Geoscience Reference
In-Depth Information
become older, tectonic reconstructions generally become increasingly difficult
and subject to error and, frequently, to speculation. The southwest-northeast
lineations extending from the Japan trench towards the Aleutian trench are called
the Japanese lineations. The northwest-southeast lineations to the south and west
of the Emperor Seamounts and the Hawaiian Ridge are called the Hawaiian
lineations. Both the Japanese and the Hawaiian lineations are identified as M1 to
M29 inclusive. The east-west lineations that straddle the equator and extend from
160
◦
Eto170
◦
W are the Phoenix lineations (so named because of their proximity
to the Phoenix Islands).
The Japanese and Hawaiian lineations form a well-defined magnetic bight,
older than, but otherwise very similar to, the Great Magnetic Bight in the north-
eastern Pacific. This older magnetic bight is thought to have been formed by an
RRR triple junction, where the Farallon and Pacific plates met a third plate, the
Izanagi plate (Fig. 3.29).
The Phoenix lineations are from anomalies Ml to M25 inclusive and were pro-
duced, between about 127 and 155 Ma, by an east-west striking Phoenix-Pacific
Ridge possibly some 40
◦
south of their present latitude. It has been suggested
that this ridge extended far to the west and joined the ridge system of the Indian
Ocean that gave rise to the magnetic anomalies north of the Exmouth Plateau
(Section 3.3.3). Any younger lineations and the symmetrical southern half of
these Phoenix lineations are no longer present here.
There are two magnetic bights in the southern Pacific, both less well defined
than their northern counterparts. The first is at the eastern end of the Phoenix lin-
eations where the lineations (M12-M4, 137-131 Ma) bend from approximately
east-west to northwest-southeast. These northwest-southeast anomalies were
probably produced by the Pacific-Farallon Ridge. The second magnetic bight is
at about 40
◦
S, 145
◦
W, w here anomalies 32-20 (45-70 Ma) bend from approxi-
mately southwest-northeast to northwest-southeast. Unfortunately, much of the
evidence required to determine the details of these western Pacific plates and their
relative motions has been swallowed by the hungry western Pacific subduction
zones.
Figure 3.29 shows a series of reconstructions of the plates in the Pacific from
110 Ma to the present. The Magnetic Quiet Zone lasted from 124 to 84 Ma, so
not much can be deduced about the motions of the plates during that period. The
Kula plate came into existence then. It is not possible for the Kula plate to be the
older Izanagi plate; rather, it is believed to have been a piece that broke off either
the Farallon plate or the Pacific plate. The Farallon plate was very large indeed:
between about 85 and 55 Ma ago the Pacific-Farallon Ridge extended for some
10 000 km.
The evolution of the western and southern Pacific was clearly very complex,
with a number of changes in spreading centres. The process probably included the
creation and subduction of whole plates. No doubt further detailed mapping of
