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Magnetotelluric investigations of the Tamar lineament


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Hermanto, MR 1992 , 'Magnetotelluric investigations of the Tamar lineament', PhD thesis, University of Tasmania.

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Magnetotelluric experiments were carried out in the Tamar lineament
area of northern and central Tasmania to study subsurface electrical structure.
These experiments have defined a high conductivity anomaly in the upper-middle
crustal depth range and confirmed the presence of the anomaly suggested by
previous authors (Buyung, 1980; Sayers, 1984; Hermanto, 1985; Parkinson,
Other geophysical techniques such as de-resistivity, SIROTEM and
two-dimensional gravity and magnetic modelling were also used. Results from
these studies and other geophysical studies such as the seismic studies of
Richardson (1985) and Vitesnik (1984) make a significant contribution to a
better understanding of the shallow structure, nature, tectonic history and, most
importantly, the setting of the conductivity anomaly.
The measured apparent resistivities are interpreted in terms of
inhomogeneous electrical structure using one- and two-dimensional modelling
indicating that the top of the high conductivity anomaly is located about 500 to
700 metres from the surface. This is about the level of the basal Permian cover
unconformity. The resisitivity of the anomaly ranges from 1 to 8 Ohm.m with a
thickness of 13.3 to 16.3 kilometres. The shape and position of the anomaly
which tapers with depth is consistent with the results suggested by the current
density and two-dimensional gravity and magnetic modelling. The gravity and
magnetic data indicate large fault or thrust bounded blocks of Lower Palaeozoic
and Upper Precambrian rocks underlie the Permian cover. The gravity and magnetic
models also show that the more magnetic Cambrian volcanic sequences, may
account for the upper part (at least 5 kilometres) of the anomaly. The deeper part
of the conductive structure underlies the more magnetic part of the Cambrian
volcanics. This suggests that the formation of the volcanic piles may be associated
with high conductivities deeper in the crust. It seems likely that the boundaries of
the conductive block are marked by tectonic slices of ultramafics. The conductive
anomaly does not extend west beyond the Tiers Fault which marks a fundamental
structural limit affecting rocks from Cambrian to Tertiary in age. This zone is
marked by a strong gravity gradient which extends SSE from Devonport to
Sorell across central Tasmania. No such feature occurs along the line of the
Tamar River or the Tamar Lineament of previous authors. The most likely cause of the high conductivity anomaly is a combination
of the presence of high conducting fluids and graphite in pores, cracks, and or
fractured rocks which provide a continuous conducting path. Geological and
geophysical evidence showing a considerable tectonic histo:ry in this region support
abnormal porosity and extensive fracturing of the deep crust providing an
environment for deep fluid circulation and graphite precipitation. Black shales
and sheared carbonates are to be expected in the Precambrian rocks implied to be
present. The velocity-resistivity relation suggests that a resistivity as low as 2
Ohm.m may be obtained from the Tasmanian crust where the velocity is about
5.8 to 6.0 km s-1. This is comparable with the resistivity associated with the
high conductivity anomaly.

Item Type: Thesis - PhD
Authors/Creators:Hermanto, MR
Keywords: Earth currents, Earth resistance, Geomagnetism
Copyright Holders: The Author
Copyright Information:

Copyright 1992 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical reerences (p. 130-140)

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