New insight into a dynamic early rift system and associated break-up volcanics; 3- dimensional geometry of Seaward Dipping Reflections (SDRs) in offshore Uruguay

Seaward Dipping Reflections (SDRs) are a commonly occurring feature of volcanic margins worldwide, representing extrusive volcanics recognised by wedges of arcuate reflections with a predominantly seaward dip. Despite their widespread occurrence their 3D geometry is poorly understood. We investigate for the first time, the three-dimensional geometry of an SDR wedge, offshore Uruguay, and consider implications for the volcanic-structure interaction at the time of continental break up. Found along the continent-ocean boundary, SDRs are associated with the opening of the South Atlantic in the early Cretaceous (~134 Ma).

A newly acquired regional 3D seismic dataset covering over 13000 km² is used in conjunction with long-offset 2D data to provide insights into the complexity of SDRs and the early rift morphology. 3D data allows the delineation of a detailed volcanic architecture through restorations, seismic facies mapping, and the identification of stratal relationships with overlying and infilling sediments. An approach of seismic facies characterisation has been adopted with terminology analogous to sequence stratigraphy. Reconstruction of the early rift palaeo-environment may be achieved by theflattening of prominent top package reflections to a pre-rotational architecture at the time of extrusion.

Early rift topography was complex and laterally variable because fault controlled subsidence was synchronous with laterally variable pulses of volcanic extrusion. These created both positive relief features and local topographic lows within the newly forming rift basin. The seismic character of basinal low fill, suggests a variety of lithologies including extrusive volcanics and hyaloclastic deposits, where subaqueous conditions have occurred, as well as lacustrine, sediment dominated sequences.

Extrusive sub-aerial highs show an erosional top, with the overlying reflections of SDR wedges also representing an erosional unconformity into which underlying reflections truncate. Volcanic extrusion was poly-phase as well as spatially variable, a result of the migrating location of both the central vent and off-axis magmatism as rifting progressed. The volcanic infill of topographic lows and isolated magmatic lenses represents off-axis magmatism, bound by transtensional faulting and laterally synchronous with SDR formation. These volcanics, though likely to be synchronous, are deemed to be distinct from SDRs on the basis of differing geometry, seismic facies and the nature of their emplacement.

Faulting within the SDRs shows evidence of minor steep faults with small displacements confined to individual packages. Local incision can be seen along the top of several individual segments of the southern wedge, where distinct periods of sub-aerial exposure are recognised between volcanic pulses. These are shown in down-lapping reflections creating incised 'valleys' and in low reflectivity, discontinuous reflections towards the top of