Geology Reference
In-Depth Information
2
10
6
n = 14.5
×
4
6
70 Ma
0
4
8
12
Crustal age Ma
1/2
Figure 8.3. Example of globally aggregated depth-age data. Straight lines are
a model subsidence (
z
336
√
t
)
two
standard deviations from 1 Myr bins. From Hillier [97]. Copyright by the American
Geophysical Union.
=
2648
+
±
500 m. Curves are the mean
±
8.2.5 There is no seafloor 'flattening'
This is a very persistent idea, but there is clear evidence against it. The evidence
was presented by Marty and Cazenave [10] in 1989, Figure 2.5 being a selection
of their results. Their results show that there are regional variations in seafloor
subsidence rates but no persistent tendency to asymptotically approach a constant
depth. The alleged evidence for such asymptotic flattening comes from globally
aggregating seafloor depths into a single plot. A recent example of an aggregated
data set is shown in Figure 8.3. As can be seen, it does not even show the asymptotic
flattening it is purported to document. Nevertheless many discussions of the flat-
tening interpretation have continued (e.g. [96-98]) even though the plots of Marty
and Cazenave show that the apparent flattening is an artefact of the aggregation.
An accurate characterisation of seafloor depth is that there is an underlying
tendency to follow a square root of age subsidence curve (Section 6.4), but there
are many regional deviations of older sea floor to higher elevations. The distinction
between these characterisations (regional anomalies versus global flattening) is
important because it implies quite different physics, as was spelt out by Davies and
