Reflectivity of the the crystalline crust: hypothesis and tests

Summary: The nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop. Geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms, but most geological information still comes from speculative interpretations of seismic experiments run in different regimes. Pronounced lower-crustal reflectivity, detected worldwide, is ascribed in various geological hypotheses to primary lithologic layering, to ductile strain banding, or to trapped fluids. A BIRPS deep crustal profile across the Atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone. Study of worldwide crustal profiles shows that exposed high-pressure terranes are not as reflective as in situ lower crust at high pressure, suggesting either that these granulite terranes are not representative of the lower crust or that physical properties, possibly the presence of fluids or thermally controlled ductile strain banding