Basalts, water, or shear zones in the lower continental crust?

Abstract

Bright, sub-horizontal, laterally extensive, multi-cyclic layered reflections have now been recorded in the lower continental crust from many areas. In contrast, this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere. With the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering.Qualitative explanations for lower crustal layering abound and include igneous, metamorphic, structural and fluid related models. Measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models. Structurally induced anisotropy, metamorphic layering, or free fluids cannot realistically produce sufficiently bright reflections. Igneous layering or the juxtaposition of pre-existing heterogeneities by shear zones can produce strong reflections.Results from crustal xenoliths, helium-isotope ratios, palaeo-geothermal gradients and chemistry of flood basalts,together with numerical modelling of lithospheric extension and asthenospheric melting, suggest that underplating and intrusion of the lower crust with mantle-derived mafic igneous rocks is common. The apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections, suggest that underplating, related to extension and/or elevated mantle temperatures, is a good explanation for lower crustal layering.