Geoscience Reference
In-Depth Information
Figure 2.20.
Absolute motions of the plates as determined from hotspot traces.
HS3-NUVEL-1A is a set of angular velocities of fifteen plates relative to the hotspots.
The hotspot dataset HS3 averages plate motion over the last 5.8 Ma. No hotspots are
in significant relative motion. The 95% confidence limit is ±20-40 km/Ma
−1
but can
be 145 km/Ma
−1
. (See Plate 1 for colour version). (From Gripp and Gordon (2002).)
The hotspot reference frame therefore is the motions of the plates relative to
the hotspots, which are assumed to be fixed in the mantle. The 'hotspot track',
the linear chain of volcanic islands and seamounts, is the path of the hotspot
with respect to the overlying plate. The slow steady motion of the oceanic plate
with respect to the hotspot (the flowline) is not marked by any feature, however.
Figure 2.19(c) illustrates the difference between the hotspot track and the flowline.
In order to use probable hotspot tracks to run the plate motions backwards, it is
necessary to know the ages of the islands and seamounts, as we do for the Hawaiian
chain. However, even if the ages of seamounts are unknown, the flow-lines can be
used. By assigning a range of possible ages to any seamount, a series of flow-lines
can be plotted (these will all follow the actual flow-line), producing a line that will
go through the present position of the hotspot. (For example, when the seamount
in Fig. 2.19(c) is erroneously assumed to be one unit old rather than two, the black
dot shows the backtracked hotspot location - this lies on the flow-line but not on
the hotspot track.) The process is then repeated for another seamount and the two
sets of flow-lines compared. If the two seamounts formed at the same hotspot then
the flow-lines will intersect at the location of that hotspot. If the seamounts were
not formed by the same hotspot, the flow-lines should not intersect. Application
of this method is improving our knowledge of hotspot locations and absolute
plate motions.
Figure 2.20 shows a determination of the present absolute plate motions. Plate
motions relative to hotspots cannot be estimated as accurately as can relative
