Geology Reference
In-Depth Information
flooding of northern Australia, while no signifi-
cant change of geoidal height is predicted for
southern Australia during this interval (Sandiford,
2007). Overall, changes in the geoid that occur
as a subduction zone is approached will be
superimposed on any large-scale tilting that is
concurrently driven by the dynamic stresses and
resultant topography associated with subduction
zones and spreading ridges. The late Cenozoic
tilting of Australia provides an illustrative exam-
ple of continental-scale dynamic topography for
which the geomorphic record provides some of
the most compelling evidence.
Sustained, large-scale continental collisions
typically produce significantly thickened crust.
A combination of conductive and radioactive
heating of the thickened crust can induce partial
melting and, therefore, significant weakening of
the lower part of the crust. As discussed earlier
(Fig. 1.9), such a condition has been deduced
for much of the Tibetan Plateau (Beaumont
et al
., 2001; Nelson
et al
., 1996).
What might be the geomorphic and tectonic
manifestations of a thick crust with a weak
lower layer? First, given Tibet's great height
(average elevation of 5 km; Fielding
et al
.,
1994) with respect to surrounding lowlands, a
strong potential energy gradient coincides with
the plateau's margin (Hodges, 2000). In combi-
nation with a warm, ductile lower crust, this
energy gradient might be expected to drive
outward crustal flow, especially of the lower
crust, towards the lowlands (Fig. 1.9). Such
flow could create a topographically descend-
ing ramp off the plateau margin. Second, if the
crust in the lowlands varies in strength, the
ease with which lower crustal flow would
occur might be expected to vary as a function
of that strength: stronger rocks would impede
flow (Clark
et al
., 2005a). Such impedance
could cause relative uplift on the edge of the
plateau and a steeper topographic gradient
at the plateau's margin (Fig. 10.48). Third, the
wavelength and magnitude of any crustal
deflection created in response to a discrete
structure, such as a normal fault, provides an
indication on the depth of compensation: shorter
wavelengths reflect shallower compensation,
1
no Cenozoic inundation
400
Uplifted Shorelines
350
Eocene
shoreline
(~41 Ma)
300
0
50
250
km
Miocene
shoreline
(~15 Ma)
2
A
200
250
Long Wave-Length
Flexure
3
200
Elevation of
15-Ma Miocene
shoreline along
Nullarbor Plain
150
100
4
250
0
B
C
50
km
Australia's Migration
across Geoid Anomaly
6
60
4
40
location of
Australia
at 15 Ma
20
2
0
0
-2
-20
-40
-4
Papua
N Guinea
Australia
Antarctica
Fig. 10.47
Topographic transects delineating
deformed shorelines and the geoidal anomaly
associated with the Antarctic-Australian region.
A. Topographic profile of uplifted Cenozoic shorelines
along transect 1-2 (Fig. 10.46). Two broad, low-relief
shoreline platforms are clearly delineated. B. Transect
3-4 (Fig. 10.46) along the mid-Miocene shoreline. The
shoreline's greater elevation in the western (left) part of
this 700-km-long transect is due to its position farther
south of the tilt axis. C. Northward migration of the
Australian continent causes it to traverse the geoidal
anomaly associated with the subduction zones along the
plate's northern margin and results in marine inundation
of northern Australia.
prevails farther from the coast is consistent with
the overall continental-scale tilting.
Some of the inundation of northern Australia
can be explained by its steady motion towards a
geoidal high over the past 15 Myr (Fig. 10.47C).
This shift could explain about 40 m of the
