Description: The Moresby Seamount detachment in the Woodlark Basin (east of Papua New Guinea) is arguably the best exposed active detachment fault in the world. We present the results of a high-resolution autonomous underwater vehicle survey of bathymetry, bottom water temperature, and turbidity. In combination with dredging and existing drillhole data, a synthesis of the tectonic geomorphology, kinematics, and mechanics of the detachment is provided. The detachment surface, which has a 30{degrees} northward dip and [~]8 km post-Pliocene displacement, is well preserved. Two major smooth areas are tectonically created, and megascopic (kilometer scale) slickensides indicate downdip direction of movement. The detachment is transected by a major sinistral strike-slip fault, suggesting deformation partitioning in the detachment zone in response to the 500 k.y. change in plate kinematics. The mainly gabbroic protoliths and cataclasites from the fault show pervasive syntectonic alteration, leading to large increases in abundance of quartz and, more important, calcite. Resulting quartz-rich and calcite-rich mylonites play a crucial role, as weak fault rocks and ductile microstructures point to detachment operation at low differential stress. A kilometer-sized anomaly in bottom water temperature and turbidity is found at the downdip end of the detachment zone, indicating that it hosts an active hydrothermal system, probably fed by overpressured fluids from a deep crustal source.

Description: The Walvis Ridge perpendicular to the African coast offshore Namibia is believed to be caused by a long-lived hotspot, which started to erupt with the opening of the South Atlantic in mid Cretaceous. The ridge in combination with the large igneous provinces (Etendeka and Parana) in South America and Namibia is today considered to be a classical model for hotspot driven continental break-up. To unravel details on how the crust and mantle were modified by such a major thermal event, a large-scale geophysical on- and offshore experiment was conducted in 2011. We present p-wave velocity models of two active seismic profiles along and across Walvis Ridge. The profile along the ridge continues onshore, has a total length of ∼730 km and consists of 28 ocean bottom stations, 50 land stations and 8 dynamite shots. This section reveals a complex structure with multiple buried seamounts, strong lateral velocity gradients and indication of a high velocity body at the crust-mantle boundary beneath the shelf area. Lower crustal velocities range from 6.5 km/s in the west to 7.0 km/s in the east while the crustal thickness is approximately 28 km at the coast thinning westwards. The second profile perpendicular to the ridge is located about 140 km west of the first profile, has a length of ∼480 km and consists of 27 ocean bottom stations. The crustal thickness is well constrained by multiple Moho reflections showing a thickness of 20km under the crest of the ridge and gradually thinning to 8km towards north and south. A seamount marks the northern termination of the ridge leading to an abrupt thickening of the crust to 14km before reaching the Angola Basin. While crustal velocities of 5.5 km/s and 6.5 km/s in the upper and lower crust are similar to the first profile, lower crustal velocities north of the crest are approximately 6% higher.

Description: The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Paraná and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (〈100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures).

Description: Voluminous magmatism during the South Atlantic opening has been considered as a classical example for plume related continental breakup. We present a study of the crustal structure around Walvis Ridge, near the intersection with the African margin. Two wide-angle seismic profiles were acquired. One is oriented NNW-SSE, following the continent-ocean transition and crossing Walvis Ridge. A second amphibious profile runs NW-SE from the Angola Basin into continental Namibia. At the continent-ocean boundary (COB) the mafic crust beneath Walvis Ridge is up to 33 km thick, with a pronounced high-velocity lower crustal body. Towards the south there is a smooth transition to 20-25 km thick crust underlying the {COB} in the Walvis Basin, with a similar velocity structure, indicating a gabbroic lower crust with associated cumulates at the base. The northern boundary of Walvis Ridge towards the Angola Basin shows a sudden change to oceanic crust only 4-6 km thick, coincident with the projection of the Florianopolis Fracture Zone, one of the most prominent tectonic features of the South Atlantic ocean basin. In the amphibious profile the {COB} is defined by a sharp transition from oceanic to rifted continental crust, with a magmatic overprint landward of the intersection of Walvis Ridge with the Namibian margin. The continental crust beneath the Congo Craton is 40 km thick, shoaling to 35 km further SE. The velocity models show that massive high-velocity gabbroic intrusives are restricted to a narrow zone directly underneath Walvis Ridge and the {COB} in the south. This distribution of rift-related magmatism is not easily reconciled with models of continental breakup following the establishment of a large, axially symmetric plume in the Earth´s mantle. Rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magma-dominated southern and an essentially amagmatic northern segment.

Description: The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism is the drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (5-7 km) of the Angola Basin lies next to the 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendaka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.

Description: Abstract The evolution of the South Atlantic in space and time is in general presented as a two plate system, where the separation of the two continent might be caused by the Tristan Hotspot. Evidence for massive subaerial and submarine volcanism is found on both margins and adjacebt basins. Namely, the Parana (Brasil) and Etendeka (Namibia) flood basalts onshore, and the Walvis Ridge and Rio Grande offshore are evidences for a long term volcanism related to the rift/drift of both continents. In science it is currently under debate, if hotspot volcanism is the driving force for the continental breakup or are e.g. the above mentioned features a by-product of the anyhow moving plates. Aeromagnetic investigations along East Antarctica show that from the beginning of the Gondwana breakup the South American and Africa plates moved in different directions and with different speeds. This most likely caused rifting along old zone of weakness between South America and southern Africa. The ridge in combination with the large igneous provinces (Etendeka and Parana) in South America and Namibia is today considered to be a classical model for hotspot driven continental break-up. For understanding the role of mantle plumes during continental breakup a large-scale geophysical experiment in 2011 on/offshore Namibia was conducted to investigate the crustal/upper mantle structure both under the Walvis Ridge and northern Namibia. Several seismic profiles in the strike and across the Walvis Ridge with in total more than 160 oceanbottom seismometers, magnetic telluric station were deployed. Onshore the number of instruments was more than 300 to map in detail the continent-ocean transition zone to enhance our understanding on how the crust was modified by the thermal anomaly. In addition, a larger number of seismological stations are operated both on- and offshore to investigate the structure of the upper mantle. Here, we reported the first results of this experiment and discuss some implications.

Description: The opening of the South Atlantic ocean basin resulted in voluminous magmatism on the conjugate continental margins of Namibia and Brazil, including the formation of the Parana and Entendeka large igneous provinces (LIPs), the formation of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDRs) near the continent-ocean transition, as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. Hence, the passive margins bordering the South Atlantic are today considered as type examples for models involving hotspot related continental break-up. However,the presence of volcanic features (SDRs, LIPs) appears to be limited south of the hotspot trails. The resulting segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question to be adressed here is whether magmatism is the driving force for continental break-up, or whether even rifting with abundant hotspot related magmatism is in principle in response to crustal and lithospheric stretching. In 2010/11, a combination of on-/offshore wide-angle seismic, marine magnetotelluric and on-/offshore seismological data were acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The set of experiments was designed to provide crustal velocity and conductivity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, the presence of interleaved sediment layers in SDR sequences as well as magmatic intrusions and underplated material near the continent-ocean transition. The sedimentary portions down to the igneous basement were additionally constrained by coincident single-channel reflection seismic data. Here, we present preliminary results for two wide-angle seismic transects and first results for a marine magnetotelluric profile. Tomographic analysis of the seismic data reveals the velocity structure of the crust down into the uppermost mantle. The probably most striking feature of our models is the sharp lateral transition in crustal structure and thickness associated with the northern boundary zone of Walvis Ridge towards the Angola Basin. Here, the rather thin oceanic crust in the basin lies opposite to the ~35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. In contrast, the southern termination of Walvis Ridge and the corresponding transition towards the adjacent 25-30 km thick crustal portions further south is much more subdued. Due to the presence of a high-velocity (6.5-7.2 km/s) lower crust we argue that the Namibian shelf south of Walvis Ridge comprises a transitional igneous origin. We suggest that the northern boundary zone close to the landfall of Walvis Ridge represents an important transtensional tectonic feature which may have provoked the preferential extraction of melts into the footwall of this structure.