NON-COAXIAL DUCTILE SHEAR ZONE AND ITS ASSOCIATED STRAIN GRADIENT: CONSEQUENCES FOR UPPER CRUSTAL REFLECTIVITY

P.F REY*, D.M. FOUNTAIN**, & W.P. CLEMENT**

*Laboratoire des Sciences de la Terre, ENS Lyon, 69364 LYON Cedex 07 and Centre Geologique et Geophysique, USTL, 34095 Montpellier Cedex 05, France

**Department of Geology and Geophysics, University of Wyoming, Laramie, 82071 Wyoming

in: JOURNAL OF GEOPHYSICAL RESEARCH (1994), Vol.99, B3, PP. 4533-4548.

ABSTRACT

In order to simulate a normal incidence reflection profile across a non-coaxial ductile shear zone, we determined P-wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile (* 27 cm long) through a transition zone and its associated strain gradient. The ductile shear zone, developed within an aplitic leucogranite, was sampled from a kilometer-wide ductile transcurrent fault in the northern French Massif Central. Strain analysis indicates the sample experienced heterogeneous and progressive simple shear deformation; shear strain (g) systematically increases from zero in the undeformed protolith to * 30 in the mylonite. The transition zone thickness (T) is about 30 cm and the mylonite thickness (M) is about 10 cm. The amount of quartz and mica increases relative to feldspar toward the mylonite indicating that a mineralogical composition change accompanied mylonitization. Mica and quartz developed a strong crystallographic preferred orientation (CPO). In the least strained domain, seismic anisotropy is low and mean Vp is 6 km/s at 600 MPa. Anisotropy increases up to 10% and Vp decreases up to 5.35 km/s for propagation normal to the mylonite foliation through the transition zone. This systematic velocity change correlates with the increasing g through the transition zone and can be directly related to the CPO of mica and the increase in volume percent mica within the mylonite zone. These results indicate that velocity and anisotropy gradients may, in some cases, be associated with ductile shear zones and that mylonite boundaries may not represent first-order discontinuities. The reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength (l) and on the T/M ratio. Synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio T/M increases. We show that layers with second order boundaries (velocity gradients in transition zones) are only seismically detectable within a narrow thickness range. Extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is, to a first approximation, scale independent. In granitic domains, ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m. Development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low-temperature conditions in the granitic upper crust. Indeed, low-temperature strain gradient may explain the high seismic reflectivity of the upper crust in the Scandinavian Caledonides, whereas high-temperature strain gradient may explain, in part, the relative transparency of the European Variscan upper crust.

ACKNOWLEDGMENTS

We wish to thank to S. Kain for valuable help in the laboratory, and G. Barruol, J.P. Burg, M. Campillo, J.M. Caron, J.P. Gratier, D. Mainprice, Y. Orengo, A. Paul, and A. Snoke for stimulating and constructive discussions. The staff of the University of Wyoming Division of Basic Research Machine and Electronic Shops maintained the pressure vessel and assisted in sample preparation for this work. Special thanks are due to P. Thomas for indicating the precise location of the studied shear zone. We are grateful to C. Hurich, T. Pratt, and D. Snyder for their helpful reviews which substantially improve the manuscript. This study was financed by the INSU-CNRS (ATP-ECORS 891705). Maintenance of the high pressure laboratory was partially supported by NSF grant EAR-9003956 while the laboratory portion of the work was carried out.

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