Isotopic dating of strain fringe increments: Duration and rates of deformation in shear zones

The time scales over which deformation in the Earth's crust remains localiz ed in shear zones are poorly known, as are the associated strain rates. We have determined the longevity and rates of deformation using rubidium-stron tium (Rb-Sr) microsampling dating of increments of fibrous strain fringes f rom a Pyrenean shear zone. The fibers grew quasi-continuously through a pro tracted deformation history between 87 and 50 million years ago, over a per iod comparable to that of an orogeny. During a short interval between 66 an d 62 million years, a rise in strain rate from 1.1 X 10(-15) to 7.7 X 10(-1 5) seconds(-1) occurred, This acceleration correlates with an abrupt change in fiber-growth direction and a stress-field inversion from gravitational collapse to renewed horizontal crustal shortening.