High‐temperature embrittlement of extensional Alpine mylonite zones in the midcrustal ductile‐brittle transition

Structural and fluid inclusion analyses of two large‐displacement extensional Alpine shear zones show that embrittlement occurred at anomalously high temperatures and indicate that factors other than gradually changing temperature and pressure can exert primary control on the ductile‐brittle transition. Most rocks within the Brenner and Simplon mylonite zones, including abundant weak schists, failed brittlely by distributed faulting, fracturing, and brittle‐ductile shearing at T = 450°–575°C and P = 400–750 MPa, conditions in which plastic or semibrittle flow rather than brittle deformation is expected, even in strong orthogneiss. Embrittlement was caused by transiently(?) high fluid pressure and local bending strain rather than by temperature or pressure decrease. Mylonitization shut off permanently in the embrittled parts of the shear zones despite continued high‐ T denudation of the footwalls. However, mylonitization apparently did continue in the structurally highest ∼50 m of the shear zones where brittle structures are absent or rare. A strength contrast evolved between these late, thin mylonite zones and the stronger, deeper parts where mylonitization ended. This contrast probably reflects both weakening of the late mylonite zones and strengthening of the deeper embrittled parts, although differential stress may have ultimately increased in the former due to strain rate increase as the shear zones thinned. The shear zones probably evolved to discrete frictional faults by T ≈ 450°C and P ≈ 400 MPa (∼15 km).