Magnetic Properties of Basalts from Deep Sea Drilling Project Leg 83: The Origin of Remanence and Its Relation to Tectonic and Chemical Evolution

Abstract

We have studied the magnetic properties of 22 samples from DSDP Leg 83 to determine the origin of remanence and its relationship to such problems as the tectonic and chemical evolution of the section, the depth of the magnetized layer, and the applicability of magnetic properties of ophiolites to the marine crust. The magnitude of natural remanence has fairly typical values in the uppermost part of the section, falls two to three orders of magnitude in the transition zone, and returns to values slightly less than the upper part in the dike complex. This behavior reflects, for the most part, variations in the amount of magnetic minerals present. Directional behavior is highly variable throughout the section and often shows complexity even on the level of a single sample. Curie temperature measurements and preliminary opaque petrography indicate that the remanence is chemical in origin and probably involves a resetting of the original thermal remanent magnetization (TRM) direction. Selective destructive demagnetization of four breccia samples shows that the remanence of the clasts was acquired prior to consolidation and did not change significantly thereafter. There are also indications that some of the remanence may be carried by secondary magnetic phases. A comparison of these samples with comparable ophiolite rocks is equivocal, with similarities in remanence characteristics but differences in magnetic mineralogy. As for magnetic anomalies, the transition zone is too weakly magnetized to contribute significantly. The available data on the dike complex are inconclusive and their contribution is still open to debate.

Magnetic Properties and Oxide Petrography of the Sheeted Dike Complex in Hole 504B

“…Faults that dip inward and strike parallel to the paleoaxis of spread are ob served in the Troodos Ophiolite (Hurst et al, 1987). It is inter esting to note, as did Smith and Banerjee (1986), that contrary to most models of crustal formation (e.g., Hall and Robinson, 1979), the magnetic data are consistent with crustal rotation at this site toward, rather than away from, the axis of spreading. The proposed listric fault, in this example, must have dipped outward, away from the spreading center.…”

“…To place the crustal section drilled during Leg 111 within the context of Hole 504B as a whole, we have compiled magnetic data from previous DSDP paleomagnetic studies, including those of Furuta and Levi (1983), Pechersky et al (1983), Facey et al (1985), Kinoshita et al (1985), and Smith and Banerjee (1985). These data are plotted vs. depth in Figure 3.…”

“…These data are plotted vs. depth in Figure 3. Because our main purpose in this paper is to examine the magnetic structure of the sheeted dike complex, we therefore present only a brief sum mary of the magnetic structure of the upper two lithologic units and refer readers to Smith and Banerjee's (1986) complete study of the upper part of Hole 504B.…”

“…Values of stable inclination are plotted vs. depth in Figure 2 for the entire section sampled at Hole 504B, including data compiled from previous investigations at Hole 504B by Smith and Banerjee (1986 ation throughout the section, with a change in mean inclination from -24° in the pillow basalts to -1° in the sheeted dikes. This change in stable inclination is also dramatically apparent in the downhole magnetic logging records acquired at this site (see Kinoshita et al, this volume).…”

Rock magnetism and magnetic anisotropy in folded sills and basaltic flows: A case study of volcanics from the Taimyr Peninsula, Northern Russia

“…Low-Ti TM has been reported from deep-buried oceanic lavas [30] and from postulated shallow-water extrusives [24,31] . The variation of magnetic properties across the Taimyr igneous rocks is not only due to grain-size variations, but also due to the concentration of magnetic minerals.…”