Weakening mechanisms along Low-Angle Normal Faults in pelagic limestones (Southern Apennine, Italy): insights from microstructural analysis

Low-Angle Normal Faults (LANFs) consist of shallowly-dipping extensional tectonic structures, whose origin relates to a mechanical paradox currently debated by a number of researches. The easy slip along these faults suggests a strain-weakening process active during fault nucleation and growth. Weakening mechanisms may include: i) presence of weak minerals; ii) high fluid pressure which, causing a drastic reduction of the effective stress, and iii) dynamic fault weakening during coseismic rupture. In the Basilicata portion of Southern Apennines, LANFs have been extensively studied by geological mapping and field structural analysis. Differently, a detailed microstructural observations are not hitherto available in the geological literature. For this reason, in this note, we summarize the results of microstructural analysis carried out on fault rock samples collected from a well-exposed mesoscopic LANFs. The present work is aimed at analyzing the weakening mechanisms that took place along the study faults. The incipient study LANFs are characterized by a narrow and discontinuous damage zone surrounding a very thin fault core that include a discrete slip-surface. The offset is in the range of tens of centimeters to few meters. At the microscope scale, the sampled rocks reveal the coexistence of different structural features such as: i) pervasive shape preferred orientation defined by elongated grains of calcite, producing a distinct foliation; ii) Crush Microbreccia (CM), formed of angular clasts locally in contact with each other; iii) several Ultracataclastic Veins (UV), departing from the slip-surfaces and cutting across the slip-zone. TEM investigation reveal the presence of ultrafine to calcite-nanoparticles (<200 nm) aggregate within UV, and iv) decarbonation features, where calcite grains exhibit irregular boundaries, vacuum and vesicles, most likely related to degassing processes. Thermal decomposition results in formation of a calcite aggregate made of lenticular calcite grains, 1 to 10 micron long. This honeycomb like structure of calcite grains shows well-defined shape- (SPO) and crystallographic-preferred orientations (CPO), consistent with the shear sense. These latter microstructural features, testify that in localized area, next to the slip-surface, an abrupt increase of temperature was accompanied by dynamic recrystallization. At least, two generation of calcite veins, approximately orthogonal to the slip-surface, show systematic cross-cutting relationships with the above illustrated structural features. Overall, the investigated features form a complex structural network resulting from ductile to brittle deformation mechanisms, and microstructural analysis provides significant insight in the static and dynamic fault weakening mechanisms acting along the shear zone.