Abstract
Two dextral faults within granitic gneiss in the Monte Rosa nappe, northern Italy reveal key differences in their evolution controlled by evolving permeability and water/rock reactions. The comparison reveals that identical host rock lithologies develop radically different mineralogies within the fault zones, resulting in fundamentally different deformation histories. Oxygen and hydrogen isotope analyses coupled to microstructural characterisation show that infiltration of meteoric water occurred into both fault zones. The smaller Virgin Fault shows evidence of periodic closed system behaviour, which promoted the growth of hydrothermal K-feldspar, whilst the more open system behaviour of the adjacent Ciao Ciao Fault generated a weaker muscovite-rich fault core, which promoted a step change in fault evolution. Effective crustal permeability is a vital control on fault evolution and, coupled to the temperature (i.e. depth) at which key mineral transformations occur, is probably a more significant factor than host rock strength in controlling fault development. The study suggests that whether a fault in granitic basement grows into a large structure may be largely controlled by the initial hydrological properties of the host rocks. Small faults exposed at the surface may therefore be evolutionary “dead-ends” that typically do not represent the early stages in the development of larger faults.
Highlights
Two dextral faults within granitic gneiss in the Monte Rosa nappe, northern Italy reveal key differences in their evolution controlled by evolving permeability and water/rock reactions
Fracturing creates higher permeability and the presence of fluids in a fault zone may control the mineralogy via fluid–rock reactions (Chester et al, 1993; Wintsch et al, 1995), which in turn influences the deformation mechanisms and fault rock strength (Janecke and Evans, 1988; Wibberley, 1999; Di Toro and Pennacchioni, 2005; Faulkner et al, 2008; Boulton et al, 2009)
This paper describes the relationships between mineralogy, fluid influx and deformation history on two adjacent but contrasting Alpine faults in granitic gneiss from the Monte Rosa nappe, northern Italy using a combination of field and petrographic evidence, fluid inclusion and stable isotope analysis
Summary
Granite pluton at 260 Ma (Bearth, 1952; Frey et al, 1976). The main Alpine metamorphic event climaxed at 38 Ma (Bocquet et al, 1974; Amato et al, 1999) and grades from greenschist facies in the SW to amphibolite facies in the NE (Dal-Piaz et al, 1972). 33 Ma the Monte Rosa nappe was at a depth of ~11–15 km (Berger and Bousquet, 2008) and subsequent deformation was dominated by brittle faulting (Hurford et al, 1991). The Monte Rosa nappe has experienced no subsequent regional metamorphic events; and mineral changes within the fault zones are a result of fluid–rock reactions during exhumation. Many small faults are exposed in the area, but this study focuses on two dextral strike-slip fault zones: the Virgin Fault and the Ciao Ciao Fault The former has accumulated ~ 10 cm of displacement while the latter has accumulated ~250 m of displacement (calculated by the offset of an aplite dyke and amphibolite sheet respectively, Fig. 1). Four dominant joint sets are oriented approximately 040/85 N; 090/90; 146/74NE and 010/33 W (Fig. 2)
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