Philip D. Rabinowitz

The separation of madagascar and Africa.

Identification of a sequence of east-west trending magnetic anomalies of Mesozoic age in the western Somali Basin helps define the position of Madagascar in the Gondwana reconstruction. The anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the Jurassic magnetic quiet zone. The motion of Madagascar relative to Africa was from the north and began in the middle Jurassic, about the same time as the initial breakup of Gondwanaland. Sea-floor spreading ceased when Madagascar assumed its present position in the Early Cretaceous.

Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa

An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.

Effects of Canary hotspot volcanism on structure of oceanic crust off Morocco

Analysis of over 6400 km of multichannel seismics (MCS) and 50 sonobuoy reflection and refraction experiments reduced both in the domain of X-T and tau-p shows that a region within the Jurassic Quiet Zone off Morocco underwent dramatic changes as a result of the passage of the lithosphere over the Canary hotspot commencing approximately 60 Ma. A seismic unit (UCF), interpreted as volcanic in origin, is observed within the sediments in a region characterized by a broad bathymetric swell. It shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous, subparallel facies. A velocity inversion is noted between the UCF (4.7km/s) and the underlying sediment (3.1 km/s). The UCF is time transgressive; it lies near the Cretaceous/s Tertiary boundary in the northern portion of the study area and is younger to the south. Kinematic studies of the movement of the Canary hotspot relative to Africa show that the hotspot first appeared off NW Africa about 60 Ma and was located beneath oceanic crust in the region where the UCF is observed. Depth-to-basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age. In areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot. The reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age. Subsidence curves of the reheated crust off Morocco show good correlation to subsidence curves of other reheated crust on a global basis. A zone characterized by high crustal velocities, (7.1–7.4 km/s) and greater crustal thicknesses (by ∼1–2 km) is observed in an area that corresponds to the bathymetric swell, the region of the UCF, and the reelevated basement. The high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the Canary thermal anomaly. The presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world. They have generally been attributed to the thermal processes associated with continental rifting. Off Morocco, we believe that similar, thermally induced phenomena have occurred but that here; the heat anomaly was the midplate volcanism associated with the Canary hotspot.

Gravity anomalies and crustal shortening in the eastern Mediterranean

Abstract Crustal shortening of the ocean floor in the eastern Mediterranean is recognized by a marked thickening of the sedimentary layer seaward of the Hellenic and Calabrian island arcs. Steep gradients and large negative free-air anomalies in the gravity field along with a highly uniform, low regional heat flow are manifestations of the thickened crust. Bodies of recently deformed sediment in and seaward of the Hellenic Trough reveal the style, polarity, and dynamics of the thickening mechanism. A linear buried anticlinal structure, inferred from analysis of surface ship gravity profiles, may mark the site of contemporary intrabasinal underthrusting. The distribution of earthquakes beneath the Mediterranean Ridge supports the interpretation that the Anaximander, Ptolomy, and Strabo Mountains are features comparable to large basement nappes. Cyprus is one such structure, offset to the south, where the oceanic crust and part of the upper mantle have been involved in the decollement.

Sediment waves on the Moroccan continental rise

Abstract A nearly continuous zone of sediment waves is present on the lower continental rise off western Morocco which parallels the regional bathymetric trends. The individual sediment waves within the zone migrate upslope with time and, in general, also trend parallel to the regional bathymetric contours. These observations suggest that geostrophic contour currents are responsible for the formation of sediment waves. Physical oceanographic measurements and sea-floor photographs indicate only a very weak bottom circulation in this region. This suggests either that strong bottom currents are not essential for the formation of sediment waves or that relatively stronger bottom currents flowed along the continental margin of Morocco in the recent past. Turbidity flows may also influence the distribution of these sediment waves.

The isostatic gravity anomaly: key to the evolution of the ocean-continent boundary at passive continental margins

Abstract Isostatic gravity highs bordering the passive continental margins are interpreted as resulting from oceanic basement highs. These basement elevations are relics of the transient phenomenon of a higher ridge axis elevation during early rifting. The steep landward gradient in the isostatic gravity field, generally associated with a magnetic edge effect anomaly, delineates the boundary between oceanic and continental basement.

Geophysical study of the continental margin of southern Africa

Marine gravity and magnetic anomalies determine a model for the boundary between oceanic and continental basement off southern Africa. A nearly linear high-amplitude positive magnetic anomaly (about 300 to 800 γ) is coincident with an isostatic gravity anomaly where the Agulhas fracture zone is bounded to the north by the African continent. The magnetic anomaly is interpreted as a magnetic “edge effect” separating oceanic basement in the south from continental basement in the north. Where deep ocean lies on either side of the fracture zone, negative magnetic anomalies occur that are modeled by assuming a broad zone of zero magnetization within the fracture zone. Mesozoic magnetic lineations M0 (∼108 m.y. B.P.) through M12 (∼128 m.y. B.P.) are adjacent to the western margin of southern Africa (Cape Sequence). West of the Orange River, the magnetic anomaly amplitudes are attenuated, which is attributed to a partial demagnetization of basement rocks due to an increase in sediment overburden. A prominent magnetic anomaly (anomaly G) borders the Cape Sequence on its landward side and is coincident with an isostatic gravity anomaly. Anomaly G is interpreted as a magnetic edge-effect anomaly separating oceanic from continental basement, similar to the magnetic anomaly associated with the Agulhas fracture zone. Southwest of Capetown, anomaly G is located on the continental slope, implying that continental basement has subsided to form the slope. Farther north, anomaly G lies nearly 125 km landward of the shelf break; this implies that the shelf break in the north was formed by a pro-grading of sediment over an oceanic basement, in general agreement with available seismic (sonobuoy) measurements.

Structure of Vema Fracture Zone

Abstract Unreversed seismic refraction profiles were recorded over the axial trough portion of Vema Fracture Zone and over the crust on opposite sides, using short- and long-range sonobuoys. Travel time—distance analysis of refracted arrivals indicates that south of the axial trough there is a 600 m thick layer of (assumed) velocity 3.6 km/sec overlying a 4 km thick 6.3-km/sec crustal layer between it and the upper mantle. Beneath the axial trough a 2.2 km thick 4.3-km/sec layer on a 2.6 km thick 5.9-km/sec layer overlies the mantle. The main crustal layer of velocity 5.9–6.3-km/sec may correspond to layer 3 of the deep-ocean basin, but its velocity and thickness are significantly less beneath the axial trough. Oceanic basement (layer 2) north of the axial trough is approximately 2 km thick and has a 3-component-velocity structure; deeper crustal layers were not detected due to insufficient profile length.

The evolution of the Rio Grande Rise in the southwest Atlantic Ocean

Abstract The Rio Grande Rise, a major aseismic rise in the western South Atlantic Ocean, is composed of two portions with distinct morphologies and geological histories. The western portion of the Rise (WRGR) is a large elliptical bulge with its crest at a mean depth of about 2000 m. Numerous guyots protrude from its main platform. The crust underlying this portion of the Rise is older than 80 m.y.B.P. and was formed at a spreading center probably situated close to the sea level. A widespread volcanic event affected this region during the Eocene and generated several volcanic islands located at the center of the present WRGR. This Eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots. Much less is known about the eastern portion of the Rio Grande Rise (ERGR). It has a north—south trend, parallel to the present South Atlantic spreading center, is bounded by fracture zones, is in a conjugate position to part of the Walvis Ridge in the eastern South Atlantic and may represent an abandoned spreading center.

The Rio Grande fracture zone in the western South Atlantic and its tectonic implications

Abstract A broad zone of linear, mappable basement structures is observed north and northeast of the Rio Grande Rise in the South Atlantic Ocean. These structures lie along the same flow line as the Sa˜o Paulo Ridge, the Florianopolis High, and onshore lineaments, suggesting that they all comprise the same tectonic trend: the Rio Grande fracture zone. The morphology developed along this fracture zone during the early opening of the South Atlantic Ocean formed a barrier to open ocean circulation during the Aptian and allowed the formation of extensive evaporite deposits to the north of it.