Patrick Unternehr

Hyper-extended crust in the South Atlantic: in search of a model

ABSTRACT The discovery of giant hydrocarbon reservoirs in the pre-salt sequence of the deep-water Brazilian rifted margin together with the new acquisition of high-quality reflection and refraction seismic surveys across many rifted margins worldwide has attracted the interest of industry and researchers to deep-water rifted margins. For the first time, the new data sets enable the imaging and description of the pre-salt structures, which indicate that deep-water rifted margins are very different from what classical models had predicted thus far. Instead of the expected fault-bounded basins and a sharp ocean–continent boundary, the new data suggest the existence of a sag basin lying on hyper-extended crust with little indication for brittle high-angle faulting, a transitional domain between continental and oceanic crust showing neither characteristics of oceanic nor continental material, and very asymmetrical distal conjugate rifted margins. These observations raise significant doubts on the validity of the classical concepts used in rheology, mechanics and isostasy to explain extensional systems leading to seafloor spreading. They also require new concepts and more data in order to understand how these rifted margins evolved in time and space. This has important implications for the exploration and evaluation of petroleum systems in the frontier areas of hydrocarbon exploration. In this study we publish two multi-channel seismic sections across the Angola and conjugate Brazilian rifted margins that we consider as ‘type’ sections for hyper-extended magma-poor rifted margins in the South Atlantic. The aim of this study is to discuss various possible interpretations and models to explain the high-resolution seismic images presented in this paper.

Anatomy and tectono-sedimentary evolution of a rift-related detachment system: The example of the Err detachment (central Alps, SE Switzerland)

The discovery of hyperextended crust in present-day magma-poor distal rifted margins has major implications for the development of rift concepts. Indeed, the occurrence of low-angle detachment faulting changes the structural style and modifies the classical architecture of rifted margins, often represented by tilted blocks and bounded by high-angle normal faults. At present, little is known about the ways in which detachment systems form in distal margins through time and space and the way in which they control the formation of supradetachment rift basins. In this paper, we discuss a Jurassic rift-related detachment system of the fossil Adriatic distal rifted margin, today exposed in the Lower Austroalpine Err nappe in SE Switzerland. A palinspastic restoration of the Alpine units places the three-dimensional postrift architecture of this detachment system over more than 200 km 2 and 34 km in transport direction. Based on the description of a preserved supradetachment basin, we can show that the synrift sedimentary evolution records the formation of supradetachment extensional allochthons and the exhumation of basement rocks. Using this well-exposed example, we will show that detachment systems are intimately related to the overall tectono-sedimentary evolution of the most distal parts of the Adriatic rifted margin and possibly other Atlantic-type magma-poor hyperextended rifted margins.

Unravelling the along-strike variability of the Angola–Gabon rifted margin: a mapping approach

Abstract We summarize here observations from the South Atlantic Angola–Gabon rifted margin. Our study was based on the interpretation of a selection of deep penetration depth-migrated seismic reflection profiles. We describe here the large-scale dip architecture of the margin under five structural domains (proximal, necking, distal, outer and oceanic) and list their characteristics. We investigated the necking domain further and discuss the architecture of the distal domain as a combination of hyper-extended crust and possible exhumed mantle. The mapping and characterization of these domains, at the margin-scale, allow us to illustrate the along-strike structural and stratigraphic variability of the margin. We interpret this variability as the result of a shift from an upper plate setting to a lower plate setting. This shift is either sharp, typified by a major regional normal fault on the northern flank of a residual hanging-wall block, identified offshore Cabinda–Zaire, or more diffuse to the south. First-order screening of conjugate profiles confirmed the segmentation and structural characteristics of the transfer zones. The dataset studied also allowed the identification of key sections that can be considered as type examples of upper plate and lower plate margins and which allow us to discuss the characteristics of these end-member settings.

Evidence for magma entrapment below oceanic crust from deep seismic reflections in the Western Somali Basin

Our understanding of melt generation, migration, and extraction in the Earth’s mantle beneath mid-oceanic ridges is mostly derived from geodynamic numerical models constrained by geological and geophysical observations at sea and field investigations of ophiolites, and is therefore restricted to the oceanic crust and the shallow part of the mantle. Here we use a >200-km-long, deep seismic reflection section to image with high resolution the sub-oceanic lithosphere within the Western Somali Basin (offshore eastern Africa) where spreading ceased at ca. 120 Ma. The location of the failed spreading axis is inferred from both seismic data and gravity data. Several groups of strong reflections are imaged to depths of >30 km below the top of the oceanic crust. We interpret the deepest reflectors, within the mantle, as resulting from frozen melt bodies which may be relicts of a paleo–melt channel system located at the base of the lithosphere and formerly feeding the failed ridge axis. Other reflectors within the mantle may correspond to melt bodies injected into major shear zones along the Davie fracture zone. Another group of reflectors, located below a 8–5-km-thick oceanic crust, is interpreted as marking a fossil melt-rich crust-mantle transition zone as much as 3 km thick. This interpretation implies an inefficient extraction of melt out of the mantle, which is favored by the combination of a slow spreading rate and a high magma budget.