Abstract The Hebron Fault in SW Namibia is associated with a <1 m to 9.6 m high scarp displacing Proterozoic basement and Middle to Late Pliocene crystalline conglomerates. The young age of strata exposed in the fault scarp together with evidence for displacement of aeolian dunes, post-dating the Middle Stone Age, suggests that latest fault displacements occurred during the Late Pleistocene to recent. Recorded historical seismic events show that the fault zone is still active. Latest movements of the fault are recorded by: down-to-the-SW offset of calcrete-cemented conglomerate; fluvially modified, asymmetric hanging wall, graben-like structures; at least two left-stepping jogs in the fault trace and structural data from basement rocks in which late-stage crush zones overprint earlier cataclasite. These features provide consistent evidence that the present scarp formed predominantly by normal dip-slip displacement on a NW-striking, steeply SW-dipping master fault with only a minor dextral strike-slip component. Strongly veined cataclastic fault rocks adjacent to the scarp in basement most probably originated at depths of 4–10 km. The conclusion is therefore that recent fault activity has reactivated a pre-existing, much older fault. Aerial photographic lineaments and similar fault scarps identified NW and SE of the present study area are interpreted as extensions of the same fault structure. Hence the total length of the Hebron Fault is at least 300 km subparallel to the Atlantic margin of southern Africa. Our observations confirm that the Hebron Fault is a neotectonic feature of regional significance that may relate to late Cenozoic and particularly Quaternary neotectonic activity in NE Namibia and NW Botswana.

During the roughly year-long Seismic Wave Exploration in the Lower Lithosphere (SWELL) pilot experiment in 1997/1998, eight ocean bottom instruments deployed to the southwest of the Hawaiian Islands recorded teleseismic Rayleigh waves with periods between 15 and 70 s. Such data are capable of resolving structural variations in the oceanic lithosphere and upper asthenosphere and therefore help understand the mechanism that supports the Hawaiian Swell relief. The pilot experiment was a technical as well as a scientific feasibility study and consisted of a hexagonal array of Scripps Low-Cost Hardware for Earth Applications and Physical Oceanography (L-CHEAPO) instruments using differential pressure sensors. The analysis of eighty-four earthquakes provided numerous high-precision phase velocity curves over an un-precedentedly wide period range. We find a rather uniform (unaltered) lid at the top of the lithosphere that is underlain by a strongly heterogeneous lower lithosphere and upper asthenosphere. Strong slow anomalies appear within ∼300 km of the island chain and indicate that the lithosphere has most likely been altered by the same process that causes the Hawaiian volcanism. The anomalies increase with depth and reach well into the asthenosphere, suggesting a sublithospheric dynamic source for the swell relief. The imaged velocity variations are consistent with thermal rejuvenation, but our array does not appear to have covered the melt-generating region of the Hawaiian hotspot.

Abstract The Judy Field pre-Cretaceous reservoirs produce from a series of tilted fault blocks that form a complex northwest-plunging horst in Block 30/7a, UK Central North Sea. The primary producing reservoir of this field is the Middle to Late Triassic Joanne Sandstone Member of the Skagerrak Formation with secondary contributions from the Middle Triassic Judy Member of the Skagerrak Formation and the Late Jurassic Fulmar Formation. Fluid properties and contacts vary throughout the field. Currently the field is producing from ten development wells. Based on core studies, the depositional environment of the Triassic Skagerrak Formation has been identified as ephemeral fluvial while the Jurassic Fulmar Formation is transgressive shallow marine. Sedimentological interpretation of core data from seven wells in the field indicates that the Joanne and Judy Sandstone Members comprise four major facies associations: fluviatile channel sandstones, sheetflood sandstones, argillaceous sandstones and lacustrine mudstones. The complex architecture of these four facies associations makes it difficult to accurately predict absolute reservoir productivity without recourse to long-term production tests. Although the reservoir complexity was appreciated from an early stage in the field development, the initial reservoir models proved to be somewhat simplistic and overestimated the length of the production plateau. A new, longer offset 3D dataset was acquired in 1999 to replace the original pre-development 1986 3D seismic volume. The 1999 data were designed to enhance the imaging of the pre-Cretaceous reservoirs and help define areas of additional potential, which could be developed through new high angle platform wells. Mapping of reflectors below the Base Cretaceous Unconformity with the new data remains problematical due to poor signal to noise ratio and short period multiples. Despite these difficulties, a revised interpretation of the crestal portion of the Judy structure was completed. This new interpretation was coupled with the results from a detailed review of the geochemical, pressure and fault seal data, to create a petroleum charge model which accounted for the variations in fluid types and contacts throughout the field and identified three fault blocks (30/7a-P16, P17 and P18) in the crestal portion of the field for infill drilling. The fault blocks were successfully drilled in 2001 and 2002 despite significant drilling challenges posed by depleted reservoir zones above the target formation.