Deflection of paleomagnetic directions due to magnetization of the underlying terrain

Abstract

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°) 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°. 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 clastic, 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 in. the inferred paleolatitude. The magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions.

Full-vector geomagnetic field records from the East Eifel, Germany

“…So-called local anomalies due to terrain effects (e.g. Baag et al, 1995;Valet and Soler, 1999) in our data would therefore not be averaged out as good as in Böhnel et al (1982) and Schnepp (1996)'s data. However, as Böhnel et al (1982) and Schnepp (1996)'s palaeodirections generally showed low scatter (α 95 usually less than 4°), it appears unlikely that these large deviations would be due to the sampling method.…”

Applications of paleomagnetism in the volcanic field: A case study of the Unzen Volcano, Japan

“…two sites (e.g., Castro and Brown, 1987;Baag et al, 1995;Urrutia-Fucugauchi et al, 2004). Small movements of the outcrop could also be the cause of the error.…”

The multimethod palaeointensity approach applied to volcanics from Terceira: full-vector geomagnetic data for the past 50 kyr

“…Furthermore, this explains why the k-values associated with the directions may be rather low for volcanic products: by sampling the cooling unit over large distances, small post-cooling movements within the outcrop or of the underlying terrain are averaged, lowering the k-values, but increasing the reliability of the directions. Moreover, potential local anomalies in the field (Baag et al 1995;Valet & Soler 1999) are suppressed by this sampling strategy.…”