The control of precursor brittle fracture and fluid-rock interaction on the development of single and paired ductile shear zones

Ductile shear zones can occur as relatively isolated single structures, as arrays, or as characteristic paired zones. In continuous glaciated exposures of metagranodiorites from the Tauern window (Eastern Alps), the control of initial dilatant brittle fracture and associated fluid-rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established. Shearing occurred under amphibolite facies conditions. Fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres. Many are healed joints without shear offset. Others show minor (mm-cm), discrete dextral offset. Such brittle faults commonly display a low-angle en-échelon arrangement, with displacement transferred between discrete fracture segments by ductile compressive bridges. The geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid-rock interaction, which is related to fluid infiltration and veining along the primary fractures. With little fluid-rock interaction, reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre-existing fracture. With increased fluid-rock interaction, a bleached halo is developed symmetrically to either side of a central epidote-quartz (±garnet±calcite) vein. Ductile shear zones commonly flank this bleached zone, to develop a characteristic paired pattern. Strain is partitioned, localizing in the central fracture/vein and the flanking shear zones. Paired zones may anastomose in accordance with changes in the width of the central bleached zone, but are always symmetrically spaced with regard to the central fracture/vein. With increasing deformation, the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone, which remains preserved as a low strain region. Paired shear zones can also develop to either side of aplite dykes. Examples of characteristic paired shear zones, usually with a clear central vein, are found in many areas ranging from greenschist to eclogite facies, suggesting that the mechanism of their formation is quite general.