Magnetic signatures of zones of fractures in igneous metamorphic rocks with an example from southeastern New England

Abstract

Elongate negative magnetic anomalies in the Northern Hemisphere can be related to zones of fractured bedrock. A magnetic model with induced magnetization of a horizontal plate that is separated by a nonmagnetic gap can reproduce observed magnetic lows. Model anomalies show the dependence of the intensity of the magnetic lows on parameters, such as magnetic susceptibility, width of the nonmagnetic gap and dip angle of the fracture zone. The magnetic low is superposed by a larger regional low which increases with the thickness of the magnetized plate. The strike direction of the fracture zone and the inclination of the ambient magnetic field also control the anomaly so that new sets of model curves are necessary for each new field case depending on location and orientation of the fractures. The model has been tested over a negative magnetic anomaly in southern New England with ground- and aeromagnetics. The coincidence with a zone of fractures, known as the Watch Hill lineament, has been documented with detailed geological mapping, remote sensing and fracture trace analysis. A good agreement between the model anomaly and the observed anomalies on the ground and from the air is possible with observed magnetic susceptibilities between 0.4 · 10 −3 and 1.2 · 10 −3 cgs. The anomaly width ranges between 1.4 km and a few tens of meters. A detailed analysis of intersecting fractures allows a distinction of older from younger anomaly trends. This zone of fractures, associated with the magnetic anomaly, is believed to be part of a major fault zone. Its northeastern extension is the margin of the Narragansett Basin, which is filled with sediments of Carboniferous age. While there have been many suggestions for its origin, the results of this study favor the formation of a pull-apart basin after the area was subjected to extensional stress.