DEFLECTION OF PALEOMAGNETIC DIRECTIONS DUE TO MAGNETIZATION OF THE UNDERLYING TERRAIN

In the summers of 1989 and 1991 we made 344 near-ground level measurements of the ambient geomagnetic field above recent basalts on the island of Hawaii using a three-component fluxgate magnetometer. We studied 12 surface features, including a lava pond, lava channels, long tilted blocks, smooth sloping surfaces, two fissures, and a deep U-shaped road cut. We observed substantial differences (up to 20 degrees) between the observed and expected (International Geomagnetic Reference Field, IGRF) magnetic field directions over these features except those composed of shelly pahoehoe and a flat (horizontally) thin lava pond. We also observed inclinations that were systematically shallower than the IGRF field by up to 5 degrees. We show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island. We found that all of the observed inclination deflections can be explained by simple two-dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain. Our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled. Since this information is rarely available, it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation (PSV). We therefore conclude that PSV dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows. We also suggest that some of the geomagnetic excursions inferred from. measurements on volcanic rocks may be at least in part due to the magnetic terrain effect. It is unnecessary to invoke ad hoc mechanisms such as elastic, block, or crustal rotations, distortion of the top crust, or flow deformation to explain the large between-site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks. Our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks, as a systematic inclination deflection due to local and regional slopes and irregular terrain, such as those we observed, would lead to a corresponding error is the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.