Integrated field and laboratory analyses of vein assemblages from the downfaulted southern Apennines fold-and-thrust belt, Italy.

Along the downfaulted axial zone of the southern Apennines fold-and-thrust belt of Italy, ongoing work focuses on field survey of high-angle extensional fault zones, and integrated microstructural, mineralogical, and stable isotope analyses of fault-related calcite veins. Two study areas are investigated. The first one lies in the southern portion of the seismically active Irpinia region, the second one along the southern flanks of the Raparo Mt., Basilicata. There, we study Mesozoic shallow-water carbonates that first underwent thrusting tectonics, and then extension and exhumation from shallow crustal depths. Within the fault zones, we select the high-angle Slip Parallel veins (SP-veins) and low-angle Comb veins (C-veins), respectively oriented parallel and perpendicular to the fault dip.

In the Irpinia region, results of microstructural analysis of the vein assemblage indicate that the high-angle faults are characterized by veins containing blocky to elongated and fibrous calcite. Blocky calcite minerals show type I and II twinning. Furthermore, inclusion bands associated with crack-and-seal processes are also present. In line with established microstructural interpretations, blocky calcite is interpreted as post-kinematics, whereas elongated and fibrous calcite is regarded as syn-kinematics. Occurrence of type I and II calcite twinning suggests that the intracrystalline deformation temperatures in these regions falls within ca. 150^o C - 300^o C.

At the Raparo Mt., microstructural data are consistent with blocky, elongate-blocky calcite textures of both SP- and C-veins. The former veins are dominated by blocky calcite with established presence of Type I and II calcite twinning, while the latter veins occasionally show blocky calcite. This area also shows widespread occurrence of both high- and low-angle veins with microcrystalline textures, which suggest that rapid cooling of the mineralizing fluids and precipitation took place in their formation process. Common tar-rich mineralization is also observed along the low-angle veins.

Aiming at deciphering the relative timing of formation and paleo stress regimes, present work is dedicated to the detailed microscale documentation of the crosscutting/abutting relations among the different vein sets. At the same time, extraction of powder samples is taking place for subsequent geochemical analyses. Results will be key to determine the source/s of the mineralizing fluids, determination of isotopic fractionation, and amount of fluid-rock interaction. These analyses will enable formulation of valuable hypotheses regarding the modalities of ingression of the mineralizing fault fluids within the study fault zones.