Rock Magnetic Characterization of the Northern Apennines Intraplate Shear Zone (NW Italy): Constraining on Fluid-Rock Interaction and Physicochemical Conditions

Fluid circulation during the seismic cycle may change the physicochemical conditions along intraplate shear zones, triggering relevant thermochemical transformations and potential changes of the magnetic properties. The occurrence of seismic slip events may induce thermal decomposition or dehydration of certain phases, promoting the generation of newly formed ferromagnetic minerals. Then, magnetic minerals are suitable indicators of changes in the physical and chemical conditions and potential tracers of the fluids migrated during the evolution of megathrust shear zones. We carried out a rock magnetic investigation across an exhumed analogue of actual intraplate shear zones, cropping out in the Northern Apennines, Italy. Here, cyclical variations in permeability and fluid sources during the seismic cycle were reported based on geochemical signatures. Our results reveal variations in the magnetic properties following systematic trends with increasing distance from the main thrust. Temperature dependent remanence experiments suggest the occurrence of goethite and magnetite assemblages with variable relative abundance depending on the distance from the basal décollement. Local evidence of iron sulfides is revealed by thermal demagnetization of three-component IRMs and transformations in the thermomagnetic susceptibility curves. In addition, different generations of iron sulfides, both in framboidal aggregates and smeared along dissolution seams, were observed using SEM-EDS. Stepwise thermomagnetic experiments of the wall-rocks revealed significant thermal generation of newly formed magnetic phases above 350 °C. Heating laboratory experiments show different thermochemical transformation depending on the structural position. We propose that variations in thermomagnetic properties might provide constraints for the spatial and temporal changes of the physiochemical conditions, providing insights into the complex evolution of intraplate shear zones from the partial preservation of the diagenetic signature to the fluid related processes (alteration, leaching and neoformation of minerals) during seismic cycles.