Thermal expansion as a mechanism for the initial breaking of Earth's lithosphere into a global plate tectonic network

Abstract:

Models for how and why Earth’s plate tectonic cooling system initiated have not converged on a mechanism or a typical early plate scale. Nonetheless, an emerging consensus suggests that plate tectonics was preceded by a single-plate lithosphere. If so, initiation of plate tectonics involved overcoming the lid strength, as proposed via: (a) mantle convective forcing, (b) gravitational instabilities formed via juxtaposition of differently weighted lithosphere, and (c) bolide impact. Geodynamic modeling of these processes has focused on continuum mechanics approximations tuned to plate scale dynamics, but without correspondence to experimentally-determined rock failure properties. Our new work offers two contributions: a new thermal expansion idea for how the lid ruptures, and solid mechanics modeling that hews directly to standard rock mechanics physical properties in order to illustrate this concept. For thermal expansion to overcome the lithosphere’s tensile strength, pre-expansion lithosphere must be cold, with a large temperature difference between the mantle and the surface. The heat-pipe cooling model supplies these conditions via dominant volcanic advection, which brings hot material from the mantle to the surface, where heat is lost to space and cold surface materials are deposited. Continual volume transfers from depth to the surface require lithospheric subsidence, thus advecting cold surface temperatures downwards. As Earth cooled, such volcanic activity and lithospheric chilling would have waned, leading to increased conductive warming of the lithosphere. This warming is postulated as the cause of thermal expansion in the new model, which uses 3D spherical shell models to demonstrate a self-organized fracture mechanism analogous to expansion-driven lithospheric uplift, in which globe-spanning rifting occurs as a consequence of horizontal extension. Resultant fracture spacing reflects lithospheric thickness and rheology, wherein geometrically-regular, polygonal-shaped tessellation is an energetically favored solution because it minimizes total crack length. Therefore, warming of the early lithosphere itself should lead to failure, propagating fractures, and the conditions necessary for the onset of multi-plate tectonics.