Abstract The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.

Abstract Diamonds have been brought to the Earth’s surface from at least 2.82 Ga onward by igneous and tectonic processes, and they have been redistributed since then by sedimentary processes into secondary diamond deposits. None of the known tectonically emplaced diamond deposits are economically viable, and only two types of igneous rock, kimberlite and lamproite, sometimes carry diamonds and can occasionally be economic to mine. Where diamonds are present in kimberlite and lamproite, the concentrations are –3 ppm, acquired by random sampling of diamond source rocks in the subcontinental lithosphere. Diamond-forming processes in the lithosphere were episodic since ~3.57 Ga, and all primary diamond deposits show evidence of two or more diamond generations. The earliest diamond-forming episode at ~3.4 ± 0.2 Ga appears to have been a worldwide metasomatic event triggered by CO2-rich, probably subduction-derived fluids that produced diamonds associated with garnet harzburgite. Further diamond populations have formed in association with craton accretion, subduction, slab melting, magmatic modifications of the lithospheric mantle, obduction tectonics, and metasomatic infiltration. In the process, diamonds formed in association with metasomatized harzburgite were supplemented predominantly by metasomatic diamond growth in eclogite, with occasional significant contributions from grospyditic, lherzolitic, and websteritic sources as well as sublithospheric ultradeep sources, notably majorite. Diamond-bearing igneous bodies exploit preexisting zones of weakness in the crust. They probably traverse most of the distance from the mantle to the surface as thin dikes or dike swarms, with nearurface expressions dictated by multiple intrusions, their volatile content, the presence or absence of cap rocks, local structures, and the ambient stress field, interaction with ground water , and degrees of preservation from erosion. Average diamond values per carat for a given diamond occurrence vary by approximately three orders of magnitude (US$1-$1,000/carat). A multistage model for the formation of diamond deposits is presented for the Kaapvaal craton that takes into account the tectonic history of the craton as well as the complexities observed within diamond populations of its various primary diamond deposits. Although the details of this model are craton specific, the general features of the model are applicable to other cratons.