Geology Reference
In-Depth Information
Fig. 5.14
Discontinuities
and duplications of a
magnetization model
caused by ridge jumps. In
this example, an eastward
jump by three blocks
occurred at the end of
chron C3An.1n. In general,
eastward (or northward)
ridge jumps add blocks to
the western (southern) side
and remove a
corresponding number of
prisms from the eastern
(northern) side. The
green
dashed lines
are
discontinuities. The
age-distance plots show the
predicted crustal age
discontinuities (After
Schettino
2012
)
The output of a forward modelling procedure
is a spreading velocity function
v
D
v
(
T
). This
function can be used in turn to determine the
offset of the anomalies along the profile, which is
the first step in the construction of
isochron maps
.
However, it is important to note that the best-
fitting spreading velocities of a magnetization
model do not necessarily represent true plate
velocities during the corresponding chrons. For
example, when a magnetic profile contains data
from one side only, the resulting velocities may
be anomalously high or low as a consequence of
local spreading asymmetry or undetected ridge
that the spreading asymmetry may change not
only between two neighbor ridge segments but
thermore, a model where the block associated
with chron
C
k
has apparent
eastern flank of a ridge cannot be distinguished
from another representation in which the same
block has been generated with velocity
v
k
/2, but
has been subsequently doubled by a westward
ridge jump. Therefore, in the analysis of magnetic
anomaly profiles that encompass one side only of
a spreading ridge, it is
not
possible to infer the
real plate velocities from the apparent spreading
rates associated with the block model. In general,
the correct determination of the relative plate
velocities requires a statistical treatment of many
velocity models from the same area, and an an-
alytical procedure of correlation of the magnetic
profiles, in order to determine stage boundaries
and average spreading rates. However, even the
analysis of a single magnetic profile can furnish
an rough estimate of the true spreading rate after
an appropriate statistical treatment. In fact, let
us consider the function
x
D
x
(
T
), which gives
velocity
v
k
on the
