Introduction to the thematic set: Rifts III: catching the wave
Our understanding of the complex interplay of geodynamic processes that operate during lithospheric extension and results in the formation of magmatic and amagmatic passive margins remains equivocal. The challenge of the third rift conference in the series was, how to better constrain interpretation of those mechanisms, contributing to the surface expression at rifted plate margins. The fundamental observation that not all extensional basins and rifted continental margins involve the development of process-related magmatism continues to court significant debate and controversy (Rifts III: Catching the Wave, Geological Society, London – this conference). Consensus remains elusive; however empirical data are providing more informed insights into the boundary conditions for input into more realistic and testable geodynamic numerical models (Lavier et al. 2016). Studied margins record their own particular complexities and causally reflect the influence of important crustal and mantle-scale heterogeneities in addition to plume-lithosphere interactions associated with core-mantle boundary hot-spot anomalies (Ryberg et al. 2015). The apparent paradoxical juxtaposition of magma-rich and magma-poor continental margin segments is now more widely recognized (e.g. Koopmann et al. 2013). By inference, we now conclude that rupture of the lithosphere is less likely to correspond to a continuous, single phase, uniform extension model (sensu McKenzie 1978). This simplistic, elegant concept, in which crust and mantle thin by the same factor, is unable to explain the diversity of documented rift margin relationships. Instead, either the crust or lithospheric mantle (or both) are more likely influenced by a combination of complementary regional or local dynamic stresses involving ‘passive’ tensional far-field plate forces and/or ‘active’ mantle upwelling. Whichever prevailing ‘cold’ or ‘hot’ mechanisms determine the nature of lithospheric rupture, the plate boundary evolution is better understood by highlighting characteristic basin margin morphologies and describing diagnostic structural/stratigraphic architectures.