ALBERT

Description: Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity, and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.8 - 4.0 km s−1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 - 6.4 km s−1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.4 - 7.2 km s−1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle, and lower crust with P-wave velocities from 5.3 - 7.0 km s−1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transi- tion zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.

Description: Antarctica represents a key component in the investigation of the geological history and reconstruction of the supercontinents Rodinia and Gondwana. Remnants of the formation and disintegration of these ancient land masses can be found, although great uncertainties remain in the location of tectonic boundaries beneath the ice sheet of Antarctica due to general lack of outcrops and the limited amount of geological data. Airborne and space measurements are the only possibility to obtain comprehensive spatial data coverage for geological studies in the remote polar areas. More than 100000 line kilometers of airborne geophysical data, including radio echo sounding, gravity and magnetic data, were acquired over an area of 1.2 million square kilometers in the center of Dronning Maud Land (DML) over four austral summer campaigns between 2001 and 2005. The data are presented here as compilations of homogeneous topographic and gravity data for the DML region, ranging from 14°W to 20°E and from 70°S to 78.5°S. With respect to older airborne geophysical investigations in DML, up to 85% of the gravity data cover unexplored regions. Analyses of the maps of topography and gravity anomalies, by filtering and isostatic analysis, reveal information about geologic and tectonic structures in DML. Different gravity maps provide hints for the general tectonic fabric of the area. For the first time the southern boundary of the continent-ocean boundary formed during the Jurassic dispersal of Gondwana is identified. Especially the mountain range in Dronning Maud Land is most likely not in isostatic balance. The area is still uplifting as a consequence of glacial rebound. Interpretations of the Jutul-Penck graben as failed Jurassic rift system can be confirmed by the gravity inversion. Thinned continental crust compared to the surrounding geological units strongly support this hypothesis. Besides on other interpreted anomalies, the data shows a gravity structure, which starts approximately at 73.25°S/006°E and strikes in southwestern direction. We speculate that this pattern represents the suture zone (eastern termination of EAAO) between southern Africa and the cratonic part of Antarctica.

Description: The Davie Fracture Zone (DFZ) evolved during the Jurassic and Cretaceous breakup and subsequent drift of Gondwana off East Africa. This old weak zone has been reactivated during the evolution of the East African Rift System. Recent faulting of Cenozoic sediments in the Kerimbas Basin off northern Mozambique shows that they are affected by the neotectonics. The question is if and howthe crustal fabric in our research area has been modified by the rifting process. We present two seismic refraction profiles acquired offshore northern Mozambique to investigate its regional crustal structure and tectonic history. The profiles show a continent– ocean transition zone that widens from around 40 km at 13◦S to more than 100 km at 11◦S. In the west the transitional crust is up to 12 km thick. To the east, around 150 km off the Mozambique coast lies oceanic crust whose thickness varies from 4.9 to 6.5 km along the northern line and from 6.5 to 7.5 km along the southern one. The latter presents an unusual high-velocity lower crustal body (7.0–7.2 km s−1), about 40 km wide and 3.8 km thick, underlying the oceanic crust. The body may consist of underplated melt with the same source as the nearby Paisley Seamount, which has not yet reached its isostatic equilibrium. Despite well documented recent seismicity along the margin, neither of the profiles reveal significant crustal modifications or reduced crustal seismic velocities that might be related to ongoing extensional tectonics as part of of the East African Rift System. Neither profile reveals seismic evidence for the presence of a major fracture zone or sheared continental margin parallel to the margin. Instead, the profiles’ broad continent–ocean transitions are consistent with their formation during an early Jurassic stage of plate divergence oblique to the margin. Later, after 157 Ma, the azimuth of relative plate motion between East andWest Gondwana changed to be parallel to the margin, and parts of the continent–ocean transitions may have been locally reactivated in a strike-slip sense. However, details on the plate movements during the directional change of the seafloor spreading between 157 and 144 Ma are not available. The oceanic crust formed by the initial divergent oblique extension became faulted/modified by the strike-slip movements between both plates. Instead of a narrow deformation zone, the DFZ is charcaterized by a broad, diffuse zone of transtensional deformation.

Description: The Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 oceanbottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here,we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for 〉300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.