Layering in the lower crust

Deep seismic-reflection profiles have shown that the lower part of the continental crust often contains many strong, sub-horizontal reflectors. Various suggestions have been made about the origin of such reflections. One explanation, which does not require the invocation of any complex tectonic process, is layering of mafic/ultramafic intrusions of partial melts derived from the upper mantle. We show that the internal layering within large mafic and ultramafic igneous intrusions can form the necessary impedance contrasts with appropriate length scales to produce seismograms similar to those observed from the lower crust, and suggest that such intrusions could be responsible for the character of the observed lower crustal reflections. Such layering could be formed at any stage of crustal formation. This suggestion is consistent with earlier geochemical proposals and does not impose any new constraints on the origin and structure of the continental crust.