Page 119 - Global Tectonics

106 CHAPTER 5

The relative motion between the mantle and the erate size and can be considered to form part of the
rotation axis, as illustrated by the TPW path, may be normal mantle convecting system. It has been pro-
interpreted as a shifting of the whole or part of the posed, however, that at least once during the history of
Earth in response to some form of internal mass the Earth there has been an episode of much more
redistribution that causes a change in the direction intense volcanic activity. The cause has been ascribed to
about which the moment of inertia of the mantle is a phenomena termed superplumes, large streams of
a maximum (Andrews, 1985). For example, Anderson overheated material rising buoyantly from the D″ layer
(1982) relates TPW to the development of elevations of at the base of the mantle (Section 2.8.6), that derived
the Earth’s surface resulting from the insulating effect their heat from the core. These spread laterally at the
of supercontinents that prevents heat loss from the base of the lithosphere to affect an area ten times larger
underlying mantle. It is possible that only the litho- than more normal plume activity.
sphere or the mantle or both lithosphere and mantle Larson (1991a, 1991b, 1995) proposed that a super-
together shift during polar wander. It is highly unlikely plume was responsible for the widespread volcanic and
that the lithosphere and mantle are suffi ciently decou- intrusive igneous activity that affected abnormally large

pled to move independently, and so it appears probable amounts of ocean floor during the mid-Cretaceous.
that shifting of lithosphere and mantle as a single unit One manifestation of this activity was the creation of
takes place during TPW. Indeed, if there is coupling numerous seamounts and ocean plateaux in the western

between core and mantle, the whole Earth may be Pacific (Fig. 7.15) at a rate some five times greater

affected. Andrews’s interpretation of TPW is supported during this period than at other times. Similarly there
by astronomical data which shows that during the 20th were extrusions of thick, areally extensive fl ood basalts
century the location of the Earth’s rotational axis has on the continents, such as the Paraná Basalts of
moved at a rate similar to that computed from paleo- Brazil.
−1
magnetic and hotspot data, namely about 1° Ma . This Phenomena attributed to the mid-Cretaceous super-
suggests that at least part of the mass redistribution plume episode are illustrated in Fig. 5.13. At 120–125 Ma
takes place in the mantle, as the continents do not move the rate of formation of oceanic crust doubled over a
this rapidly. Sabadini & Yuen (1989) have shown that period of 5 Ma, decreased within the next 40–50 Ma, and
both viscosity and chemical stratification in the mantle returned to previous levels about 80 Ma ago (Fig. 5.13d).

are important in determining the rate of polar wander. The additional production of crust required increased
Another mechanism proposed for driving TPW is the subduction rates, and it is significant that major batho-

surface mass redistribution arising from major glacia- liths of the Andes and the Sierra Nevada were emplaced
tions and deglaciations (Sabadini et al., 1982). However, at this time.

mantle flow is required to explain TPW during periods Coupled to the increased crust production, and
with no evidence of significant continental glaciation, caused by the consequent general rise in the level of the

and, indeed, may be responsible for the majority of sea floor, was a worldwide increase in sea level to an
TPW. It has also been suggested that TPW is excited by elevation some 250 m higher than at the present day
the mass redistributions associated with subduction (Fig. 5.13b). At high latitudes the surface temperature
zones (Section 12.9) (Spada et al., 1992), mountain build- of the Earth increased by about 10°C, as shown by
ing, and erosion (Vermeersen & Vlaar, 1993). oxygen isotope measurements made on benthic fora-

minifera from the North Pacific (Fig. 5.13a). This effect
was probably caused by the release of large amounts of
5.7 CRETACEOUS carbon dioxide during the volcanic eruptions, which
created an enhanced “greenhouse” effect (Sections 13.1.1,
SUPERPLUME 13.1.2). During the superplume episode the rates of
carbon and carbonate sequestration in organisms
increased due to the greater area of shallow seas and
the increased temperature, which caused plankton to
Certain hotspots, as described in Section 5.5, are thought thrive. This is reflected in the presence of extensive

to be the surface manifestation of plumes of hot mate- black shale deposits at this time (Force, 1984) and in the
rial ascending from the deep mantle. These are of mod- estimated oil reserves of this period (Tissot, 1979;