Magnetic parameters of rocks from the Red Sea rift zone

Results of detailed geomagnetic and geomorphological studies carried out by R/V Akvanavt together with data obtained by a side-scanning sonar and high-frequency profiles from a towed Zvuk-4 vehicle plus results of visual observations of deep-sea manned Pisces submersible have shown that the spreading axis is divided into segments, whose strike (330°) differs from the overall strike (310°) of the axial magnetic anomaly. In the study area segments are about 1 km long and transform displacements are 0.5 km. Calculations on a model have shown that spreading is asymmetric: during the Brunhes epoch accretion rate of the African Plate was 6 mm/yr and that of the Arabian Plate 7 mm/yr. Earlier it had been 9 and 11 mm/yr, respectively.

Magnetic properties of plutonic rocks at DSDP Leg 82 holes

Ten samples of gabbro and peridotite, with varying degrees of serpentinization, were studied by magnetic techniques and reflected light microscopy. Evidence from these methods suggests that the natural remanent magnetization is primarily of chemical origin. It is generally weak for the gabbros and much stronger for the peridotites. This difference is offset by the fact that the peridotites have generally lower magnetic stability and Koenigsberger ratios. There is a considerable variation in both magnetic parameters and petrology even among closely spaced samples, which suggests that some combination of source heterogeneity and tectonic mixing was involved in the production of these rocks. However, the small number of samples makes this conclusion tentative. There may also have been significant postemplacement alteration involved. All samples show a significant anisotropy of weak field susceptibility that appears to be related to deformation. This anisotropy may be useful in defining petrofabrics.

Magnetic susceptibility of bottom sediments from the Middle and South Caspian Sea

Dependence of magnetic susceptibility of bottom sediments from the Caspian Sea on composition of magnetoactive minerals contained in the heavy subfraction of fine-grained sand (0.125-0.100 mm grain size fraction) was established. Changes in the curve shape and magnetic susceptibility values reflect a pulsating pattern of input of different (in magnetic properties) magmatic and metamorphic clastic minerals into sediments, as well as different intensities of formation of authigenic magnetoactive iron sulfides under conditions of multiple alternation of transgressive and regressive phases in marine basins. Values of magnetic susceptibility and shapes of magnetic susceptibility curves for studied sedimentary sequences show that sediments in the South and Middle Caspian Basins are characterized by different specific features.

Magnetic properties of Triassic sediments of the Wombat Plateau

During drilling at Sites 759, 760, and 761 of Leg 122 (Exmouth Plateau, northwest Australia), a thick section of Upper Triassic sediments was recovered. Paleomagnetic analyses were made on 398 samples from Holes 759B, 760A, 760B, and 761C. Progressive thermal demagnetization, alternating field demagnetization, or mixed treatment removed an initial unstable component and isolated a characteristic remanent magnetization which is of normal or reversed polarity. The magnetostratigraphic results allow us to propose a magnetic polarity sequence which extends from the upper Carnian to lower Rhaetian. This sequence reveals many more reversals than previously suggested from paleomagnetic studies. The magnetostratigraphic data also allow us to suggest correlations between Sites 759 and 760.

Physical, geochemical and magnetic properties of several ODP Leg 192 sites, Ontong Java Plateau

In this manuscript, we present the results of a physical properties investigation carried out on basaltic cores recovered from the four Leg 192 basement sites, focusing on the relationship between physical properties and alteration in basalts. Variations in physical properties in the Leg 192 basement sites closely resemble each other and reflect the amount of alteration and vein formation in the basement basalts. P-wave velocities, magnetic susceptibilities, and densities for the dense massive basalts are higher than those of more altered and heavily veined basalts. Porosity-dependent alteration is observed at Leg 192 basement sites: P-wave velocity displays a general decrease with increasing loss on ignition and potassium content. These trends are consistent with trends documented for typical alteration of oceanic crust and suggest that basalt alteration is largely responsible for the variation of the physical properties exhibited by rocks at Leg 192 basement sites. Our physical property data support the conclusion that only low-temperature seawater-mediated alteration occurred in the lava flows of the Ontong Java Plateau (OJP). This lack of higher-temperature hydrothermal alteration is consistent with the idea that the OJP basement sites are far from their eruptive vents.

Paleomagnetic and rock magnetic characterization of ODP Leg 210 cores

We conducted an integrated paleomagnetic and rock magnetic study on cores recovered from Ocean Drilling Program Sites 1276 and 1277 of the Newfoundland Basin. Stable components of magnetization are determined from Cretaceous-aged sedimentary and basement cores after detailed thermal and alternating-field demagnetization. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier. In view of the normal polarity of magnetization and radiometric dates for the sills at Site 1276 (~98 and ~105 Ma, both within the Cretaceous Normal Superchron) and for a gabbro intrusion in peridotite at Site 1277 (~126 Ma, Chron M1), our results suggest that the primary magnetization of the Cretaceous rocks is likely retained in these rocks. The overall magnetic inclination of lithologic Unit 2 in Hole 1277A between 143 and 180 meters below seafloor is 38°, implying significant (~35° counterclockwise, viewed to the north) rotation of the basement around a horizontal axis parallel to the rift axis (010°). The paleomagnetic rotational estimates should help refine models for the tectonic evolution of the basement. The mean inclinations for Sites 1276 and 1277 rocks imply paleolatitudes of 30.3° ± 5.1° and 22.9° ± 12.0°, respectively, with the latter presumably influenced by tectonic rotation. These values are consistent with those inferred from the mid-Cretaceous reference poles for North America, suggesting that the inclination determinations are reliable and consistent with a drill site on a location in the North America plate since at least the mid-Cretaceous. The combined paleolatitude results from Leg 210 sites indicate that the Newfoundland Basin was some 1800 km south of its current position in the mid-Cretaceous. Assuming a constant rate of motion, the paleolatitude data would suggest a rate of 12.1 mm/yr for the interval from ~130 Ma (Site 1276 age) to present, and 19.6 mm/yr for the interval from 126 Ma (Site 1277 age) to recent. The paleolatitude and rotational data from this study are consistent with the possibility that Site 1276 may have passed over the Canary and Madeira hotspots that formed the Newfoundland Seamounts in the mid-Cretaceous.

The downhole variation of the rock-magnetic properties of DSDP Leg 73 Holes

The magnetic stability and mean intensity of the natural remanent magnetization (NRM) of Leg 73 sediments (Holes 519 to 523) decreases with the age of the sediment. We demonstrate that these variations are linked with physical and chemical changes in the magnetic grains themselves. Alteration of the magnetic component occurs most rapidly shortly after deposition. A significant magnetic alteration over the topmost few meters of the sediments is thought to be the result of oxidation. The modification of the NRM characteristics through the partial dissolution of the carbonate is largely accounted for by the effects of concentraion of the magnetic minerals. We apply the techniques of rock-magnetism and X-ray fluorescence analysis to clarify the physical and chemical mechanisms that affect the magnetic character of the sediment.