Tuesday, August 21, 1:00pm

Mr Scott Reynolds

PhD Candidate, NCPGG, The University of Adelaide

Characterization and Modelling of the Regional In Situ Stress Field of Continental Australia


The Indo-Australian plate is unique when compared to most other plates in
that the maximum in situ horizontal stress orientation is not uniform and
does not parallel the direction of absolute plate velocity. Consequently,
the Australian continent provides an ideal setting to study the interaction
between tectonic forces and the in situ stress field. The focus of this
talk is on (1) defining the regional stress field of continental Australia,
and (2) the analysis of the origin of the Australian stress field in terms
of plate boundary forces.

In order to compare observed in situ stresses with those based on
modelling, the regional stress field throughout continental Australia has
been defined using two different methods: stress province definition and
stress trajectory mapping. These techniques have been applied to a greatly
increased Australian stress database comprising a total of 331 reliable
tectonic stress indicators. In general there is broad agreement between the
two techniques, which reveal continental scale rotation of the stress field
across Australia. Stress orientations in Australia vary from east-west in
the western and southern parts of the continent (Perth region, Carnarvon
and Cooper Basins) rotating to northeast-southwest across the northern
margin (New Guinea, Bonaparte and Canning Basins) and northwest-southeast
through southeastern Australia (Otway and Gippsland Basins). The central
(Amadeus Basin) and northeastern (Bowen Basin) parts of Australia are
characterised by a north-northeast to south-southwest stress field.

The tectonic forces controlling the present day regional intraplate stress
field in continental Australia have been evaluated through finite element
analysis of the intraplate stresses in the Indo-Australian plate.
Constraints for the modelling are provided by the 'observed' regional
stress field based on the stress province definition. This observed dataset
is greatly improved compared to that used in previous modelling efforts. A
weighted 'basis-set' method has been employed to quantitatively assess the
misfit between the observed and predicted intraplate stresses, providing an
efficient means to evaluate a very large number of boundary and potential
energy force combinations acting on the plate.

The modelling results indicate that the major features of the regional
stress field in continental Australia can be explained in terms of a
geologically plausible array of plate boundary and potential energy forces.
These results indicate that modelling of the Australian intraplate stress
field is inherently non-unique such that a large number of different
boundary force combinations can produce a very similar predicted stress
fields. Nevertheless a number of fundamental conclusions can be made about
the tectonic settings along the principal plate boundary segments including
the following:
(1) Compressional forces act along the Himalayan and Papua New Guinea
boundaries to produce stress focussing normal to those boundaries and
stress rotation between them.
(2) The observed stress field in the Bowen Basin requires compressional
forces along the Solomon and New Hebrides subduction zones.
(3) East-west compression in eastern Australia requires moderate
compression acting along the Tonga-Kermadec subduction zone.
(4) Modelling stresses in southeastern Australian requires compressional
forces along the New Zealand, Puysegur Trench and Macquarie Ridge boundary

Biography of the Speaker

Scott Reynolds completed his BSc(hons) from the University of Adelaide in
1997. Just recently he has submitted his Ph.D. for which he has been
studying for since 1997. His research has focused on characterization and
modelling the regional stress field of continental Australia. During this
research a detailed investigation of the stress field in the Bowen, Sydney
and Perth Basins was conducted. Member: ASEG, GSA and PESA.