Origin of the Natural Remanent Magnetism of Sheeted Dikes in Hole 504B Cored during Legs 137 and 140

Abstract

The natural remanent magnetism of samples from the sheeted dike complex generated at the Costa Rica Rift, drilled in Deep Sea Drilling Project/Ocean Drilling Program Hole 504B, can be resolved into a vertical drilling induced remanent magnetization and a shallow stable remanence by alternating field and thermal demagnetization. We investigate the unblocking temperature and coercivity spectra of these components, and compare these to spectra of thermal, induced, and viscous magnetizations imparted in the laboratory. The drilling-induced remanence is not a simple thermal, induced, or viscous remanence, but probably a piezoremanence acquired in an elevated field. The stable remanence may be a thermal remanence. Comparison of the stable remanence in altered and fresh samples indicates that natural remanent intensity and bulk susceptibility are reduced in more altered samples, but the stability and direction are similar to those in fresh samples. Whereas the stable components of the sheeted dike complex as a whole have a mean inclination of -18° ± 17°, individual lithologic units exhibit much less scatter and significantly different inclinations. A plausible explanation is that the stable remanence is a thermal remanence or a rapidly acquired chemical remanence, which does not average secular variation. The distribution of the inclination data suggests that dikes may be intruded as multiples.