Palaeozoic Palaeomagnetism of South-Eastern Australia

The drift history of Gondwana following the break-up of Rodinia (or
perhaps Pannotia) to the amalgamation into Pangaea has great
implications in many disciplines in Earth sciences, but remains
largely unknown. Among the apparent polar wander (APW) paths
published for Gondwana in the last few decades, large discrepancies
exist (sometimes up to thousands of kilometres). The mid Palaeozoic
segment of the APW path is particularly problematic, and two
primary schools of thought arise. Some authors favour a Silurian –
Devonian loop in their APW path passing through southern South
America (on a reconstruction of Gondwana), whereas others draw a
path directly through Africa during this period. The main
controversy stems essentially from whether or not palaeomagnetic
data from eastern Australia are incorporated in order to compensate
for the lack of mid Palaeozoic data. Determining whether the
terranes of the Southern Tasmanides are (para-)autochthonous or
allochthonous in origin is therefore of crucial importance and a
matter of intense debate. The aim of the work presented herein is
to palaeomagnetically define the positions of these terranes
throughout the Palaeozoic in order to better constrain the complex
tectonic history of this region and to help clarifying the APW path
of Gondwana. The construction of an APW path is discussed herein.
An attempt is made to determine whether only “objective” criteria
can be employed to select data used to draw an APW path. However,
it is shown that the palaeomagnetic database has not enough
entries. Subjective data selection must be introduced leading to
two end-members: the X-type and the Y-type, thought to be best
illustrated by the X-path proposed by Bachtadse & Briden (1991)
and the Y-path proposed by Schmidt et al. (1990). These two models
are, therefore, used in the discussion of the results obtained for
this study. The Southern Tasmanides had a complex tectonic history
with several orogenic events throughout the Palaeozoic. The
sampling coverage carried out for this study comprises fifty
localities (289 sites, 1576 cores, 3969 specimens; see table 1,
pages 54-55) distributed along an east-west transect across most of
the subdivisions of the Southern Tasmanides. The sampled localities
are gathered in three main areas: the Broken Hill area, the Mount
Bowen area, and the Molong area, which are situated where no
published palaeomagnetic studies were previously available
providing, therefore, new information. Sampling and laboratory
procedures have been carried out using standard techniques. In
particular, detailed stepwise thermal demagnetisation, principal
component analysis, anisotropy of magnetic susceptibility and rock
magnetic measurements have been systematically employed. The
routine measurement of the anisotropy of magnetic susceptibility
allowed drawing the first maps of the magnetic fabrics throughout
the region. A strong correlation between the magnetic fabrics and
the main tectonic structures corroborates the existence of
cross-structures (E-W) in the Southern Tasmanides. The directions
of magnetisation obtained yielded much information, despite poor
quality. The effects of weathering are deep, intense and
widespread. For example, most of the samples from the Mount
Arrowsmith Formation (localities ARR & ARO) and the Funeral
Creek Limestone (FUN) in the Broken Hill area (western New South
Wales) are totally remagnetised, as well as some from the Mitchell
Formation (MIT) in the Molong area (eastern New South Wales).
Secondary magnetisations are also largely responsible for the bad
results obtained in most of the fifty localities studied.
Intermediate directions of magnetisation are common and often
result in significant data scattering, as illustrated for instance
by results from the Kandie Tank Limestone (KAN; Broken Hill area)
or the Ambone and Ural Volcanics (HOP, BOW, SHE; Mount Bowen area).
In general, it has not been possible to precise the remagnetisation
process leading to those scattering. Nevertheless, a major
remagnetisation event, probably thermo-chemical in origin, has been
also recognised. This event is thought to be Oligocene in age and
triggered by changes in geothermal gradient prior to the onset of
hot spot volcanism in the Molong area. The existence of Jurassic
overprints are also suggested, in particular in the Broken Hill
area, possibly in association of intrusion of mafic dykes. All
other magnetic components described herein are considered
Palaeozoic in age, but further constraints on age are very
difficult to establish since field tests are most often not
significant. Palaeopoles obtained from three localities, however,
are believed to correspond to primary magnetisations. The pole from
the Late Cambrian Cupala Creek Formation (CUP), confirmed by a
positive unconformity test, implies that this zone can be regarded
fixed relative to the craton since the Late Cambrian. In the Early
Devonian Mount Daubeny Formation (DAU), the applied fold test,
contact test and conglomerate test indicate the primary origin of
the magnetisation carried by haematite. The corresponding pole
(DAU) is, however, significantly distinct from the VGP deduced from
the Early Devonian Ural Volcanics (MER) showing that at least one
of the two localities has been rotated. The MER pole agrees with
the remagnetisation pole associated with the Cupala Creek
Formation, and favours the X-type of APW path proposed by Bachtadse
& Briden (1991) for Gondwana. The outcome of this agreement
contradicts the Y-type path and the existence of a Silurian –
Devonian loop mainly anchored on the Early Devonian Snowy River
Volcanics pole obtained by Schmidt et al. (1987). Invocation of
terrane rotation, arising possibly from a pull-apart basin, may
explain the discrepancy between the pole from Mount Daubeny
Formation and the X-path. The most significant finding of this
study is the widespread terrane rotation. This conclusion is based
upon the inability of intermediate directions of magnetisation,
alternate APW path for Gondwana, true polar wander or non-dipole
field contribution to correctly explain the distribution of these
new data. Consequently, one has to admit that block translation and
rotation occurred in the Southern Tasmanides in the first half of
the Palaeozoic Era and perhaps up to the Early Carboniferous. A
possible scenario concerning the tectonic arrangement of blocks in
the Southern Tasmanides is presented in conclusion. This
palinspastic model involves block translation in the
Siluro-Devonian, and rotation in the Early and more probably Middle
Devonian, with late tectonic displacements and rotations in the
South-Western Belt of the Lachlan Orogen in the Late Devonian to
Early Carboniferous.