Page 418 - Global Tectonics

400 CHAPTER 12

(a)
4 Ma 43 Ma 83 Ma 100 Ma 121 Ma 176 Ma

(b)
78 Ma 94 Ma 98 Ma 106 Ma 114 Ma 137 Ma

0 Temperature (°C) 1846

Figure 12.14 Sequences from numerical models, scaled approximately to the mantle, in which a plume grows from a
thermal boundary layer. In (a) the viscosity is a function of temperature, and in (b) the viscosity also increases by a
factor of 20 at 700 km depth. In (b) the plume slows and thickens through the 700 km discontinuity but then narrows
and speeds up in the low viscosity upper layer (from Davies, 1999. Copyright © Cambridge University Press, reproduced
with permission).

core–mantle boundary. It has been estimated that a African superswell although, as in the Pacifi c, there
major plume may be fed for 100 Ma from a volume of are several potential deep mantle, or primary, plumes
layer D″ only tens of kilometers thick and 500–1000 km around it (Figs 5.7, 12.11c). This contrast is also
in diameter. refl ected in the marked difference in the seismic veloc-
If major upwellings, such as those beneath south- ity anomalies in the upper mantle beneath the two
ern Africa and the south Pacific, reach the base of areas (Plate 12.2b,d between pp. 244 and 245). The

the transition zone they may well form a thermal differing characteristics of the African and Pacifi c super-
boundary layer at this depth from which secondary swells may arise from the fact that the south Pacifi c
plumes may originate (Brunet & Yuen, 2000; Courtillot upwelling is the remnant of the Cretaceous superplume
et al., 2003). These would be relatively short-lived and in this area (Section 5.7). The uplift of southern Africa
without initial flood basalts but may well account for was also initiated in the mid-Cretaceous, suggesting

the hotspots on the south Pacific superswell such as that major mantle upwellings or superplumes may

the Society and Cook-Austral islands, Samoa, Pitcairn, have a life cycle analogous to, and perhaps related
and Caroline (Fig. 5.7) (Adam & Bonneville, 2005). to, the life cycle of the assembly and break-up of
By contrast, there are no plumes within the southern supercontinents.