High-temperature embrittlement of extensional Alpine mylonite zones in themidcrustal ductile-brittle transition

Structural and fluid inclusion analyses of two large-displacement extension al Alpine shear zones show that embrittlement occurred at anomalously high temperatures and indicate that factors other than gradually changing temper ature and pressure can exert primary control on the ductile-brittle transit ion. Most rocks within the Brenner and Simplon mylonite zones, including ab undant weak schists, failed brittlely by distributed faulting, fracturing, and brittle-ductile shearing at T = 450 degrees -575 degreesC and P = 400-7 50 MPa, conditions in which plastic or semibrittle flow rather than brittle deformation is expected, even in strong orthogneiss. Embrittlement was cau sed by transiently(?) high fluid pressure and local bending strain rather t han by temperature or pressure decrease. Mylonitization shut off permanentl y in the embrittled parts of the shear zones despite continued high-T denud ation of the footwalls. However, mylonitization apparently did continue in the structurally highest similar to 50 m of the shear zones where brittle s tructures are absent or rare. A strength contrast evolved between these lat e, 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 differenti al stress may have ultimately increased in the former due to strain rate in crease as the shear zones thinned. The shear zones probably evolved to disc rete frictional faults by T approximate to 450 degreesC and P approximate t o 400 MPa (similar to 15 km).