Reflectivity of the crystalline crust: hypotheses and tests

Abstract

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, are more likely responsible for observed reflectivity than are simple lithologic boundaries. The argument for the importance of physical properties in causing observed lower-crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat-flow. 1. Geophysical Constraints The notion that geometric effects (thin-layer interference and focussing) in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature. Because many reflections are observed on deep crustal profiles, velocity (and density) cannot increase monotonically with depth while remaining within bounds set by refraction experiments, implying the existence of a funtion of velocity with depth that is oscillatory. While the concept, thus developed, of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust, seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity. For normal incidence, no structure embedded in a constant impedance medium gives a stronger reflection than a quarter-wavelength layer which produces an amplitude twice that due to a simple interface. Complex structures can increase the total energy reflected, but the energy is spread over longer times and the peak reflected amplitude is not increased. Observed reflection amplitudes from the lower crust vary by factors u2, so that the presence or absence of 'thin-layer interference' cannot be the only effect causing lower crustal