Understanding fluid-flow during tectonic reactivation: an example from the flamborough head chalk outcrop (UK)

Flamborough Head chalks are located at the termination of E-W and N-S trending fault systems along theYorkshire coast (UK). Rock deformation is mainly expressed in Selwicks Bay where two faults, with displacements up to 20 m, are exposed and a high density of calcite vein swarms is observed. The main fault is characterized by an E-W fault damage zone composed in its center of a large calcite cemented hydraulic breccia. Cross-cutting relationships allowed to differentiate three vein generations: a network of thin pseudoparallel veins perpendicular to the stratigraphy (Group I), hydraulic breccia with typical puzzle structure of chalk pieces including crack and seals (Group II), and a third generation of calcite veins crosscutting the two previous generations (Group III). Selective sampling was performed on these three sets for petrographic observations (optical and cathodoluminescence microscopy) and for geochemical analysis including stable isotopes measurements (δ13C and δ18O, Sr isotopes) and ICPOES analyses for trace elements quantification. Geochemical analyses revealed that all 3 generations have the same chemical signature and reflect successive pulses from the same fluid source. Strontium isotope analyses show that veins reached 87Sr/86Sr ratios up to 7.11 while ratios of the chalk matrix equals 7.707. The latter is in agreement with the signature of Late Cretaceous seawater. These results indicate that the fluid originated from an external source, and reflect an open system. The radiogenic Sr-isotope ratios, combined with low iron concentration (Fe < 400 ppm) suggest fluids migrated through sandy deposits. Fluid inclusion salinities determined by microthermometry range from 0 to 12 wt% NaCl equiv. with a dominant meteoric water signal. Meteoric fluids were thus possibly stored in an underlying confined sandstone aquifer and could be remobilized due to Early Cenozoic tectonic activity. The wide range of salinities could result from mixing of the meteoric fluid with a more saline fluid or from the dissolution of salts in the subsurface. In addition to the understanding of the local paragenetic evolution of the Flamborough Head chalks, this study offers an insight into how fluid flows and mineralizes along fault zones (e.g. formation of hydraulic breccia). This may help understand fault zones behavior in subsurface tight chalk deposits.status: publishe