ALBERT

Description: The ‘magnetic quiet zone’ in the eastern Gulf of Aden is located between the oceanic crust of Sheba Ridge and the continental crust of Arabia and Somalia, and is separated from both by important structural boundaries. The seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity. The landward boundary is marked by escarpments made up of a series of normal faults. These escarpments extend from 2–3 km below sea-level to 1500 m above sea-level and are equivalent of the ‘hinge zone’ found at mature continental margins. The magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography. The basement lacks the topographic slope characteristic of mid-ocean ridge flanks and is characterized by moderately rough relief. The crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined, they are slightly greater than those of oceanic crust. New heat flow measurements show high values (95.7–123.3 mW m−2) in the quiet zone with values decreasing from Sheba Ridge toward the coast. The unusual structure of the quiet zone and the observations that more opening appears to have occurred between Arabia and Somalia than can be accounted for by the oceanic crust of Sheba Ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational (listric) faulting and dyke injection. This possibility is investigated using both a ‘stretching’ or ‘lithospheric attenuation’ model and a model in which a portion of the extension occurs through dyke injection. It is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension (β=4–6) are required in the deep seaward portion of the quiet zone. This results in more extension than is compatible with the documented motion between Arabia and Africa. However, formation of the magnetic quiet zone occurred over a period of 10–15 Myr rather than instantaneously as assumed in the simple models. When the effects of a finite length rifting episode are considered, less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone.

Description: Although motion between Arabia and Africa is presently occurring along the entire length of the Red Sea, the morphology and tectonics that result from this motion vary greatly along its length. South of 21°N, the main trough is bisected by a deep axial trough which has formed by sea-floor spreading during the past 4 m.y. and is associated with large-amplitude magnetic anomalies and high heat flow. North of 25°N, an axial trough is not present and the floor of the main trough has an irregular faulted appearance. The magnetic field in the north is characterized by smooth low-amplitude anomalies with a few isolated higher amplitude magnetic anomalies commonly associated with gravity anomalies and in many places probably due to intrusions. Between these regions, the axial trough is discontinuous with a series of deeps characterized by large-amplitude magnetic anomalies alternating with shallower intertrough zones which lack magnetic anomalies. It is argued that the different regions represent successive phases in the rifting of a continent and the development of a continental margin. An initial period of diffuse extension by rotational faulting and dike injection over an area perhaps 100 km (60 mi) wide is followed by concentration of extension at a single axis and the initiation of sea-floor spreading. The main trough in the southern Red Sea, away from the deep axial trough, formed during the Miocene by the same processes of diffuse extension that are still active in the northern Red Sea. This model explains the available geologic and geophysical data and reconciles previous models for the formation of the Red Sea which emphasize either the evidence for considerable motion between Arabia and Africa or the evidence for down aulted continental crust beneath much of the Red Sea. The initial pre-sea-floor spreading stage results in considerable extension (160 km or 100 mi) at 25°N in the Red Sea), can last for several tens of millions of years, and is an important factor in the development of the continental margin. Such an extended phase of rifting and diffuse extension must be taken into account in studies of sedimentation, subsidence, and paleotemperatures.

Description: Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.