Paleomagnetism of Basement Rocks, Leg 37

Abstract

Close to 400 semioriented subsamples of oceanic basement rocks from DSDP Sites 332, 333, 334, and 335 have been tested for paleomagnetic properties. Eight nonbasaltic samples consist of a variety of plutonic rocks from a melange penetrated at Site 334. Basalt in situ remanence intensity, giving unit weight to each 9.5meter core length, is very close to 40 × 10~4 emu/cm 3 , or 40% of the value normally assumed in model studies. The ratio of remanent to induced magnetization is high, averaging 37. In terms of remanent intensity and induced magnetization, the basalts fit the requirements of the Vine-Matthews hypothesis. However, in terms of vertical polarity layering, and abundant very shallow, nondipole NRM inclinations at Sites 332 and 333, they do not fit the hypothesis. Equivalent magnetic source layer thickness varies from 570 to 820 meters for dipole NRM inclinations to in excess of 2.5 km for the observed shallow inclinations. Systematic alternating field NRM analysis results in significant inclination change in 44% of samples. Shallow original magnetization is confirmed for many samples from Sites 332 and 333. The explanation appears to lie between tectonic tilting or recording of an excursion of long duration of the geomagnetic field.Cleaned magnetic inclinations are tightly grouped when divided on the basis of lithologic units. However, in many cases, sequences of lithologic/magnetic units have indistinguishable (or nearly so) average cleaned inclinations (the magnetic superunits of this chapter), suggesting formation during single eruptions or short (~10 2 yr) eruptive episodes. It seems that a 2-km-thick section of oceanic layer 2 could be formed by about 20 major eruptions or eruptive episodes, together with some minor intrusives. The occurrence of many anomalous inclinations does not permit reliable absolute plate motion estimates to be obtained from the data.

Comparison of Chemical and Magnetic Stratigraphy of Holes 396 and 396B

“…Four units in Hole 332A do not extend to Hole 332B, 107 meters away but one much thicker unit does (Natland, in press). One other unit in Hole 332B does not reach to Hole 332A and has a different polarity than the top of 332A (Hall and Ryall, 1977). Holes 395 and 395A, drilled on Leg 45, are so close together that the difference in their location is not resolvable by satellite fixes (Melson, Rabinowitz, et al, in press).…”

Comparison of the Chemical and Magnetic Stratigraphy of DSDP Sites 332 and 395

“…But phyric basalt P B is relatively less depleted in light rare-earth elements (Figure 11) than phyric basalt P 2 (Figure 10), so the two types do not match chemically. Hall and Ryall (1977) speculated that low inclinations of Site 332 and the poor correlation between the upper parts of Holes 332A and 332B result from a combination of "tectonic breaks" and possible eruption during a long-term departure from an axial dipole field. They pointed out that to explain the low inclinations entirely by faulting would require a dip to the south of 30° to 40°, whereas, so far as is known, only small dips (3°-10°) to the west occur in the area.…”

“…There are no olivine-rich basalts or Hall and Ryall (1977) Bougault (1977) and Blanchard et al (1976), normalized as in Tables 1 and 2 (see text). Rare-earth element data from Blanchard et al (1976) and Schilling et al (1977). oceanites at Site 395, and as argued earlier, Site 395 basalts appear to represent smaller degrees of melting in the mantle than basalts of Site 332.…”

“…Hall and Ryall (1977) first determined mean inclinations on each lithologic sub-unit (as defined by the Leg 37 shipboard party), in several cases dividing these into further magnetic units, then grouped as many of these as possible into magnetic superunits. These superunits are essentially equivalent to the magnetic units of Site 395, and it is their mean magnetic inclinations which are shown on Figure 2.…”

“…MAGNETIC UNITS Magnetic units were defined in much the same way on Legs 37 and 45 (Hall and Ryall, 1977; Johnson, Paleomagnetism of igneous rocks, this volume). The magnetic inclinations of minicore samples taken from one or more oriented basalt pieces per core were measured.…”