias inferieur, une extension e-w a ene-wsw datee du trias superieur a l'oxfordien inferieur, une extension n-s a nne-ssw associee a un regime decrochant e-w a wnw-ese, tous deux dates du malm moyen au cretace inferieur puis enfin, deux regimes decrochants compressifs successifs nw-se puis n-s, respectivement dates du paleocene a l'eocene inferieur et de l'eocene moyen jusqu'a une limite superieure incertaine au cours du cenozoique. Les trois premieres extensions ont pu etre correlees avec l'evolution polyphasee du rifting atlantique, alors que les regimes decrochants compressifs ont pu etre eux rattaches aux periodes d'inversion des bassins sedimentaires lors des phases compressives d'origine pyreneenne et/ou alpine. Dans l'arc de castellane, l'etude de nombreuses failles synsedimentaires a permis de mettre en evidence un regime d'extension e-w date du neocomien au barremien inferieur, suivit des l'aptien par un regime decrochant compressif submeridien favorisant des mouvements decrochants senestres relativement importants le long de failles ne-sw a ene-wsw. Le premier regime tectonique s'inscrit dans un large mouvement de deplacement relatif senestre entre les plaques europe et afrique avec au milieu un mouvement des blocs iberique et apulien et pourrait s'apparenter a la presence du bassin liguro-piemontais. En revanche, le regime decrochant plus tardif enregistre dans le bassin du sud-est traduit la rotation du bloc iberique et le deplacement vers le nord du bloc apulien, expliquant ainsi la presence d'un rifting dans la baie de biscaye en meme temps que de la compression en autriche
Abstract Deformations observed within Quaternary alluvium in the Champagne region (Paris Basin) comprise faults, folds and soft-sediment deformation structures. Their occurrence is linked to the subjacent weathered chalk. Previously interpreted as neotectonic features, the deformations are reinterpreted as karst subsidence features or/and soil displacements due to periglacial processes. Dissolution of chalk has produced superficial subsidence, explaining the geometry of some faults and their large offsets within surface deposits. The freezing-thawing cycles in the porous superficial layers have also favoured gravity instability and deformations, and this can explain local small-scale deformations but also mass movement (sliding). The seismotectonic hypothesis is rejected, because of the absence of regional faults able to generate such large co-seismic offsets. The fault directions and the apparent vertical offsets are not homogeneous at regional scale and they are often inconsistent with the Quaternary stress field. Moreover, the rooting of faults into the basement is not documented and therefore, the neotectonic origin is very doubtful.
