The mobilized sediments expelled by the mud volcanoes in Trinidad correspond to liquefied argillaceous and sandy material in which the solid fraction is systematically polygenic and originating from several formations (Cretaceous to Pliocene). The mud is notably rich in thingrained quartz that is angular and frequently mechanically damaged related to shearing at great depth, during the sedimentary burial, and/or hydraulic fracturing processes. The exotic clasts are mostly fractured fragments from various formations of the tectonic wedge (mostly Palaeocene to Miocene). The origin of the solid particles of the mud is polygenic, including deep Cretaceous-Palaeogene horizons close to the décollement, and various materials from the stratigraphic pile pierced by the mud conduits. Moreover, the fluids expelled by the mud volcanoes have a deep origin and notably the gas phase is thermogenic methane generated probably below a depth of 5000 m. The effusions occur either during cycles of moderate effusion of mud and fluids (quiescence regime), or during catastrophic events responsible for the expulsion of huge volumes of mud, clasts and fluids (transient regime). Available subsurface data suggest that the deep structure of the mud volcanoes includes: (1) a focused deep conduit at depth in the zone of overpressure; (2) a mud chamber intruding the surrounding formations around and above the top of the abnormal pressure zone; and (3) a superficial outlet leading to the surface vents.

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Aspects of the initial stages of a collision between European and African plates may have been documented in a geophysical survey of the central Mediterranean Ridge (MR) conducted last year. The idea of an incipient collision was first suggested by Finetti [1976],and details of the seafloor and tectonic deformation along the MR, revealed for the first time in the survey, seem to point in that direction. A unique opportunity may therefore exist for studying the beginnings of such a collision—between the passive margin of a major plate (Africa) acting as a continental indenter against the active margin of another plate (Europe). More wide angle data, deep penetrating multichannel seismic data, and drilling data are crucial to better assess the nature and the architecture of the underlying lithosphere, the styles of sedimentary deformation, and the consequences on fluid releases. Such data will make it possible to establish, or reject, a geodynamic collision model.

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