ESTIMATING STRAIN RATE IN A MID-CRUSTAL SHEAR ZONE, AND ITS VARIATION AS SHEAR ZONE WIDTH CHANGES WITH DEPTH

Strain rate in ductile shear zones is commonly estimated using piezometrically-calculated stresses and experimentally-calibrated flow laws; however, to test such flow laws, strain rate must be independently constrained. We calculate strain rate at a range of depths in the well-studied, crustal-scale, normal-sense Simplon Shear Zone (SSZ) in the Central Alps, using samples from a transect extending into the exhumed footwall. Once shear zone width and deformation temperature are calculated for each sample, they can be correlated with cooling curves, compiled from a range of thermochronometers, to estimate when each sample was deformed. Because published thermomechanical modeling shows that the footwall exhumation rate changed over time, this age is needed to tie each sample to a specific exhumation rate. Assuming a constant fault dip over time, this exhumation rate can then be converted to a slip velocity along the fault and used, together with the width of the shear zone active at the time and depth that each sample was deformed, to calculate a strain rate for each sample.

Our calculations show that strain rate in the crustal-scale SSZ varies from ca. 6x10^-14 s^-1 at depths of ca. 16 km, where shear was distributed over a width of 2000 m, to ca. 1x10^-11 s^-1 at ca. 8 km, where the shear zone had narrowed to only 3 m. The slower strain rate at greater depth compares well with that predicted by several published flow laws for dislocation creep in quartz, calculated using the specific PT-stress conditions measured in the SSZ. However, most of these flow laws consistently predict slower strain rates (stronger rock) in the narrow, high-strain rate portion. This suggests an additional weakening mechanism is active in the narrower, shallower portions of the SSZ.

The change in footwall exhumation rate (from 0.71 to 1.36 mm/yr, Campani et al. 2010) had only a minor impact on the strain rate within the shear zone (less than an order of magnitude), which is significantly more sensitive to shear zone width (spans 3 orders of magnitude).