50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

TECTONICS OF THE GALATEA BASIN
USING GEOPHYSICAL TECHNIQUES
Suzannah Toulmin & Tim Stern
Institute of Geophysics, Victoria University of
Wellington, P O Box 600, Wellington.
(suzannah.toulmin*vuw.ac.nz)
Unusually deep basins can occur in regions
subjected to both active extension and strike-slip.
Such “pull-apart” basins are known to occur in
Southern California, where they have been explored
for more than 100 years. In New Zealand, examples
of pull-apart basins include the Hamner Basin
(Hope fault, Canterbury) and the Dagg Basin
(Alpine fault, Fiordland). It is arguable that the
dextral strike-slip Wellington fault also exhibits
pull-apart character at eastward en echelon
releasing steps as it passes through the Hutt Valley
into the Wairarapa.
The Galatea Basin, located in southern Bay of
Plenty, straddles the eastern margin of the
extensional Central Volcanic Region and is
juxtaposed against strands of Waiohau and Te
Whaiti faults. These faults define the western limit
of the North Island Dextral Fault Belt. Although
part of a principally dextral strike-slip system of
faults, motion on the Waiohau fault has been
identified recently as being dominantly normal,
with an average dip-slip rate of 0.6-0.9 mm/yr.
Over 170 new gravity measurements were acquired
the Galatea Basin with the aim of better defining
the dimensions of an associated Bouguer anomaly
and determining the sub-surface structure of the
basin. In addition, a single 5 km seismic refraction
profile and a 2.5 km reflection profile were
recorded along a line perpendicular to the Waiohau
fault and strike of the Galatea Basin.
The residual gravity anomaly is –27 mGal at the
centre of the basin. Results from ray-tracing have
identified that the Waiohau fault dips in excess of
45° at depth, and, combined with other methods,
we conclude that basin depth reaches at least 1000
m. Basin fill is Quaternary volcanics, including
Matahina Ignimbrite and Rangitaiki Ignimbrite
underlain by Pleistocene sediments including Lukes
Farm Formation and possibly older Tertiary units.
The distinctly deep gravity anomaly combined with
the steeply faulted eastern margin suggests a pullapart
origin for the Galatea Basin.
ORAL
CONSTRUCTING A BRITTLE–DUCTILE
RHEOLOGICAL MODEL OF
CONTINENTAL LITHOSPHERE
John Townend
School of Earth Sciences, Victoria University of
Wellington, PO Box 600, Wellington
(john.townend*vuw.ac.nz)
Idealised rheological models — elastic, ductile,
viscoelastic etc. — provide physically reasonable,
mathematically useful descriptions of the
lithosphere’s large-scale mechanical characteristics
in different circumstances. However, these models
do not directly represent small-scale deformation
mechanisms known to operate under particular
geological conditions. In particular, the parameters
describing common rheological models are not
straightforwardly reconciled with the brittle and
ductile modes of deformation thought to operate at
shallow and deep levels within the crust and
lithospheric mantle. Moreover, it is difficult to
account for the vertical and horizontal distributions
of crustal seismicity using existing rheological
models in which the lithosphere’s mechanical
properties are uniform.
In this presentation, I will discuss progress towards
constructing a rheological model for continental
lithosphere that combines geological evidence for
brittle and ductile deformation mechanisms, and
seismicity, with geophysical measurements of the
lithosphere’s long-term, long-wavelength
mechanical properties.
If the brittle–ductile transition in continental crust is
controlled by the crust’s thermal structure and the
ambient strain rate (e.g. Sibson, 1983), then the
large-scale vertically averaged rheology of the
lithosphere as a whole is temperature-dependent.
Based on borehole observations indicating that the
brittle crust is in a state of frictional failure
equilibrium, Zoback and Townend (2001)
hypothesized that the rate at which continental
lithosphere deforms can be expressed in terms of
heat flow if the total strength of the lithosphere is
known. The corollary is that for a given crustal
structure and heat flow, the corresponding
lithospheric strain rate and the depth of the brittle–
ductile transition (and the base of the seismogenic
zone) can be estimated. In other words, any two of
four key data sets listed in the schematic below can
be used to compute the two others. All four data
sets exist for California, providing a rare
opportunity to test these ideas.
50 th Kaikoura05 -87- Kaikoura 2005