Plate Motions - Quantitative Plate Tectonics - page 51

Fig. 2.21 Pattern of magnetic anomalies along the east-
ern North American margin and in the western central
Atlantic, extracted from the global data set of Korhonen
et al. ( 2007 ). Positive anomalies are shown in dark green .
The location of the ECMA ( vertical hatch ) marks a zone

of transitional crust (or anomalous oceanic crust) along the
volcanic margin. The yellow line shows the location of the
COB according to the gravity data of Fig. 2.20 (maximum
horizontal gradient of the gravity anomalies)

Another class of geophysical markers that are
useful to identify COBs is represented by the
magnetic anomalies that border volcanic OCTs
(see Sect. 1.3 ) . A classic example is represented
by the East Coast Magnetic Anomaly (ECMA) of
eastern North America (e.g., Schettino and Turco
2009 ). In this instance, a strong linear magnetic
anomaly associated with the extrusives and in-
trusives of the initial magmatic pulse marks the
site of transition from the rifting stage to drifting,
hence the location of the COB, as illustrated in
the example of Fig. 2.21 . However, a comparison
between the location (and the geometry) of the
ECMA and the COB defined on the basis of grav-
ity anomaly data (Fig. 2.20 ) shows that the co-
incidence of these features is only approximate,
and that differences of up to 70-80 km exist be-
tween the two lineaments. Therefore, even when
based on a geophysical approach, the definition
of COBs remains to some extent qualitative.
A major problem in the definition of both
COBs and tectonic boundaries that are placed
along rift zones is represented by the considerable
thinning that characterizes the passive margins
of the corresponding tectonic elements. If we
use one of the geophysical techniques described
above to define a conjugate pair of COBs, then a
reconstruction based on the fit of the margins will
be representative of the onset of sea floor spread-
ing, not of the pre-rift configuration. In fact, tec-

tonic elements whose extensional boundaries are
defined using potential field data (either gravity
or magnetic data) are stretched elements, which
should be restored to their original size when
the objective is to make a pre-rift reconstruction.
There are three approaches to the solution of
this problem, which clearly does not affect the
reconstruction of the spreading history of oceanic
basins. All these methods require an estimation
of the amount of stretching that occurred dur-
ing the rifting stage. This is usually expressed
in terms of stretching factor “ (see Sect. 2.4 ).
A determination of this quantity can be made
when a set of crustal profiles along the continental
margins, obtained from seismic refraction exper-
iments, is available (e.g., Schettino and Turco
2009 ). The first step consists into an estimation
of the directions of stretching, for example by
landward prolongation of the first post-rift direc-
tions of sea floor spreading. We shall see that
these directions can be easily calculated on the
basis of a kinematic model. Then, the seismic
cross-sections are projected onto the directions of
stretching, to avoid an incorrect determination of
the continental margin width. At the next step, the
upper and lower boundaries of the stretched crust,
the latter coinciding obviously with the Moho,
are identified on the cross-sections. Assuming
that seismic profiles always start on unstretched
crust, then these boundaries are two functions,