Ultra-slow transverse waves during continental breakup

Continental rifting is one of the four fundamental geological processes of the Wilson cycle. Rifting results from the continuous stretching of continental lithosphere and involves mechanical, thermodynamic, and rheological processes. It may be followed by a catastrophic breakup stage, which determines cessation of extensional deformation and the final separation of a continent into two distinct tectonic plates that grow by accretion of oceanic lithosphere. To date, the transition to sea−floor spreading and the conditions for the development of a new ocean have not been fully understood. We propose that a consistent description of this process must consider the existence of long−term retarded elasticity in the mantle lithosphere, the superadiabatic conditions of this layer, and the combined action of such elastic forces with the localized buoyancy arising from thermal anomalies. We present a solution of the rheological equation for a nonlinear viscoelastic model of the lithosphere mantle and numerical experiments showing that transient thermal anomalies are generated during the extension, which lead to the formation of transverse waves having wavelengths of the order of hundreds to thousands km and periods of several tens kyrs. These waves induce oscillating topography and influence the relief. Therefore, they could be responsible for eustatic cycles both in the axial rift lacustrine system and in off−axis (dendritic) lakes placed in areas of reversed drainage. At sufficiently high extension rates, deformation localizes and these ultra-slow waves determine cyclic shear failure, with formation of X−shaped cross structures through the lithosphere that prelude to the final rupture.