An integrated thermomechanical model for transform continental margin evolution

 A 2D numerical thermal model for transform continental margin evolution is presented that calculates thermally driven uplift and subsidence profiles across the margin, for any margin segment assuming both regional and local isostasy. Lateral variations in the magnitude of continental uplift along the transform are predicted. For a margin with a length of 900 km, with a spreading rate of 1 cm yr^-1, maximum continental uplift of 1300–1400 m is calculated, assuming local isostasy. Using a regional isostatic approximation, maximum uplift is reduced substantially to 335–470 m, and the exact magnitude, location, and timing of the maximum effect depends strongly on the assumption of a coupled or decoupled continent–ocean boundary. The length of time a margin point experiences continent–ocean shearing prior to ridge passing is also shown to be very significant. Received: 23 February 1995 / Revision received: 17 August 1995     

Palaeozoic corals: their evolution and palaeoecology

In recent years, our understanding of Palaeozoic corals has increased enormously. Several new groups have been discovered, extending the range of corals back to the early Cambrian, and we now have a clearer idea of the relationships between them. None of them was a direct ancestor to the post‐Palaeozoic corals. Similarly, the ecology of Palaeozoic corals differed from that of their living relatives in important respects. Palaeozoic corals were principally adapted for life on soft substrates and were not the spectacular reef‐builders that modern corals have become. However, their contribution to Palaeozoic ecosystems is nonetheless fascinating. Their structural adaptations and growth‐forms can tell us much about life and processes on Palaeozoic sea floors.     

Design and application of digital filters for the Graf-Askania Gss2, No. 11 sea gravity meter

With knowledge of the response characteristics of the Graf-Askania Gss2, No. 11 gravity meter it has been possible to design a digital filter to correct for the attenuation and time lag caused by the heavy magnetic damping of the beam. Other filters have also been designed to ensure that all parameters related to the measurement of gravity at sea have a similar cut-off frequency. The filters are able to operate in an on-line reduction mode if required. The method has been used on data recorded at sea to illustrate attenuation and time lag corrections and its suitability in rough weather.     

Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea

 Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT. Received: 6 February 1995/Revision received: 24 July 1995     

Davie fracture zone and the movement of Madagascar

A curvilinear feature extending over 21° of latitude can be recognised in the western Indian Ocean. Its structure and relationship to Madagascar suggest that it is a fracture zone that may have controlled the motion of Madagascar as the island drifted southwards to its present position. The pole of rotation defined by the fracture zone does not allow a good fit between the late Palaeozoic to early Mesozoic palaeomagnetic poles for Africa and Madagascar [3] if the latter was in a position against Kenya at the time. The likely presence of the fracture zone needs to be reconciled with the palaeomagnetic results.     

Structure and evolution of the sea floor south of South Africa

Geophysical results from the continental margin south of South Africa are consistent with a fault-controlled origin for the steep, linear continental slope. Accepting that the faulting was most probably of the shear type, caused by the transcurrent motion of the Falkland Plateau past South Africa, a model for the evolution of the sea floor adjacent to the margin is proposed. In this model the Agulhas Plateau is considered to be an oceanic feature, possibly an abandoned sea-floor spreading centre.     

Long-term changes in the rotation rate of the Earth

In the last decade, major advances in both observational and theoretical fields have taken place in our understanding of the rotation of the Earth. With the adoption of Atomic Time, the rotation rate can now be measured with unprecedented precision and independently of planetary or lunar motions. Information from earlier observations is also being recovered more accurately: independent analyses and new data from ancient eclipse observations have increased confidence in the controversial results obtained earlier. Perhaps the most outstanding development is the tentative measurement of the mean rotation rate in the remote geological past, made possible by the study of periodicities in the skeletal growth of fossil organisms.The improved data have exposed new geophysical problems and accentuated earlier ones. An unexplained acceleration of the Earth's rotation is confirmed by the re-examination of ancient eclipse records. In addition, Darwin's theory of tidal friction now has the arbitrariness of its time-scale replaced by data from modern astronomical observations so that, in spite of various theoretical refinements, it leads to the unacceptable prediction of a catastrophic period in the history of the Earth-Moon system in the mid-Precambrian. Information from the study of fossil growth increments can make a vital contribution in both these problems.The palaeontological data have been gathered mainly from corals and bivalves, although several other groups with accretionary skeletons provide a few figures and are potentially useful. The unexpected cosmogonic applications of these data have stimulated research into the physiological and particularly the ecological factors controlling incremental growth in these organisms, especially the bivalves. The main problems centre on the recording accuracy and the definition of the growth increments. The difficulty of eliminating irregular interference and of recovering geophysically useful measurements from the fossil record is discussed and we have tried to assess the reliability of the information already derived from this source.     

Siphonostomatoid copepods infecting Squalus acutipinnis Regan, 1908 off South Africa

Squalus acutipinnis (previously included in S. megalops) individuals caught off the east, south and west coasts of South Africa were examined for siphonostomatoid copepod infection. Collected siphonostomatoids were fixed and preserved in 70% ethanol and studied under microscopes using both transmitted and incidental light. The copepods were identified and the host-parasite associations estimated by calculating their prevalence, mean intensity and mean abundance. The siphonostomatoids represented four families and six species, and included new host records for Nemesis sp., Lernaeopoda sp. and Eudactylina acanthii, and a new geographical record for E. acanthii. Squalus acutipinnis individuals were infected by relatively low loads of siphonostomatoids. The most commonly found species was Achtheinus pinguis, the only species found off all three coasts. There was no difference in the prevalence of A. pinguis in sharks sampled off the three coasts but the highest mean intensity was found on the East Coast followed by the West Coast. The overall prevalence of A. pinguis was 33.6%, and was an order of magnitude higher than that of the other siphonostomatoids recorded. At such low levels of infection it will thus be necessary to examine a large number of hosts to determine the biodiversity of siphonostomatoids and geographical differences in prevalence in South African waters.