Marine Magnetic Anomalies - Quantitative Plate Tectonics - page 163

Fig. 5.17 An example of data coverage for the construc-
tion of isochron maps. Black lines are ship tracks, blue
lines are fracture zones. The spreading center is divided
into 11ridge segments. The coverage is good for segments

1-4, 8, 14, 16, and for fracture zone segments, while it is
incomplete for segment 12 and scarce for segments 5, 6,
and 10

building block for the construction of a sea floor
spreading isochron. To this purpose, we usually
group the crossings according to the side and to
the ridge segment. Further subdivision is made
when a ridge segment has variable strike. The
reason is that we are going to search the finite
reconstruction pole and rotation angle that best
fits the set of all conjugate groups of crossing
and fracture zone lineations , namely, the set of all
groups of crossings or fracture zone points that
are placed on the opposite flanks of a ridge and
are approximately aligned along great circle arcs.
As an example, Fig. 5.18 shows the conjugate
groups of crossings for anomaly 2A relative to
thedatasetofFig. 5.17 .
Therefore, at the next step we shall perform
a statistical fitting of the magnetic lineations
and the intervening fracture zone segments. This
procedure will be described in the next section.
It furnishes the Euler pole of closure of the
ocean floor younger than the selected anomaly.
Then, we proceed creating two additional sets
of crossings by application of the resulting fi-
nite reconstruction matrix (see Sect. 2.7 ) tothe
eastern groups, and the conjugate matrix to the

western groups. The result is a more dense set of
crossing points that delineates better the magnetic
lineations corresponding to the selected anomaly.
Furthermore, this technique allows to perform a
visual inspection of the goodness of fit. An ex-
ample of this approach is illustrated in Fig. 5.19 .
The last step requires tracing the representative
magnetic lineations on one of the two flanks (the
choice is generally arbitrary). These lineations
are prosecuted to intersect the fracture zones,
where they are linked together by intervening
segments of the digitized fracture zones. The
result is a digital isochron. To obtain the conju-
gate isochron, we do not repeat the procedure on
the opposite flank. Instead, we simply rotate the
isochron using the reconstruction matrix deter-
mined during the previous step. This technique
ensures that the two isochrons will match per-
fectly when making plate reconstructions.
The final result of the procedure illustrated
above is represented by a pair of conjugate
isochrons like those shown in Fig. 5.20 .
Repeating the steps for older anomalies gives
the complete set of isochrons describing the
evolution of an oceanic basin.