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The structural and metamorphic history of the Wilmot and Franklin ranges, South-west Tasmania.


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Boulter, Clive Anthony 1978 , 'The structural and metamorphic history of the Wilmot and Franklin ranges, South-west Tasmania.', PhD thesis, University of Tasmania.

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Tidally dominated, shallow-shelf sea conditions are indicated
for the deposition of much of the late Precambrian sequence of the
Frankland and Wilmot Ranges, Southwest Tasmania. Predominantly
pelitic units may reflect tidal-flat or deltaic situations, and
detritus in marine quartz sand horizons was derived from an environment
of prevailing aeolian conditions perhaps by transgression.
Overprinting proves five cleavage-forming deformation events
followed by at least two conjugate kink sets and a regional 'faultdrag'
rotation of all structural surfaces. The major geometrical
features are the result of D 1 and D4 whilst D2 and D3 locally
produce macroscopic folds; all D 1 to D5 events are essentially
coaxial. Major D4 folds are generally tight upright structures with
wavelengths and amplitudes of about four kilometres and they
rotate all earlier events. Gravitational gliding was responsible
for the emplacement of F 1 structures which were subsequently, in
the main, only slightly modified by further overriding during the
same event. D 1 folds root to the west and southwest and though
F2 show the same direction of overthrusting it is uncertain whether
D1/D 2 is a continuous event. D1/D 2 show orthotectonic characteristics
whilst D4 is of a paratectonic style. D5 and the later conjugate
kink bands are minor in scale. The regional rotation of deformations
before and including those of D5 and the kinks is considered to
have been caused by pre-Ordovician transcurrent movement on the
Lake Edgar Fault. Quartz arenite, in D 1' first deformed by plastic
deformation in hydrolytically weakened diagenetic quartz overgrowth
but soon stress difference, in the grain-supported arenite, was
taken up by intragranular plasticity of the detrital grains. Structural
grains become apparent in outcrop at less than 10% shortening and
good cleavages require less than 20% shortening. Penetrative
fabrics developed in pelitic rocks in D l . All phases are extremely
heterogeneous, with unstrained zones surviving to the present day.
Post-D 1 cleavages usually involve microfolding of earlier fabrics,
pressure dissolution and/or intracrystalline plasticity of quartz
and mica. The relative importance of each mechanism is dependent
on the pre-existing fabric and mica content of the rock.
Investigations of strain in quartz arenite revealed a need
to measure sedimentary fabrics from non-orogenic areas to provide
a sound basis for work in deformed material where initial marker
ratios and orientations were variable. Methods chosen must also
allow for an independent check on the validity of two-dimensional
strain ratios. The geometry of deformed cross-bedding shows
that flexural slip was important in the formation of major F 1
structures which were modified by an average 25% flattening.
As sedimentary structures are commonly modified or mimicked by
deformation, care in interpretation is emphasised. Soft sedimentary,
pre-tectonic clastic dykes are often planar and sub-parallel to
cleavages where both structures are at an angle to bedding.
Ready convergence of sedimentary and tectonic elements is thus
demonstrated and the use of the approximate parallelism of dykes
and cleavage to support tectonic dewatering is considered unsound.

Item Type: Thesis - PhD
Authors/Creators:Boulter, Clive Anthony
Keywords: Geology, Structural, Metamorphic rocks, Geology, Geology
Copyright Holders: The Author
Copyright Information:

Copyright 1978 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, 1978. Bibliography: (v. 1) l. 218-238

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