The role of small-scale horizontal convection in lithosphere-asthenosphere interaction

I present a set of numerical experiments, which show the formation, evolution, and influence of small-scale rolls at the lithosphere-asthenosphere boundary (LAB). The rolls originate from lateral gradients of temperature and are not related to classical large-scale Rayleigh–Bénard convection. Rather, they represent a form of horizontal convection, arising from the circumstance that the lower part of the lithosphere (both oceanic and continental) can contribute to the advection of material (due to a relatively low viscosity) but is characterized by a non-adiabatic thermal regime. The formation of convection rolls indicates that the process is relatively steady, with a relaxation time of several hundreds kyrs. Although the LAB geometry influences the formation of convective cells, these features form even in presence of a flat LAB surface, whenever there is a lateral thermal change within the lithosphere, for example at the transition between oceanic and continental lithosphere along continental margins. An important observation is that the thermal structure of the oceanic lithosphere close to a spreading center induces an ascending flow even in absence of extension. Consequently, an active component of spreading exists regardless of whether two plates are moving apart. In these experiments, the active component of spreading induces a velocity between 0.6 and 1.2 cm/yr, which adds to the velocity imposed with boundary conditions. Such active component develops even in the case of closed systems and determines a state of compressional stress within the lithosphere. The structure of the ascending flow in the melting regime below a spreading center suggests that it results from the superposition of two small-scale rolls with opposite polarity, associated with horizontal convection.