The origins and geodynamic implications of mid-lithospheric discontinuities

Abstract

Geophysical, petrological and geochemical studies have shown that the lithospheric mantle is highly heterogeneous in its physical properties and chemical compositions. They provide critical information on the evolution of the lithosphereasthenosphere boundary (LAB). Considerable efforts by geophysical approaches have demonstrated that LAB is characterized by profound changes in physical properties such as seismic velocities and electrical resistivity at an average depth of ~70 km in the oceanic areas. In the continental regions, however, the variations in the physical properties at LABs are not that significant and their depths vary greatly with tectonic environments (e.g., orogens vs. cratons). Midlithospheric discontinuities (MLDs; ~60 to 120 km depths) have been equally well documented by various techniques, including deep seismic reflections and refractions, receiver functions analyses of teleseismic Ps and Sp phases, teleseismic and ambient-noise based surface-waves tomography. Except in some regional cases, MLDs are usually featured by reduced velocities (i.e., negative velocity gradients). It appears that MLDs are statistically correlated with the low-resistivity layers, as imaged for the European continent. Many hypotheses have been proposed to explain the origin of MLDs, including the accumulation of volatile-bearing assemblages due to metasomatism of magmas/fluids and/or metamorphism; the change of deformation patterns of rocks in the lithospheric mantle and finally a "frozen" LAB. Here we review the related advances and try to reconcile available data and models. We first provide a brief introduction about the discovery of MLDs, by summarizing their velocity, radial and azimuthal anisotropies as well as their internal structures across Australian, North American and Euro-Asian continents. Then we discuss the possible origin of MLDs, by considering mantle heterogeneities in mineralogy and petrology and in particular the potential role of volatile-rich minerals such as phlogopite, amphibole and carbonates. Finally, we conclude that MLDs may be frozen LABs. It sheds light on the potential importance of MLDs in Earth's evolution and especially the global tectonic and the changes over time. We present a working hypothesis using a dynamically-evolved LAB model. MLDs began as oceanic LABs, with their depths increasing gradually from the mid-ocean ridges to subduction zones due to change in temperature and age. During continental accretion, the "inherited" oceanic LAB merged with the continental lithosphere and became deepened along with the cooling of the new continental lithosphere. In brief, the old LAB was "frozen" into the continental lithosphere and produced MLDs. Several possible mechanisms supporting this hypothesis are proposed, and the conditions that are tenable from recent results of geodynamic modeling studies in Africa are presented. The advantages of this hypothesis are highlighted. Its potential in explanation of scientific puzzles of solid earth is evaluated.