Direct measurement of strain rates in ductile shear zones: A new method based on syntectonic dikes

We describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone. The method is tested in a well-constrained shear zone: the Ailao Shan-Red River shear zone, for which global strain rates can be calculated from published fault rates. The strain rate was determined by measuring independently the shear strain (gamma) recorded by the dikes and the age (t) of dikes emplacement. The shear strain was quantified by three different methods that take into account either the stretching of the dikes or their angle variations during deformation or both of them. The values of minimum shear strains range between 0.2 and 9.7 for the less to the most deformed dikes, respectively. The ages of dike emplacement were obtained by Th-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of monazites. We obtained three groups of ages: the younger age is 22.55 +/- 0.25 Ma, the intermediate age is 26.81 +/- 0.66 Ma, and the oldest ages are 29.89 +/- 0.46 Ma and 29.93 +/- 0.38 Ma. The geochronological data are in agreement with the structural data, the most deformed dikes being the oldest. The minimum strain rates deduced from these measurements are 3 to 4 x 10(-14) s(-1), which is consistent with previous estimates of geological strain rates in ductile shear zones.