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Science 23 June 2000:
Vol. 288. no. 5474, pp. 2195 - 2198
DOI: 10.1126/science.288.5474.2195
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Reports

Isotopic Dating of Strain Fringe Increments: Duration and Rates of Deformation in Shear Zones

Wolfgang Müller, 1* Domingo Aerden, 2 Alex N. Halliday 1

The time scales over which deformation in the Earth's crust remains localized in shear zones are poorly known, as are the associated strain rates. We have determined the longevity and rates of deformation using rubidium-strontium (Rb-Sr) microsampling dating of increments of fibrous strain fringes from a Pyrenean shear zone. The fibers grew quasi-continuously through a protracted deformation history between 87 and 50 million years ago, over a period comparable to that of an orogeny. During a short interval between 66 and 62 million years, a rise in strain rate from 1.1 × 10-15 to 7.7 × 10-15 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.

1 Institut für Isotopengeologie und Mineralische Rohstoffe, Department Erdwissenschaften, Sonneggstrasse 5, ETH Zürich, CH-8092 Zürich, Switzerland.
2 Departamento de Geodinamica, Universidad de Granada, E-18071 Granada, Spain.
*   To whom correspondence should be addressed. E-mail: wolfgang.mueller{at}erdw.ethz.ch

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