Geology Reference
In-Depth Information
Fig. 5.15
Age-Distance plot illustrating the relation be-
tween the offset,
x
, of each block included in a magnetiza-
tion model and the corresponding chron upper boundary
age
T
(
small black dots
). These points furnish the crustal
age as a function of the distance from the ridge axis. The
black line
shows the linear spline regression fit using a
three-stages model. Large black dots are the knots of the
regression curve
the distance
x
from a spreading centre segment,
where we can find oceanic crust of age
T
:
we called
stages
. This principle implies in turn
that the spreading rate along a ridge segment is
approximately constant during a stage, which is
effectively what we observe through the analy-
sis of marine magnetic anomalies. In general, a
rough estimate of the true full spreading rates can
be obtained from the apparent velocity function
v
D
v
(
T
) through a linear spline regression fit
(Schettino
2012
). A better estimate requires the
more complex procedure that will be described in
the next section.
Z
T
1
2
x.T/
D
v
.t/dt
(5.63)
0
This age-distance function can be easily built
on the basis of the velocity function
v
D
v
(
T
).
A key observation in plate kinematics is that
the function (
5.63
) can be always approximated
fairly well by a sequence of straight lines, that
is by a first-order piecewise polynomial, in spite
of the apparent full spreading rates variability
through the geological time. As an example,
Fig.
5.15
shows the age-distance plot associated
with a magnetic profile in the central Atlantic.
This plot suggests a change of spreading rate at
anomaly 5, 6, and 13 times, associated with a
change of slope of the regression lines.
Therefore, even the statistical analysis of a
single magnetic profile can furnish an estimate of
the true spreading rate over long time intervals.
In fact, we have mentioned in Sect.
2.7
that a ba-
sic principle of plate kinematics establishes that
the Euler vector describing the relative motion
between two plates is approximately constant for
long time intervals (of the order of tens Myrs) that
5.6
Construction of Isochron
Maps
Sea floor spreading
isochrons
are lines formed
by a combination of points with the same age
and fracture zone segments in an oceanic basin.
They can be considered as determinations of the
spreading ridge geometry in the geologic past. If
the ocean is not yet subducting, we always have
two
conjugate isochrons for each possible time,
placed on the opposite sides of the spreading
center at a more or less equal distance. Although
in principle we can build isochrons of any age
less than the age of onset of sea floor spreading,
it is common practice to construct isochron maps
