Reasons for the high reflectivity of the lower continental crust in ultra deep reflection seismic sections

Ultra-deep reflection seismic lines (American COCORP, diverse European projects such as ECORS, some industrial surveys such as those from ION-GXT) have conspicuously shown that the lower part of the continental crust is highly reflective. Strong, short, discontinuous, sub-horizontal, wavy reflections are characteristic, imparting an undulating highly reflective pattern to the lower continental crust. Most of the times, the Moho is interpreted at the base of such reflections, at the boundary with the seismically transparent upper mantle. The other important crustal discontinuity, the Conrad, is usually interpreted at the top of such reflectors, at the boundary with the seismically transparent upper crust. In this manner, ultra deep seismic sections usually display the lower continental crust as a strongly reflective wavy layer of varying thickness sandwiched between the transparent upper crust and the transparent sub-continental upper mantle. The reasons for such high reflectivity include the development of abundant sub-horizontal ductile shear zones and the dominant sub-horizontal foliation so characteristic of exposed high-grade metamorphic rocks of the lower crust.