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Electrical structure of the crust in Southeast Tasmania


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Dwipa, Sjafra 1993 , 'Electrical structure of the crust in Southeast Tasmania', PhD thesis, University of Tasmania.

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In the south-east of Tasmania, Jurassic dolerite forms a partial cover over
much of the area. The sedimentary and volcanic rocks in this region are mainly
Permian or younger. The topography is largely dominated by the local structure of the
Utilizing the magnetotelluric and magnetovariational methods, an
investigation was made of the electrical structure along two cross-sections to lower
crust / upper mantle depths. In conjunction with this study the potential field and
transient electromagnetic methods were used. Two-dimensional gravity and magnetic
modellings delineated the geometry and possible structural origin of several rock
sequences associated with a basin structure. The use of the transient electromagnetic
method placed constraints on the thicknesses and resistivities of the surface layer.
As part of the magnetotelluric analysis package, a new rotation angle and
dimensionality calculation method is introduced and tested with different geometrical
structures. The results when compared to other conventional and the Mohr circle
methods show this new method works well and is simpler and faster. Two types of anomalies, inland and coastal effect, are revealed from
magnetovariational observation. Correction for the ocean were applied to induction
vectors at periods of 10 and 60 minutes. The inland anomaly is characterized by
corrected in-phase induction vectors at stations to the east of the Huon River, pointing
north-west. Those to the west of the river point in a north-east direction, indicating
the presence of a gradient anomaly zone which lies along the Huon River. Meanwhile
the gradual swing in direction from southeast for observed vector to east and almost
perpendicular to the coast-line for corrected vector at the eastern-most station of crosssection
II illustrates the remaining effect of the coast. This can be explained by high
conductivity contrasts between the resistive block in the eastern part of Tasmania and
the conducting ocean floor.
The results from the one and two-dimensional modelling of magnetotelluric,
gravity and magnetic data indicate that the base of the Permo-Triassic cover with its
stockwork of massive dolerite intrusions, is probably never less than 500 metres
below the surface at the northern cross-section and dips south reaching a depth of
about 800 metres on the southern cross-section. The Ordovician limestone, which
may be a possible source of hydrocarbon deposits, has a bulk apparent resistivity
value of 40 Ohm-m inferred from transient electromagnetic modelling. This rock has a
thickness of about 300 metres and its distribution is restricted to the western part of the
study area which is consistent with the results of the two-dimensional gravity and
magnetic modelling. The dipping discontinuity needed in the models to match the
magnetotelluric data, results in a trough-like structure with depth from the surface to
the bottom of about 6 kilometres. This structure is reflected by a broad and large
gravity anomaly together with slightly negative magnetic anomaly and is believed to be
associated with a trough of Cambrian volcanics. This trough has a northwest -
southeast direction and becomes wider and has more conductive flanks to the south. The magnetotelluric results also indicate the presence of a low resistivity layer
at middle-lower crustal depths. There is broad but not exact correlation with the
position of the Cambrian trough above. In combination with other geophysical
evidence gained from the magnetovariational method, i.e. the short corrected induction
vectors at all stations at periods of 60 minutes, the layer is inferred to exist beneath the
entire study area. The likely cause of the low resistivity associated with this layer is
believed to be the presence of free carbon along grain boundaries or fractured rocks
which provide a continuous conducting path. Another possible cause is the presence
of anomalously high temperatures in the deeper crust expected from previous
geothermal measurements.

Item Type: Thesis - PhD
Authors/Creators:Dwipa, Sjafra
Keywords: Earth currents
Copyright Holders: The Author
Copyright Information:

Copyright 1993 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 references (p. [187]-198)

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