What controls the width of ductile shear zones?

Abstract

It is widely recognized that major brittle faults in the upper crust transition downwards into ductile shear zones that then widen with depth. However, the controls on shear zone width at any specific depth, and the mechanisms that may cause the width to change over time, are less well understood. This study therefore reconstructs the geometry and rheology of the crustal-scale, normal-sense Simplon Shear Zone between the depths of ~3 and ~ 26 km, by combining field- and microstructural mapping, quartz paleopiezometry, and published estimates for the pressure, temperature, and timing of deformation at different crustal levels. The geometry is complex, with multiple strands actively deforming at any one depth, although overall the cumulative width does increase downwards. A scaling law is developed relating shear zone width to rheology and displacement rate, assuming simple shear and neglecting stretching parallel to the shear plane, and rheological parameters for granitoids with a quartz, feldspar, or mica dominated microstructure are quantified. Localization during exhumation was primarily accompanied by hydrous retrogression of feldspar to white mica, and the associated formation of interconnected weak mica layers. This weakened the shear zone, allowing the degree of strain localization to increase (and the shear zone width to decrease) by a factor of ~4–9.