NON-COAXIAL DUCTILE SHEAR ZONE AND ITS ASSOCIATEDSTRAIN 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 07and 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-coaxialductile shear zone, we determined P-wave velocities of samples cut paralleland normal to mylonite foliation along a closely spaced profile (* 27 cmlong) through a transition zone and its associated strain gradient. Theductile shear zone, developed within an aplitic leucogranite, was sampledfrom a kilometer-wide ductile transcurrent fault in the northern FrenchMassif Central. Strain analysis indicates the sample experienced heterogeneousand progressive simple shear deformation; shear strain (g) systematicallyincreases 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 tofeldspar toward the mylonite indicating that a mineralogical compositionchange accompanied mylonitization. Mica and quartz developed a strong crystallographicpreferred orientation (CPO). In the least strained domain, seismic anisotropyis 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 mylonitefoliation through the transition zone. This systematic velocity changecorrelates with the increasing g through the transition zone and can bedirectly related to the CPO of mica and the increase in volume percentmica within the mylonite zone. These results indicate that velocity andanisotropy gradients may, in some cases, be associated with ductile shearzones and that mylonite boundaries may not represent first-order discontinuities.The reflectivity of a ductile shear zone depends on the thickness of thetransition zone relative to the seismic wavelength (l) and on the T/M ratio.Synthetic seismograms show that for a given seismic wavelength the reflectivitydecreases when the transition zone thickness increases and when the ratioT/M increases. We show that layers with second order boundaries (velocitygradients in transition zones) are only seismically detectable within anarrow thickness range. Extrapolation to thicker shear zones is based onthe assumption that the strain gradient thickness relative to shear zonethickness is, to a first approximation, scale independent. In graniticdomains, ductile shear zones with similar geometrical and petrophysicalfeatures to the example studied here will be detected on deep seismic profilesonly if their width is between 20 and 400 m. Development of ductile shearzones with strain gradients of the appropriate thickness to enhance reflectivityis favored under low-temperature conditions in the granitic upper crust.Indeed, low-temperature strain gradient may explain the high seismic reflectivityof the upper crust in the Scandinavian Caledonides, whereas high-temperaturestrain gradient may explain, in part, the relative transparency of theEuropean 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 Machineand Electronic Shops maintained the pressure vessel and assisted in samplepreparation for this work. Special thanks are due to P. Thomas for indicatingthe precise location of the studied shear zone. We are grateful to C. Hurich,T. Pratt, and D. Snyder for their helpful reviews which substantially improvethe manuscript. This study was financed by the INSU-CNRS (ATP-ECORS 891705).Maintenance of the high pressure laboratory was partially supported byNSF grant EAR-9003956 while the laboratory portion of the work was carriedout.

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