Misoriented faults in exhumed metamorphic complexes: Rule or exception?

Low angle normal faults and other weak faults are common in the metamorphic core of collisional orogens. They frequently show a phyllosilicate-rich mylonitic foliation that was reactivated under brittle conditions. Experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on shear failure and frictional sliding, eventually inhibiting the nucleation and propagation of new Andersonian shear fractures and favoring the localization of brittle failure along the pre-existing foliations. Metamorphic phyllosilicate-rich rocks may show a friction coefficient varying from 0.6, at high angles to the foliation, to 0.2–0.4, for shear along the inherited foliation. To test the influence of mechanical anisotropies on the development of non-Andersonian faults, we have applied a modified slip tendency analysis to three misoriented phyllosilicate-rich faults of the European Alps. The analysis accounts for anisotropy in friction coefficients, and has been named “Anisotropic Slip Tendency analysis”. Here we show that brittle deformation along misoriented phyllosilicate-rich foliations is more probable than the development of new Andersonian faults. The presence of a well developed network of weak, phyllosilicate-rich faults may influence the overall structural style and mechanical properties of the brittle lithosphere in collisional orogens.