Extension of the Eyre Peninsula Conductivity Anomaly

Abstract

Twenty-two digitally recording fluxgate magnetometers measuring short-period natural time variations of the geomagnetic field were deployed during 1988 at 54 sites to define the northward extension of the Eyre Peninsula Conductivity Anomaly (EPCA). The southern portion of this zone of high electrical conductivity was initially mapped during the early 1980s; it trends inland just to the east of north from the southern extremity of the peninsula, at nearly 90° to the continental shelf edge.The 1988 data show that the axis of the anomaly in the north initially swings to the east, and then turns NNW, following the major structural trends in the Precambrian basement of the Gawler Block. Short-period geomagnetic variations observed in stacked magnetograms display a significant amplitude and phase change in the vertical (Z) and total field (F) components across the anomaly axis, while the horizontal variation fields (D and H) are enhanced along the axis. These features are consistent with a concentration of current in the crust.Transfer-functions relating the anomalous vertical to normal horizontal geomagnetic variation fields are expressed in the form of vectors and hypothetical event contours, and these clearly delineate the surface position of the anomaly axis.The EPCA is coincident with a broad band of seismic activity, and a continuous conductive zone such as this must be formed along a significant structure within the Precambrian crust. The zone probably delineates the boundary between the essentially non-conductive Archean crystalline basement to the west and the multiply deformed Proterozoic metasediments of the Hutchison Group to the east. Its presence is important in understanding the tectonic development of this potentially economic zone.Zones such as the EPCA can also introduce errors of several nT into aeromagnetic surveys; data from closely-spaced magnetometer arrays may be used to accurately predict and eliminate such errors.