Recognizing detachment-mode seafloor spreading in the deep geological past.

Large-offset oceanic detachment faults are a characteristic of slow- and ultraslow-spreading ridges, leading to the formation of oceanic core complexes (OCCs) that expose upper mantle and lower crustal rocks on the seafloor. The lithospheric extension accommodated by these structures is now recognized as a fundamentally distinct “detachment-mode” of seafloor spreading compared to classical magmatic accretion. Here we demonstrate a paleomagnetic methodology that allows unequivocal recognition of detachment-mode seafloor spreading in ancient ophiolites and apply this to a potential Jurassic detachment fault system in the Mirdita ophiolite (Albania). We show that footwall and hanging wall blocks either side of an inferred detachment have significantly different magnetizations that can only be explained by relative rotation during seafloor spreading. The style of rotation is shown to be identical to rolling hinge footwall rotation documented recently in OCCs in the Atlantic, confirming that detachment-mode spreading operated at least as far back as the Jurassic

Time and Origin of Cichlid Colonization of the Lower Congo Rapids

Most freshwater diversity is arguably located in networks of rivers and streams, but, in contrast to lacustrine systems riverine radiations, are largely understudied. The extensive rapids of the lower Congo River is one of the few river stretches inhabited by a locally endemic cichlid species flock as well as several species pairs, for which we provide evidence that they have radiated in situ. We use more that 2,000 AFLP markers as well as multilocus sequence datasets to reconstruct their origin, phylogenetic history, as well as the timing of colonization and speciation of two Lower Congo cichlid genera, Steatocranus and Nanochromis. Based on a representative taxon sampling and well resolved phylogenetic hypotheses we demonstrate that a high level of riverine diversity originated in the lower Congo within about 5 mya, which is concordant with age estimates for the hydrological origin of the modern lower Congo River. A spatial genetic structure is present in all widely distributed lineages corresponding to a trisection of the lower Congo River into major biogeographic areas, each with locally endemic species assemblages. With the present study, we provide a phylogenetic framework for a complex system that may serve as a link between African riverine cichlid diversity and the megadiverse cichlid radiations of the East African lakes. Beyond this we give for the first time a biologically estimated age for the origin of the lower Congo River rapids, one of the most extreme freshwater habitats on earth

Continental break-up of the South China Sea stalled by far-field compression

The outcome of decades of two-dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean. However, geological observations unequivocally show that continental break-up propagates in the third dimension at rates that do not scale with the rate of opening. Here, we perform three-dimensional numerical simulations and compare them with observations from the South China Sea to show that tectonic loading in the direction of propagation exerts a first-order control on these propagation rates. The simulations show that, in the absence of compression in that direction, continental break-up propagates fast, forming narrow continental margins independently of the coupling. When compression is applied, propagation stagnates, forming V-shaped oceanic basins and wide margins. Changes in out-of-plane loading therefore explain the alternation of fast propagation and relative stagnation. These new dynamic constraints suggest that the west-to-east topographic gradient across the Indochinese Peninsula prevented continental break-up propagation through the 1,000-km-wide continental rift of the central and west basin of the South China Sea, until the direction of stretching changed 23 million years ago, resulting in bypassing and acceleration of continental break-up propagation