Growth of primordial continents by cycles of oceanic lithosphere subductions: Evidence from tilted seismic anisotropy supported by geochemical and petrological findings

Abstract

Formation of the first continents belongs to fundamental questions regarding the evolution of the Earth. Though the growth of early crust is often debated, role of the mantle lithosphere that represents the biggest volume of continents is often overlooked, particularly in geologic interpretations of tectonic processes. This is mainly due to difficulties in its imaging and uncertainty in its rheology. Investigation of seismic anisotropy from propagation of teleseismic P and S waves in three dimensions (3D), i.e., with no limitation imposed on the symmetry axis orientation into the horizontal or vertical directions, provides a unique constraint on tectonic fabrics and character of past and present-day deformations of the continental lithosphere. In this paper, we collect independent findings from seismology, petrology and geochemistry to support our 3D anisotropic model of mantle lithosphere with tilted symmetry axes, derived from data of passive seismic experiments organised in tectonically different domains of Archean, Proterozoic and Phanerozoic provinces of Europe.Olivine preferred orientation, formed by mantle convection in the oceanic mantle lithosphere due to its spreading on both sides of the mid-ocean ridges, is a prerequisite for the tilted anisotropies that we model in the continents. We have explained the systematically oriented dipping fabrics in the continental mantle lithosphere by successive subductions of ancient oceanic plates and their accretions enlarging primordial continent cores. Consequent continental break-ups and assemblages of wandering micro-plates preserve “frozen” anisotropic fabrics and create patchwork structures of the present-day continents. Supporting arguments for such model arise from petrological and geochemical studies indicating that continental mantle peridotites formed in oceanic environments and became “continental” after significant thickening or underthrusting. Combining seismological, petrologic and geochemical findings can help to bridge the gulf between the different viewpoints and evoke further discussions on possible growth mechanisms of the continental lithosphere. Keywords: Continental growth, Mantle lithosphere, Seismic anisotropy, Subduction cycles, Dipping olivine fabrics, Relict oceanic subductions