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Abstract

The emplacement of the Jurassic dolerites of
Tasmania is considered in terms of the tectonic
structure of Tasmania at the time of intrusion. At
this time Tasmania consisted of a Permo-Triassic
sedimentary cover draped in gentle downwarps over
an uneven basement of folded Precambrian-Palaeozoic
rocks, with uplifted highs in the west, northeast,
and east.
Characteristics required for the recognition of
dolerite feeders in Tasmania, particularly cones,
are discussed and the structures are examined in
the light of these. In form, the cones are generally
asymmetrical structures. In many cases there is
insufficient evidence present to determine whether
they represent rootless conical sheets emplaced
under a flotational mechanism, or whether they
represent conical feeders. Possible modes of
emplacement are considered. Some of the cones
appear to be secondary structures developed in the
sedimentary cover at about the base of the Triassic
sequence, possibly from cupola-like structures on
underlying sill sheets.
The intruding dolerite was controlled by, and
also caused, extensive Jurassic faulting. The
recognisable tectonic pattern is shown. The faulting
in places suggests the development of horstlike
and graben-like structures in the downwarps
between the main pre-existing highs. Some of the
major Tertiary faults of the State were originally
lines of Jurassic movements, and there is a suggestion
that the Tertiary Midlands, Cressy, Tamar,
and possibly the Derwent and Oyster Bay trough
structures were inherited, in part, from Jurassic
structures. It is considered east-north-easterly
and north-easterly tensional forces were important
in producing the fault pattern and that it was controlled
to a certain extent by the margins of the
main pre-existing highs and structural trends in
the basement. The greatest fracturing took place
in the central trough of the main downwarp
between the highs of western and eastern Tasmania.
Dolerite magma probably rose here from feeder•
dykes and its emplacement was controlled by compensation
surfaces, giving rise to transgressive sills,
meeting in numerous large dykes, with considerable
rafting of sedimentary blocks. Away from the main
downwarp, sill sheets were more extensively
developed and in the broad scheme their emplacement
apparently followed compensation paths.
Cone structures were emplaced, apparently mostly on the margins of the downwarps and on small
intervening highs. On the upper flanks of the
main highs, feeders were mainly plug and dyke-like
plugs. Where basement highs exceeded heights
of about 4,000 feet above the basement level in the
downwarps, dolerite feeders were lacking.
At the time of the dolerite intrusions Tasmania
was probably situated below the Antarctic Circle,
just east of the Ross Sea, and comparisons are
made with the similar Ferrar dolerites of
Antarctica. Differences in the emplacement of the
dolerite appear to be due to structural differences
between the two regions at the time of emplacement.
Only limited comparisons can be made between
Jurassic trends associated with the dolerites of the
two regions, but maxima in the trends of Tasmanian
dykes were apparently similarly orientated
to those of Antarctica.

Item Type:	Article
Authors/Creators:	Sutherland, FL
Keywords:	Royal Society of Tasmania, RST, Van Diemens Land, natural history, science, ecology, taxonomy, botany, zoology, geology, geography, papers & proceedings, Australia, UTAS Library
Journal or Publication Title:	Papers and Proceedings of the Royal Society of Tasmania
ISSN:	0080-4703
Collections:	Royal Society Collection > Papers & Proceedings of the Royal Society of Tasmania
Additional Information:	Copyright Royal Society of Tasmania
Item Statistics:	View statistics for this item

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