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Fig. 2.20 Eastern North
American COB, based on
the maximum horizontal
gradient of the free-air
gravity anomaly field
(Sandwell and
Smith 1997 )
Some tectonic element boundaries are defined
on the basis of geophysical constraints. An
important example is represented by COBs (see
Sect. 1.3 ) . In the first plate reconstructions based
on rigorous computational methods, Bullard
et al. ( 1965 ) matched the conjugate continental
margins around the Atlantic on the basis of a
fitting algorithm. They applied the method to
the 100, 500, 1,000, and 2,000 fm isobaths (1
fathom D 6 ft), then chose the best fitting set of
curves (500 fm D 914.4 m) as most representative
of the conjugate COBs.
Although this method produced a set of
reconstructions that resisted through time and
can be considered milestones in the history
of plate kinematics modelling, none of the
selected isobaths was really representative of the
true boundary between continental and oceanic
crust. The shape of bathymetry contour lines is
strongly affected by the sedimentary cover. For
instance, deposits associated with the delta of
a great river (such as the Nile or the Ganges)
can progressively shift the 1,000 mt isobath
seawards by hundreds of kilometers in a few
million years. The existence of such processes
implies that even though conjugate isobaths may
fit well at the beginning of the ocean opening,
subsequent sedimentation generally decreases
this initial goodness of fit. Therefore, more recent
studies have adopted a geophysical definition of
the COBs. For example, Schettino and Scotese
( 2005 ) assumed that the COBs were associated
with the maximum horizontal gradient of the
gravity anomaly field. This assumption was based
on the observation that marine gravity anomaly
maps adequately filter the sediment cover,
and that COBs are associated with important
lithological discontinuities, which give rise to
sharp variations of the gravity anomaly field.
In this context, free-air gravity anomaly maps
such as those proposed by Sandwell and Smith
( 1997 ) represent an invaluable tool for modern
plate tectonic modeling. Figure 2.20 shows a
map of the horizontal gradient of the gravity
anomaly field along the eastern margin of North
America. The line of maximum gradient can
be
easily
used
to
trace
the
North
American
COB.
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