Abstract

Geomagnetic variation data for all available stations in eastern North America are presented here in the form of induction arrow maps at periods of 6, 20, and 64 min. The region is subdivided on the basis of the induction patterns revealed by these maps.The near-coastal stations are influenced by conductivity contrasts associated with the continent – ocean water interface, although the considerable variation in amplitude and in inland persistence of the coast effect reflects variations in the conductivity contrast across this transition. The predominantly southward-pointing arrows of the shield areas appear to reflect their source fields rather than subsurface conductivity. This pattern is disrupted by the high conductivity associated with the anomalously high crustal temperatures in the vicinity of the White Mountain heat flow anomaly and is terminated abruptly in the south by a well-defined reversal through northern New York State. We suggest that the conductivity boundary defined by this line of reversal continues southward to join a similar boundary observed near the Ohio – West Virginia border. These boundaries, with the crystalline Appalachians to the east, bound a segment of the crust (or upper lithosphere) whose conductivity structure is quite distinct from its surroundings.This region lies within the Central Geophysical Province of Diment et al.; however, if the poorly defined western and northern boundaries of that province are replaced by the line of reversal the two regions coincide. Some tentative models for its origins are discussed. We also suggest that the subdued geomagnetic coast effect in Virginia may be related to the extensive overthrusting of Paleozoic sediments by the crystalline Appalachians, which has been proposed on the basis of deep seismic reflection studies further to the south (Cook et al.).

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