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Gravity and magnetics of the West Tamar district, Northern Tasmania


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Zengerer, M 1999 , 'Gravity and magnetics of the West Tamar district, Northern Tasmania', Honours thesis, Universtiy of Tasmania.

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The West Tamar District of northern Tasmania is a region of diverse geology and
complex structure. The area comprises an allochthonous block of Precambrian basement,
the Badger Head Block, a Lower Palaeozoic thrust belt, and a large Tertiary graben, the
Tamar Graben. A gravity survey was carried out in the area in order to model the
subsurface geology and structure.
The residual Bouguer anomaly reveals some large and small scale trends. To the east
steep symmetrical west and east directed gradients mark the down thrown block that is
the Tamar Graben, whilst to the west a long, N-S trending, east-directed gradient marks
the western boundary of the Badger Head Block and is considered to mark a major
structure. An ultramafic complex near the centre of the study area causes a large negative
anomaly due to its level of serpentinisation, whilst positive anomalies over the low
density Badger Head Block suggest denser lithologies at depth.
Gridding and re-interpretation of pre-existing aeromagnetic data sets has enabled the
complex faulting structure of the area to be outlined. Comparisons of faults inferred from
the magnetics with a digital elevation model and local geology reveals that much of the
present topography and outcrop geology is controlled by northwest and northeast trending
faults. The northwest trending faults form the major structures in the region. The
ultramafic complex is comprised of two main bodies, with a third inferred to branch west
from the main body beneath the Badger Head Block.
Three structural cross sections have been created using forward modelling of gravity and
magnetics. Two of the sections model east-dipping, thrust-bound packages of Lower
Palaeozoic strata along the eastern part of their sections. The western sections are best
modelled with Cambrian ultramafics and sedimentary rocks dipping west beneath the
Neoproterozoic Badger Head Block.
The tectonic implications of the outcome of structural modelling are that a NE-directed
thrusting event is inferred to have occurred during the Cambrian or Early-Middle
Devonian prior to SW -directed thrusting in the Middle Devonian, which emplaced
Palaeozoic strata in the Beaconsfield area into thrust fault-bound packages.

Item Type: Thesis - Honours
Authors/Creators:Zengerer, M
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Copyright 1999 the Author

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