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Studies in the geometry of folding and its mechanical interpretation

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Powell, C (1967) Studies in the geometry of folding and its mechanical interpretation. PhD thesis, University of Tasmania.

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Abstract

Analysis of thickness variation of folded layers in terms
of apparent flattening indicates the relative 11viscositiesrr of
the layers, and hence modes of deformation, Orthogonal-thickness
ratios, which are unique for each percentage of flattening,
are convenient indicators of the equivalent flattening of an
initially concentric profile. Axial-thickness ratios,
intrinsi cally more sensitive, are less useful because there is a
spread of values for each percentage of flattening.
Measurements of fold profiles from diverse sedimentary,
diagenetic and low-grade metamorphic environments show that the
higher the t emperature and pressure at which the rocks were
deformed the smaller the range of fold style. At Port Moresby,
Papua, progressive syntaphral sliding has folded Eocene cherts
and less competent argillite in a complex polyclinal style.
Individual layers, then groups of layers up to one metre thick,
and finally slip sheets tens of metres thick slid towards the
west-southwest.
Structural analysis of linear and planar fold elements in
diagenetic or lovr-grade metamorphic environments in Tasmania
leads to the conclusion that cleavage developed contemporaneously
in both sandstone and slate as a planar feature. The cleavage in the slate is penetrative to the scale of detrital grains,
but non-penetrative in the sandstone where pelitic ribbons
anastomose through the rock enclosing non-cleaved lenses of
normal greywacke fabric. The most satisfactory hypothesis
to account for the observed mesoscopic configurations is that
the cleavage formed during deformation as pelitic deposits in
channels along which excessive water was forced out of the
rock at very high pore pressure.The ratio between pore pressure and confining pressure is
a significant parameter determining fold style. Heterogeneities
necessary for concentric folding are effective where the
pore pressure is relatively low, and the fold style is similar
where the pore pressure approaches the pressure on the grain fabric.
Pore-pressure ratios are most readily varied in superficial deposits,
and many different fold styles occur in the intrastratal contortions
in the terrestrial, Pleistocene, preglacial deposits in Tasmania,
and in the intraformational slumps of the Pleistocene Lisan
Formation in Israel. Irregular "flow" folds in the contact aureoles
of two intrusive bodies in Tasmania grade outwards into more
regular, concentric and disjunctive folds. Both the folding
and the variation in fold style may have been caused by water
pressures approaching the lithostatic load in the intrusions,
and decreasing outwards.

Item Type: Thesis (PhD)
Additional Information:

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Date Deposited: 23 Jun 2012 13:19
Last Modified: 11 Mar 2016 05:56
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