Mylonites in the continental crust and their role as seismic reflectors

Abstract

Shear zones in the continental crust can accommodate much of the regionally imposed shortening or extension. An analysis of the flow patterns and flow history in active shear zones and of their geometry and orientation contributes to the understanding of large-scale crustal deformation. Information on the flow behaviour in 'fossil' ductile shear zones can be obtained from deformation fabrics in exposed mylonitic fault rocks. Unfortunately, the geometry and orientation of such exposed zones have often been modified by uplift and overprinting, masking the original aspect. However, in situ information on the shape and orientation of active ductile shear zones can be obtained from deep reflection seismograms. Provided the zones have a gentle dip, the unique homogeneous nature of mylonitic fabrics in them can generate high amplitude seismic reflections of large lateral continuity.