Across the brittle-ductile transition: the role of fluids and anisotropy

The meta-granitoids of the Zillertal unit of the Tauern window (eastern Alps) record a sequence of Alpine deformations, developed during exhumation, ranging from ductile (at amphibolite-upper greenschist facies metamorphic conditions) to brittle (at conditions close to the base of the seismogenic crust).

In the core of the Zillertal unit, the high grade deformation (stage₁) is common and localized to steeply-dipping strike-slip shear zones, mainly striking around E-W and hierarchically organized in thick (up to several metres), km-long mylonitic major shear zones (MSZs), and small-scale (mm-dm-thick) shear zones (SSZs). SSZs are strictly associated with precursor tabular heterogeneities (e.g. dykes) and fractures/veins^{1, 2}. Stage₁ deformation occurred (i) in presence of fluids, recorded by cyclic vein formation and extensive alteration haloes surrounding fracture/veins, (ii) at low differential stress, and (iii) during shortening at 345° (i.e. at a high angle to the orientation of most shear zones)³. Stage₂ deformation is recorded by very discrete, local shear reactivation of the core of SSZs and of the mylonitic foliation of MSZs. Stage₂ shear zones have a similar strike-slip shear sense as the overprinted stage₁ shear zones, but developed (i) under fluid-deficient conditions, and (ii) high differential stress.

At lower temperature the meta-granitoids were involved into 2 stages of brittle deformation (stage_3A and stage_3B). Stage_3A is represented by thin (mm-thick) cataclasites and pseudotachylyte veins formed by slip along the mylonitic foliation of MSZs with the same strike-slip kinematics of the exploited stage₁ and stage₂ shear zones. Cataclasites are not associated with any significant alteration and pseudotachylytes do not show ductile reactivation. Stage_3B is represented by a pervasive system of vertical extensional chlorite-quartz-filled veins, epidote-filled hybrid fractures and faults, that crosscut and offset stage_3A structures. The stage_3B structures are surrounded by haloes of alteration of the host rock. The mineral filling of fractures (chlorite, epidote, albite) indicates conditions close to the base of the brittle crust. The orientation and kinematics of Stage_3B structures constrain shortening as horizontal, oriented ca. N-S³.

We interpret this structural sequence as the result of deformation at decreasing temperature and, basically, under constant orientation of tectonic shortening. At ductile/brittle transition conditions yielding occurred by (i) seismic slip along the highly misoriented planes of anisotropy provided by the persistent (km-scale) foliation of MSZs, under fluid-deficient conditions and high differential stress (stage_3A); and (ii) formation of new extensional and shear fractures, that disregard previous anisotropy, under fluid-present conditions and transient low differential stress (stage_3B). This indicates that the fluid availability dramatically modifies the rock strength and the type of mechanical response of anisotropic rock systems.  

 

¹Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid–rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology 27, 645–661.

²Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780

³Pennacchioni, G., Mancktelow, N.S., 2018. Small-scale ductile shear zones: neither extending, nor thickening, nor narrowing. Earth-Science Reviews 184, 1-12.