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

Author:

Müller Wolfgang1,Aerden Domingo2,Halliday Alex N.1

Affiliation:

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.

Abstract

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.

Publisher

American Association for the Advancement of Science (AAAS)

Subject

Multidisciplinary

Reference18 articles.

1. O. A. Pfiffner and J. G. Ramsay J. Geophys. Res. 87 B1 311 (1982).

2. Schmid S. M., Eclogae Geol. Helv. 68, 247 (1975).

3. W. Müller N. Mancktelow M. Meier Earth Planet. Sci. Lett. in press. Only a brief summary is given emphasizing the modifications necessary because of the peculiar sample mineralogy. After cutting the fiber increments with a microscope-mounted microdrill subsequent cleaning and weighing the quartz + chlorite microsamples were leached twice to ensure separation of silicates from residual carbonates with 500 μl of ∼8 M acetic acid in an ultrasonic bath (∼10 min) and on a warm hotplate (∼15 min) with a 500-μl rinse of water in between. Subsequently the sample was leached with warm 2.5 M HCl for ∼5 min followed by another rinse with 500 μl of water. All leachates and rinses were collected and analyzed; they contained ≤6‰ of Sr that previously resided in the silicates. After addition of a 84 Sr- 85 Rb tracer the silicate samples were dissolved in closed fluorinated ethylene propylene (FEP) vials at ∼80°C with 400 μl of concentrated HF and 10 μl of concentrated HNO 3 (3 days) and equilibrated with 400 μl of 6M HCl overnight. The solutions were scrutinized with a binocular microscope and found to be free of undissolved particles or precipitates. Carbonate samples were dissolved with 500 μl of ∼8 M acetic acid. All sample weights of PFC-2 and the silicate sample weights of PFC-1 are given according to the estimated modal abundances deduced from thick section. Measured values for the SRM987 Sr standard were 0.710292 ± 13 (1σ; static mode Faraday; 10 to 100 ng of Sr; n = 23).

4. J. G. Ramsay and M. I. Huber The Techniques of Modern Structural Geology vol. 1 Strain Analysis (Academic Press London 1983).

5. Dietrich D., Tectonophysics 170, 183 (1989).

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