A Geomagnetic Variation Anomaly Across the Northern Gulf of California

Abstract

Summary Variations in the geomagnetic field have been observed along a profile perpendicular to the northern Gulf of California. The profile extends from the Pacific Ocean (including two sea-floor stations off the coast) to Tucson, Arizona, and includes stations at San Felipe and Puerto Penasco on opposite sides of the Gulf. At these two stations there is a large anomaly in the vertical component variations in the frequency range 0.025–4 c hr-1 which amounts to almost complete reversal from one station to the other at the higher frequencies. Analysis of the variations demonstrates that the anomaly is due to induced currents flowing in or beneath the Gulf of California. Using a modelling technique developed by Greenhouse, Parker and White the anomaly may be interpreted either in terms of very high Gulf sediment conductivity or an uplifted zone of hot mantle material beneath the Wagner basin. A suitable combination of these models is compatible with the horizontal field observations. Both of these factors support the suggestion that the Wagner basin is the site of a spreading centre similar to those found at ocean ridges. Observations along a profile of stations across the Imperial Valley indicate that the upwelling of mantle material does not necessarily occur continuously along the Gulf of California—Salton Trough axis and is probably a local phenomenon associated only with such spreading centres. Because of the consequent lack of two-dimensionality in the actual situation, the predicted height of the upwelling will be too small. It is thought that a conductivity of 0.02 (Ωm)-1 could be present at 20–30 km depth in such areas.