The role of tectonic shear strain on the illitization mechanism of mixed-layers illite–smectite. A case study from a fault zone in the Northern Apennines, Italy

Abstract

The influence of tectonic strain on the diagenetic degree and illitization process of mixed-layers illite-smectite at shallow crustal conditions was studied. For this purpose, the modal composition of clay fraction and illite FWHM parameters of argillites deformed by a regional-scale fault zone were studied in detail by XRD, chemical analyses and by SEM observations. Analyses were performed on deformed samples of the fault rock and compared with the non-deformed rocks off the fault zone. In addition, this paper reports a detailed comparative analysis of deformed (shear surfaces and cleavage domains) and non-deformed domains (lithon cores) of a scaly fabric in the fault rock. A systematic increase in illite concentration, a decrease of Kubler index and FWHM((002)) values, and an enrichment of K(+) ions were observed in cleavage domains with respect to the non-deformed sediments off the fault zone and the lithon cores within the fault rock. Migration of K(+)-rich fluids along scaly cleavage domains causes progressive conversion of smectite-rich I-S to illite-rich I-S and thickening of illite crystallites along the c-direction. Changes in mineralogical and crystallographic parameters, therefore, seem to be strongly controlled by shear plane development in highly sheared rocks

Late Oligocene–early Miocene olistostromes (sedimentary mélanges) as tectono-stratigraphic constraints to the geodynamic evolution of the exhumed Ligurian accretionary complex (Northern Apennines, NW Italy)

“…These minerals may have undergone low-temperature illitization processes in the fault zones by fluid diffusion and tectonic shear strain with a further significant water release. These processes have been described by Dellisanti et al (2008) in fault zones at relative low burial (i.e., less than 2 km), which is comparable with that of the studied areas. The combined effects of fluid expulsion by both dewatering of sandstones and mineral transformation and its migration along fault surfaces may have induced significant fluid flow with consequent overpressure likely causing dike injection (Codegone et al 2012b).…”

Orogen-parallel variability in 3D seismicity distribution, Northern Apennines (Italy): Evidence for a slab tear fault?

“…1). Such morphological and seismological discontinuities coincide with the aforementioned Livorno-Sillaro Line (LSL), which is traditionally interpreted as a system of sinistral (Bortolotti, 1966;Fazzini and Gelmini, 1982) strikeslip/transpressive faults (Dellisanti et al, 2008;Eva et al, 2005). Although univocal interpretations of the NA seismicity are still missing, there is large consensus in describing its pattern in terms of internal and external domains (e.g.…”

Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

“…Additionally, they could derive from clay mineral transformation. This mechanism has been reported for the transformation of smectite and/or mixed-layer illite-smectite to illite in other fault gouges (e.g., Vrolijk and van der Pluijm, 1999;Dellisanti et al, 2008). The shaly OPA facies contains 5-20 wt % mixed-layer illite-smectite .…”