High-grade metamorphism and migmatizaof the Namaqua metamorphic complex around Aus in the Southern Namib Desert, South West Africa

Bibliography : pages 261-277.Rocks of the Namaqua Metamorphic Complex are exposed in an area of 10 000 km² in the northwestern part of the Namaqua Mobile Belt east of Liideritz. The Garub sequence represents the oldest rocks in the Aus area and comprises a diverse group of layered rocks of mainly semi-pelitic, pelitic, mafic, calcareous and quartzose composition. These rocks have been metamorphosed to form marbles, calcitic gneisses, metaquartzites, biotite schists, sillimanitecordierite garnet gneisses, amphibolites and granolites with minor amounts of iron formation and magnesian rocks. The principal metasediments are concentrated in west-trending zones. A central zone of calcareous rocks broadens westwards and contains the largest known bodies of carbonate rocks in the Namaqua Metamorphic Complex. The calcareous zone is bounded in the north by a narrow beit of quartzose rocks and in the south, by a broad zone of aluminous rocks. Gradational rock types between these zones are compatible with original sedimentary lithofacies changes. Layered biotite gneiss of psammitic composition has been interfolded with units of the Garub sequence. Both these rock units are present as inclusions within a tonalitic augen gneiss

Global dynamic topography observations reveal limited influence of large-scale mantle flow

Convective circulation of the Earth’s mantle maintains some fraction of surface topography that varies with space and time. Most predictive models show that this dynamic topography has peak amplitudes of about ±2 km, dominated by wavelengths of 104 km. Here, we test these models against our comprehensive observational database of 2,120 spot measurements of dynamic topography that were determined by analysing oceanic seismic surveys. These accurate measurements have typical peak amplitudes of ±1 km and wavelengths of approximately 103 km, and are combined with limited continental constraints to generate a global spherical harmonic model, the robustness of which has been carefully tested and benchmarked. Our power spectral analysis reveals significant discrepancies between observed and predicted dynamic topography. At longer wavelengths (such as 104 km), observed dynamic topography has peak amplitudes of about ±500 m. At shorter wavelengths (such as 103 km), significant dynamic topography is still observed. We show that these discrepancies can be explained if short-wavelength dynamic topography is generated by temperature-driven density anomalies within a sub-plate asthenospheric channel. Stratigraphic observations from adjacent continental margins show that these dynamic topographic signals evolve quickly with time. More rapid temporal and spatial changes in vertical displacement of the Earth’s surface have direct consequences for fields as diverse as mantle flow, oceanic circulation and long-term climate change.This research was supported by a BP-Cambridge collaboration. We are grateful to ION for permission to publish partial seismic reflection profiles shown in Fig. 2 from their IndiaSPAN and Greater BrasilSPAN data sets

Initiation of gravitational collapse of an evaporite basin margin: The Messinian saline giant, Levant Basin, eastern Mediterranean

Regional 2D and 3D seismic data from the Levant Basin reveal the updip extensional component of thin-skinned gravity tectonics on this continental margin. Because of its youth (5-7 Ma), the earliest stages of deformation of this salt basin are preserved without the severe structural overprinting common on more mature, giant salt-tectonic systems. Extension detaches onto and within Messinian evaporites up to 1800 m thick. By structural restoration, we reconstruct the tectonic evolution of the Messinian evaporites using paleobathymetric constraints from wells and seismic profiles. This analysis suggests that the Mediterranean drawdown during the Messinian Salinity Crisis was ~800 m. The 10- to 15-km-wide extensional domain tracks the landward pinch-out of the mobile Messinian evaporites against an older Late Miocene scarp. Diachronous extension began in the center of the margin in the mid-Pliocene, then spread northward in the late Pliocene, then finally southward in the early Pleistocene. Extension continues today on many of the most landward faults. Extensional strain varies greatly along strike from <1 km to as much as 12-15 km. Comparing observations with four end-member conceptual models, we infer that both extension and seaward salt flow thinned the evaporite margin and its overburden. Both processes were triggered by a combination of uplift of the continental shoulder of the Dead Sea Rift and subsidence in the Mediterranean Basin. At least three factors controlled variations in extension: (1) degree of filting of the salt wedge, related to the interplay of coastal uplift and basin subsidence; (2) presence of pre-Messinian canyons overlain by landward salients of salt; and (3) variations in evaporite facies and the proportion of siliciclastic admixture. © 2007 Geological Society of America

The origin of salt-encased sediment packages: Observations from the SE Precaspian Basin (Kazakhstan)

Intrasalt sediment packages containing siliciclastic sediments, carbonate sediments, or non-halite evaporites such as gypsum or anhydrite are common within most salt sequences. Intrasalt sediment packages may have been deposited before, during, or after salt deposition and be incorporated into the salt by various processes. Understanding the origin and evolution of intrasalt sediment packages may yield important insights into the tectonic and geodynamic history of the basin, and also into the understanding of salt tectonics. Despite the importance of intrasalt sediment packages, currently there is no systematic description of their possible origins and their distinguishing criteria. This work is divided into three parts. The first part outlines the possible origins of intrasalt sediment packages, as well as criteria to determine if they originated as subsalt, suprasalt or intrasalt sequences. The second part examines how sediment packages that originated on top of salt, such as minibasins, can be encased within salt. Four key encasement processes are proposed: a) salt expulsion from beneath a minibasin experiencing density-driven subsidence; b) salt expulsion from beneath adjacent subsiding minibasins; c) salt expulsion associated with lateral shortening; d) override of minibasins by a salt sheet sourced from elsewhere. The third part of the paper presents a case study from the SE Precaspian Basin, Kazakhstan, where, using a borehole-constrained 3D seismic reflection dataset, the proposed criteria are applied to an area with abundant, newly discovered sediment packages within salt