Page 115 - Global Tectonics

102 CHAPTER 5

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within the “oceanic hemisphere,” i.e. the Pacifi c ocean, 50 mm a . Intriguingly, this corresponds with a period
and have produced distinctive traces across the Pacifi c of major reorganization of global plate motions (Rona
plate (Fig. 5.7). The Louisville Ridge originates at the and Richardson, 1978), the age of the major bend in the

Ontong Java Plateau of the western Pacific. This formed Hawaiian–Emperor seamount chain, and a period in
approximately 120 Ma ago and is the largest LIP in which the rate of true polar wander (Section 5.6) was
terms of the volume of mafi c igneous material much greater than during the period 10–50 Ma ago,
emplaced. The Hawaiian–Emperor seamount chain when it was virtually at a standstill (Besse & Courtillot,
may well have had a similar origin but the earlier part 2002).
of this track has been subducted, the oldest seamounts If hotspots remain fixed, and provide a framework

in the chain dating at approximately 80 Ma. The Easter for absolute plate motions, then paleomagnetic studies
Island–Line Islands track originated about 100 Ma ago, should be able to provide a test of their unchanging
not as an LIP, but in an area with an unusually high latitude. Paleomagnetic data for the oceanic plates of
density of submarine volcanoes known as the mid- the Pacific are sparse, and subject to greater uncertain-

Pacifi c mountains. ties than those obtained for continental areas. Neverthe-
The relative positions of the continents around the less preliminary results (Tarduno & Cottrell, 1997)
Atlantic and Indian oceans, for the past 200 Ma, are well suggest that the Hawaiian hotspot may have migrated
constrained by the detailed spreading history contained south through as much as 15–20° of latitude during the
within these oceans (Section 4.1.7). If one or more period 80–43 Ma. Paleomagnetic results obtained from
hotspot tracks within this Indo-Atlantic hemisphere are Ocean Drilling Program drill core, from which any
used to determine the absolute motions of the relevant latitudinal change in of the Reunion hotspot could be
plates in the past, tracks for the remaining hotspots in deduced (Vandamme & Courtillot, 1990), suggest that
this hemisphere can be predicted. Comparison of the this hotspot may have moved northwards through
observed and predicted tracks provides a test of the approximately 5° of latitude between 65 and 43 Ma.
fixed hotspots hypothesis, and a measure of the relative These latitudinal shifts are compatible with the discrep-

motion between the hotspots. Such an analysis by ancy between the two hotspot reference frames prior to
Müller et al. (1993) suggests that the relative motion 43 Ma ago, and support the assumptions regarding
between hotspots in the Indo-Atlantic reference frame Cenozoic – late Mesozoic plate boundaries within and
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is less than 5 mm a , i.e. an order of magnitude less than around the Antarctic plate. These results also imply that
average plate velocities. A similar analysis for Pacifi c the major bend in the Hawaiian–Emperor seamount

hotspots by Clouard & Bonneville (2001) yields a similar chain at approximately 43 Ma does not reflect a major

result for the Pacific reference frame. However, there change in the absolute motion of the Pacifi c plate, as
are problems in linking together the two reference originally thought, but can be accounted for almost
frames; in other words, in predicting Pacifi c hotspot entirely by the southward motion of the Hawaiian
traces using the Indo-Atlantic reference frame or vice- hotspot (Norton, 1995).
versa. This is because, for most of the Mesozoic and Predicted hotspot traces in the Atlantic and Indian
Cenozoic, the oceanic plates of the Pacifi c hemisphere Ocean (Müller et al., 1993) are shown in Fig. 5.9, super-

are surrounded by outward dipping subduction zones, imposed on volcanic structures on the sea floor and on
except in the south. This means that in order to deter- land. The correlation between the two is excellent. For
mine the motion of the Pacifi c Ocean plates relative to example, the Reunion hotspot began beneath western
the Indo-Atlantic hemisphere one must have a detailed India and was responsible for the Deccan Traps fl ood
knowledge of the nature and evolution of the plate basalts: India’s northwards motion was then recorded
boundaries around and within the Antarctic plate in the by the Maldive-Chagos Plateau and the Mascarene
South Pacific area. Unfortunately there are still uncer- Plateau. The gap between these two features results

tainties about this, but an analysis based on the model from the passage of the mid-ocean ridge over the hot
of Cande et al. (1999) for the evolution of these bound- spot approximately 33 Ma ago. The hotspot is currently
aries suggests that the two reference frames or domains beneath a seamount 150 km west of the volcanically
are not compatible, despite the compatibility of hotspot active island of Réunion.
tracks within each domain (Fig. 5.10). The discrepancy It will be noted that Iceland has not been included
is greatest before 40–50 Ma, when the relative motion in Fig. 5.9. If one assumes that this hotspot was initi-
between the two hotspot frames is approximately ated 60 Ma ago beneath East Greenland then its track