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SUMMARY A crustal-scale N130°E strike-slip fault in the Ubaye-Argentera area, southwestern French-Italian Alps, was the locus of a seismic swarm in 2003-2004. Its reactivation is examined by a 2-D frictional fault analysis. The regional stress tensor in the vicinity of the fault is determined by inversion of focal mechanisms of the 38 events with largest magnitudes of the 2003-2004 swarm. Inversion shows that the axis of the maximum principal stress σ1 is oriented nearly horizontal and at 63° from the fault plane and that the intermediate principal stress σ2 is almost parallel to the fault plane. A 2-D analysis with a static coefficient of friction of 0.4 (consistent with the presence of phyllosilicate-rich gouges at depth) shows that the N130°E fault is unfavourably oriented and that its reactivation is possible only with pore fluid pressure excess in the hypocentral region (6-7 km below surface). A calculation based on a pore fluid factor-differential stress failure mode diagram shows that the required excess pressure is comprised between 7 and 26 MPa. The presence of thermal springs in the Argentera area indicates that the pore fluid is water. The pore water overpressure is likely achieved by fault zone compaction and hydraulic barriers. The hydraulic barriers can be provided by hydrothermal sealing of the fault damage zone and by the 1-2-km-thick sedimentary lid (marl-rich autochtonous sedimentary cover and Embrunais-Ubaye sedimentary nappes) whose permeability is lower than that of the underlying crystalline rocks. [ABSTRACT FROM AUTHOR]

Detecting the presence and geometry of crustal shear zones by geophysical methods relies on our understanding of the intrinsic parameters controlling the seismic properties of these deformed rocks, over the range of pressure-temperature conditions expected in the Earth's crust. To this end, we aimed to track changes in P-wave propagation velocity (V P), anisotropy (AV P) and S-wave splitting (AV S) across a natural shear zone using experimental and electron backscatter diffraction methods. Five samples were collected across a meter-scale shear zone developed under amphibolite- to greenschist-facies in the Neves metagranodiorite (Tauern Window, Eastern Alps). With increasing strain, seismic properties show a non-linear evolution resulting from interactions among structural, textural and mineralogical changes. Geometry and magnitude of seismic anisotropy are controlled by the interference of strongly anisotropic phyllosilicates with less anisotropic but more abundant quartz and plagioclase. The amphibolite-facies shear zone margin (> 500 °C) is free of major mineral transformations, and rather characterized by the development of S-C structure, inducing an orthorhombic symmetry to the V P distribution and up to 12 % AV P , with the fast axis parallel to lineation. AV S is maximized (6 %) in the two directions parallel to the C-planes. Further deformation under greenschist-facies (450–500 °C) induces significant crystallization of muscovite, chlorite, and albite. A dominant hexagonal symmetry develops, with a fast V P plane and a fast S-wave polarization plane along the C-planes. AV P and AV S exceed 10 %. The computed elastic tensor of the whole shear zone emphasizes the importance of the strain gradient on the seismic signature of the overall structure. In our model, the bulk seismic properties are dominated by the orthorhombic symmetry of the protomylonitic zone rather than by the ultramylonite core with hexagonal symmetry (C-plane), which may explain why seismic studies of shear zones commonly involve a fast velocity plane oblique to the layer boundaries. • Non-linear evolution of P- and S- wave anisotropy with ductile strain. • Unexpected anisotropy destruction in mylonite due to a change in mica texture. • Magnitude and geometry of seismic anisotropy controlled by metamorphic conditions. • Bulk shear zone seismic properties controlled by the S-plane dominated margins. [ABSTRACT FROM AUTHOR]