INVESTIGATING THE CONTROLS ON FAULT ZONE WIDTH BELOW THE BRITTLE-TO-DUCTILE TRANSITION

No consensus currently exists for the structure and geometry of plate boundary-scale shear zones below the brittle-to-ductile transition. Estimates for shear zone width as a function of depth in the middle to lower crust vary significantly. Here, we present temperature, pressure, differential stress, and field-based strain rates from two crustal-scale shear zones to investigate what extrinsic or intrinsic variables control shear zone width. Data from the Simplon shear zone (SSZ) in the central Alps and Scandian thrust zone (STZ) in northwest Scotland indicate that strain rates in the STZ are upwards of one order of magnitude faster than those in the SSZ, the result of a difference in displacement rate between the two systems. In comparing the two shear zones, the effect of stress, temperature, or depth on shear zone width, and hence strain rate, seems to be negligible. These observations indicate that the primary control on shear zone width is likely a difference in rheology. In this case, the difference between the rheologies of the SSZ and the STZ translates to an order-of-magnitude difference in effective viscosities.

The footwall of the SSZ deforms granitic gneisses of the Monte Leone and Antigorio nappes, whereas the hangingwall of the STZ deforms a thick sequence of chiefly psammitic schists of the Moine Supergroup. The difference in observed rheology is likely due to a change in the dominant deformation mechanism; deformation in the Moine Supergroup is primarily controlled by quartz dislocation creep while the granitic gneisses of the SSZ are likely controlled by a polyphase mixture of quartz and feldspar. Water activity may also play an important, yet currently unconstrained, role; rocks from both shear zones experienced amphibolite facies metamorphism, but shallower regions record deformation at more hydrous greenschist facies conditions.