TOPOGRAPHY
AND TECTONICS
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Mike Sandiford School of Earth Sciences,
University of Melbourne, Victoria 3010, Australia. m.sandiford@earthsci.unimelb.edu.au http://jaeger.earthsci.unimelb.edu.au |
SUMMARY
The Earth is a hot, dense planet in a cold, sparse universe.
Many of Earth's psychoses, such as volcanism, earth-quaking and other forms of
anti-social behaviour, can be understood in terms of a competition between heat
loss, which strives to disperse the thermal energy anomaly, and
self-gravitation which holds the mass anomaly together. This competition is
manifest as tectonics and is perhaps most spectacularly observed through the
topography of the Earth's surface. As is well understood, plate tectonics
provides a framework that accounts for most of the large-scale surface
topographic features. Increasingly it is able to account for subtle features!
Topography of the ocean
basins (pinks are high, blues are low) as measured and estimated from gravity
data derived from satellite altimetry and shipboard depth soundings.
Plate
tectonics provides a framework for understanding the connection between the
processes that facilitate heat loss and the forces that drive plate motion. One
of the great successes of plate tectonics lies in its explanation of most of
the major surface topographic features of the Earth, particularly in the ocean
basins (Figures 1 & 2). In recent years, this success has extended to many
subtle aspects of topography at the continental-scale that potentially provide
important insights into the dynamics of the hot interior of the planet. This talk discusses the links between
topography and tectonics, highlighting recent advances that help relate surface
topography to dynamic processes in the mantle.
In recent years
large-scale digital databases have revolutionised our approach to Earth
sciences. This is particularly true for tectonics where global topographic and
gravity datasets provide a level of analysis hitherto unimaginable. This talk
will use visualisations based on a number of different datasets including
digital terrain models (DTMs) such as GTOPO30[1],
the GEOSAT synthetic topography and gravity datasets.
These images are available at my web site: http://jaeger.earthsci.unimelb.edu.au/msandifo/Talks/2000/Selwyn/Selwyn.html.
The many topographies of the plate tectonic
world
The lithospheric plates
represent the surface manifestation of large-scale flow in the mantle. Much of
the surface topography reflects the density distribution within the plates
themselves, related to the way in which they grow and age, and contributes to
what we term the "isostatic topography". The isostatic response of the lithosphere reflects the way
in which the lithosphere accommodates lateral variations in density by creating
topography, not only at the
Figure 1.
Shaded
relief map of the world from the ETOPO5 dataset.
surface, but also on
internal density interfaces such as the crust-mantle boundary. Density variations in the lithosphere
represent internal loads and, at length-scales less than several hundred
kilometers, such loads flex the lithosphere, resulting in the production of
"flexural topography", with the characteristic flexural response depending
on the size and age of the load and the strength of the lithosphere. In the
deep mantle, the large-scale convective flow involving both upwelling and
downwelling deforms the Earth's surface thereby creating "dynamic
topography".
Dynamic topography is best seen in the "residual topographic" field obtained by
removing the isostatic contribution from the observed topography (Figure 3).
Isostatic and
dynamic topography in the ocean basins
Plate tectonics finds its
greatest success in the ocean basins. The ocean floors are dominated by the
mid-ocean ridges, with their symmetrical, gently curved ridge flanks descending
into the abyssal plains (Figure 2).
To a very good approximation the bathymetry of the ocean floors
increases with square root of the age of the ocean floor, at least for ocean lithosphere
younger than about 80 Ma[2].
A corresponding decrease in the heat flow with the square root of age gives
rise to the remarkable age-bathymetry-heatflow relationship for the ocean floor.
This relationship is probably the most profound observation pertaining to the
behaviour of the Earth and provides one of the cornerstones of plate tectonics:
the ocean lithosphere forms by the conductive cooling of hot mantle! The increase in density as the
lithosphere cools causes it to "sink" giving rise to variations in
topography that are essentially isostatic in nature. The resulting topography
developed around the mid-ocean ridges generates horizontal buoyancy forces,
helping to drive flow away from the ridges and consequently stabilising the
plate divergence at the ridge axis.
Figure 2. Topographic field of the
ocean basins. Note the anomalously low segment of the mid-ocean ridge on the
Australian-Antarctic-discordance to the south of Australia.
Figure 3. The residual topography
of the ocean basins is obtained by subtracting the isostatic contributions from
the topographic field in Figure 2. This effectively removes the age-dependence
of oceanic bathymetry, such that mid-ocean-ridges disappear. Variations in the
residual topography reflect dynamic and flexural contributions at long at short
wavelengths, respectively. Note
the low in the residual topography on the Australian-Antarctic-discordance to
the south of Australia, and the high in the western Pacific. Australia is
currently in transit from a dynamic topographic low to a dynamic topographic
high.
The age-bathymetry-heatflow relationship implies
that the ocean lithosphere has a simple structure that changes in a very
predictable way with time. Therefore we can easily unroll the effects of time,
allowing us to separate the "isostatic topography" from the "residual
topography". Variations in residual topography
reflect dynamic processes associated with convection deep within the Earth. The
buoyancy flux associated with upwelling of hot mantle contributes to broad
swells around many hot-spot or plume-related volcanic islands, where the
contributions of dynamic and flexural topography are relatively obvious (Figure
4).
One of the most dramatic,
large-scale residual topographic features occurs south of Australia on the
Australian-Antarctic discordance (Figure 3). The anomalously deep nature of the
residual topography in this region suggests the possibility of mantle
downwelling, while the apparent slope in the residual topography from the
Antarctic to the Australian margin points to large-scale density anomalies in
the sub-lithospheric mantle (Figure 5).
The geoid and
continental flooding
The geology of the ocean
crust is very simple, consisting of a thin layer of sediment (siliceous ooze
and red clay) overlying a 5-7 km thick mafic igneous crust consisting mainly of
basalt, "sheeted" dyke complexes and gabbro. Simply on the basis of the age we can
predict the major geophysical features of the ocean such as its bathymetry and
heat flow. In comparison, the
continents are much more heterogeneous, and plate tectonics sensu stricto has found more
difficulty in accounting for the subtleties in continental topography. Many subtle features are however
explicable in terms of the links between dynamic topography and sea level as
the continents circuit above a "lumpy mantle" (Gurnis et al., 1998).
Figure
4. Line
profiles of topography and free-air gravity across Hawaii, showing the flexural
response of the lithosphere to the seamount.
Figure 5. Bathymetric profile from
Antarctica (left) to Australia (right) across the Southern Ocean. Top panel
shows age of ocean floor. Bottom panel shows the observed and residual
topography. The residual
topography is anomalously low (~ -1 km), with the suggestion of a slight
gradient across the Southern ocean.
Variations in the density
structure at depth create variations in gravitational potential inducing
anomalies in the equipotential surfaces relative to a spherically symmetric
Earth. The geoid is a reference equipotential surface that equates to the
mean sea level. There is a general correlation between the geoid anomalies and
the large-scale pattern of dynamic topography (Figures 3 & 6). If the geoid
height changed at exactly the same rate as dynamic topography, then the
"lumpy mantle" would have no effect continental sea level. On the other hand, when geoid changes
at a different rate to the dynamic topography, the extent of continental
flooding should change as the continents transit across the mantle. The ratio
of the amplitude of the variation in the geoid to the amplitude in the
variation in the dynamic topography (the so-called admittance) is still
uncertain but is estimated to be about 0.3 - 0.5 (Gurnis, 1990). The range in the geoid anomalies on the
modern Earth is about 150m, suggesting that dynamic topography can readily
account for 150-300 m of relative sea-level change.
Over the last 150 Ma,
Australia has moved northward over the dynamic topographic low now centered
beneath the Australian-Antarctic discordance. It is now moving towards a
dynamic and geoid high centered over the western Pacific (Figure 7). The impact
of this dynamic topography has left its imprint in the flooding record and
drainage systems on the Australian continent. For example, the flooding of the
Mesozoic inland seas can be attributed to the passage of the Australian
continent over a developing dynamic topographic low associated with
downwelling, the remnants of which are now located beneath the
Australian-Antarctic discordance (Gurnis et al., 1998). The re-emergence of the
continent and the development of the ~1000 km scale west-flowing drainage
systems of the Murray-Darling basin can be linked to the changing topography of
the eastern sea-board as it exited this dynamic topographic low.
Figure 6. Global geoid map. There
is general correlation between the geoid highs (light colours) and the residual
topographic highs in Figure 3. The
total amplitude in the geoid is about 150m from the low of Sri Lanka, to the
high near Papua-New Guinea. He ghosted outlines of the continents reflect the
fact that sources due to lithospheric density variations are subordinate to
deep mantle density anomalies.
Figure 7. The" lumpy
mantle" effect illustrated by draping a shaded relief image of the Indian
Ocean on the geoid surface. The
view is to the east, with north to the left. A broad geoid trough extends from India (lower left) to
southern Australia (upper right). Papua-New Guinea (top center) is riding the
crest of the western pacific geoid high. The admittance between the geoid and
the dynamic topography I estimated to be ~0.3 -0.5 implying that transit across
a lumpy mantle will modulate relative sea level.
The role of dynamic
topography in the Australian Neogene sea-level record is yet to be
satisfactorily elucidated. Nevertheless, it is clear from the current setting
of Australia that dynamical processes are likely to have helped shaped this
record. The elucidation of the dynamic topographic signal of the continents as
distinct from ice, and sea-floor spreading rate, controlled sea-level changes
remains a major challenge for the Earth sciences, in part because it will help
unravel the dynamic story of the Earth's interior.