ALBERT

Description: Early Jurassic sheet-like intrusions (sills and dykes) are abundant in the Karoo Basin in South Africa, and were emplaced as a part of the Karoo Large Igneous Province. Here we discuss the evolutionary history of dolerite sills and dykes in different parts of the basin on the basis of new major and trace element analyses of dolerite samples collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the Golden Valley Sill Complex (GVSC). In addition, we present Sr–Nd isotope data for selected samples. The dolerites are subalkaline tholeiitic basalts and basaltic andesites characterized by enriched trace element patterns, variable degrees of depletion in Nb–Ta relative to light rare earth elements, negative to positive Pb anomalies, and mild to moderate enrichment in initial Sr–Nd isotopic ratios. The aim of this study is to unravel the evolutionary history of the melts that gave rise to the dolerites. We propose that the primary melts were derived from sub-lithospheric mid-ocean ridge basalt (or ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb–Ta, mildly enriched Sr–Nd isotopic ratios) through interaction with metasomatized lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up to 10% assimilation of granulites with strong arc-type geochemical signatures. The AFC processes may alternatively have taken place in the uppermost mantle. Distinct geochemical characteristics among the GVSC and drill-core units reflect different amounts of AFC. During and/or after intrusion into the sedimentary rocks in the Karoo Basin the magmas underwent a second stage of fractional crystallization (50–60%) and local contamination by their sedimentary wall-rocks. High U concentrations and U/Th ratios in some dolerites in the southwestern part of the Karoo Basin were probably caused by fluids released from shales rich in organic material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect interaction with Ta–Th–U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have interacted with such rocks. Considering that continental flood basalts are emplaced through continental crust and sedimentary basins, it is likely that other LIPs have similar evolutionary histories to that proposed for the Karoo Basin.

Description: Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.

Description: Submarine landslides can be several orders of magnitude larger than their terrestrial counterparts and can pose significant hazards across entire ocean basins. The landslide failure mechanism strongly controls the associated tsunami hazard. The Tampen Slide offshore Norway is one of the largest landslides on Earth but remains poorly understood due to its subsequent burial beneath up to 450 m of sediments. Here, we use laterally extensive (16,000 km2), high‐resolution processed 3D seismic reflection data to characterize the upper Tampen Slide. We identify longitudinal (downslope, movement‐parallel) chutes and ridges that are up‐to‐40 m high, as well as extensional and compressional (cross‐slope) ridges. This is the first time that longitudinal ridges of such size have been imaged in a deep marine setting. The first phase of the Tampen Slide involved the simultaneous translation of over 720 km3 of sediments along a single failure plane. This was followed by spreading along the head‐ and sidewall, and the formation of a retrogressive debris flow and slump, the volumes of which are insignificant compared to the first failure. The process responsible for movement of such a large sediment volume along a single glide plane differs significantly from that of other passive margin megaslides, which typically comprise numerous smaller landslides that fail retrogressively along multiple glide planes. The trigger mechanism (e.g. an earthquake), the presence of mechanically strong obstructions (e.g. igneous topographical high), and the number and location of weak layers may be key factors that determine whether megaslides develop along a single plane or retrogressively.