The Moho as a transition zone: A revisit from seismic and electrical properties of minerals and rocks

Abstract

The seismic Moho discontinuity is not always consistent with a chemical boundary (the “petrological Moho”) that separates the mafic lower crust from the ultramafic upper mantle. We summarize the effects of pressure, temperature and metamorphism on seismic and electrical properties of lower crustal and mantle rocks. The sharpness of the Moho discontinuity is generally related with the petrological transition from olivine-poor to olivine-rich compositions. Due to serpentinizat]ion, the seismic Moho may not correspond with the petrological Moho beneath the oceanic crust and forearcs. More precise velocity–density relationships are established for typical oceanic and continental crust–mantle boundaries, as well as for serpentinization of peridotites at low and high temperatures. Combined with Poisson's ratio, the occurrence of continental lowest crust with Vp > 7.2 km/s may reflect pyroxenites and garnet gabbros by magma underplating, or eclogitization of gabbros and mafic granulites by crustal thickening. On the other hand, the “electrical Moho” is defined as the bottom of the relatively conductive continental lower crust (10 − 4 to 10 − 1 S/m). Electrical conductivity of granulites and peridotites is mainly controlled by temperature, composition (iron content and water content) and oxygen fugacity, but weakly depends on pressure. Even a small amount of hydrogen can significantly enhance electrical conductivity of olivine, pyroxene and garnet. Although amphibole and serpentine are resistive under their stability conditions, their electrical conductivity will be remarkably increased by dehydration reaction at temperature > 600 °C. Granulites and eclogites show similar conductivity of 10 − 4 to 10 − 2 S/m at 500–1000 °C, about one order more conductive than water-poor garnet pyroxenites and spinel peridotites. Therefore the depth of the electrical Moho varies with temperature and water content at the crust–mantle boundary. The discrepancy between the seismic, electrical and petrological Moho in different tectonic provinces highlights the Moho as an active transition zone in the crust–mantle system. • We summarize seismic and electrical properties of lower crustal and mantle rocks. • We discuss the petrological nature of the Moho as a velocity gradient. • We establish electrical profiles of the lithosphere in cratons, orogens and rifts. • Conductivity contrast near the Moho depends on temperature and water content.