P-WAVE VELOCITY ACROSS A NONCOAXIAL DUCTILE SHEAR ZONE AND ITS ASSOCIATED STRAIN GRADIENT - CONSEQUENCES FOR UPPER CRUSTAL REFLECTIVITY

In order to simulate a normal incidence reflection profile across a no ncoaxial ductile shear zone, we determined P wave velocities of sample s cut parallel and normal to mylonite foliation along a closely spaced profile (almost-equal-to 27 cm long) through a transition zone and it s associated strain gradient. The ductile shear zone, developed within an aplitic leucogranite, was sampled from a kilometer-wide ductile tr anscurrent fault in the northern French Massif Central. Strain analysi s indicates that the sample experienced heterogeneous and progressive simple shear deformation; shear strain (gamma) systematically increase s from zero in the undeformed protolith to almost-equal-to 30 in the m ylonite. The transition zone thickness (T) is about 30 cm, and the myl onite thickness (M) is about 10 cm. The amount of quartz and mica incr eases relative to feldspar toward the mylonite, indicating that a mine ralogical composition change accompanied mylonitization. Mica and quar tz developed a strong crystallographic preferred orientation (CPO). In the least strained domain, seismic anisotropy is low and mean V(p) is 6 km/s at 600 MPa. Anisotropy increases up to 10% and V(p) decreases up to 5.35 km/s for propagation normal to the mylonite foliation throu gh the transition zone. This systematic velocity change correlates wit h the increasing gamma through the transition zone and can be directly related to the CPO of mica and the increase in volume percent mica wi thin the mylonite zone. These results indicate that velocity and aniso tropy gradients may, in some cases, be associated with ductile shear z ones and that mylonite boundaries may not represent first-order discon tinuities. The reflectivity of a ductile shear zone depends on the thi ckness of the transition zone relative to the seismic wavelength (lamb da) 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 la yers with second-order boundaries (velocity gradients in transition zo nes) are only seismically detectable within a narrow thickness range. Extrapolation to thicker shear zones is based on the assumption that t he 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 i f their width is between 20 and 400 m. Development of ductile shear zo nes with strain gradients of the appropriate thickness to enhance refl ectivity is favored under low-temperature conditions in the granitic u pper crust. Indeed, low-temperature strain gradient may explain the hi gh seismic reflectivity of the upper crust in the Scandinavian Caledon ides, whereas high-temperature strain gradient may explain, in part, t he relative transparency of the European Variscan upper crust.