Page 112 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 99

stationary during the past 25 Ma. Following a long
period of quiescence, in terms of tectonic and volcanic N Am
activity, large parts of Africa have been subjected to 74
uplift and/or igneous activity during the late Cenozoic. 65
This was considered to be a result of the plate becom- 40°N
ing stationary over hot spots in the upper mantle. 43 HE
Another proposal was that the Caribbean plate is likely
to be stationary as it has subduction zones of opposite 100 25 5 0
polarity along its eastern and western margins. Sub- 20°N
ducting plates would appear to extend through the 74 100 Pacific Ocean
asthenosphere and would be expected to inhibit lateral 65
motion of the overlying plate boundary. Similar rea- 0° A - C 74
soning led Kaula (1975) to suggest a model in which OP 100 43 65
the lateral motion of plate boundaries in general is
25 43 L
minimized. 25
20°S 74 5 0
65 5 0
Aus plate
5.5 HOTSPOTS 43
40°S LS
25
5
The major part of the Earth’s volcanic activity takes 0
place at plate margins. However, a signifi cant fraction
occurs within the interiors of plates. In oceans the
160°E 180° 160°W 140°W 120°W
intra-plate volcanic activity gives rise to linear island
and seamount chains such as the Hawaiian–Emperor
Figure 5.7 Hotspot tracks on the Pacific plate. HE,

and Line Islands chains in the Pacific (Fig. 5.7). More- Hawaiian–Emperor chain; A-C, Austral-Cook islands; L,
over, several of these Pacific island chains appear to be Line islands; LS, Louisville chain; OP, Ontong-Java

mutually parallel. Where the volcanic centers in the Plateau. Numbers on chains indicate the predicted age
chains are closely spaced, aseismic ridges are con- of seamounts in Ma (redrawn from Gaina et al., 2000, by
structed, such as the Ninety-East Ridge in the Indian permission of the American Geophysical Union.
Ocean, the Greenland–Scotland Ridge in the North Copyright © 2000 American Geophysical Union).
Atlantic, and the Rio Grande and Walvis ridges in the
South Atlantic. These island chains and ridges are asso-
ciated with broad crustal swells which currently occupy
about 10% of the surface of the Earth, making them alous feature that will eventually become welded to a
a major cause of uplift of the Earth’s surface (Crough, continental margin as a suspect terrane (Section
1979). 10.6.1).
The island chains are invariably younger than the An example of an oceanic island chain is the
ocean crust on which they stand. The lower parts of Hawaiian–Emperor chain in the north-central Pacifi c
these volcanic edifices are believed to be formed pre- Ocean (Fig. 5.7). This chain is some 6000 km long and

dominantly of tholeiitic basalt, while the upper parts shows a trend from active volcanoes at Hawaii in the
are alkali basalts (Karl et al., 1988) enriched in Na and K southeast to extinct, subsided guyots (fl at-topped sea-
and, compared to mid-ocean ridge basalts, have higher mounts) in the northwest. Dating of the various parts

concentrations of Fe, Ti, Ba, Zr, and rare earth ele- of the chain confirmed this trend, and revealed that
ments (REE) (Bonatti et al., 1977). Their composition is the change in direction of the chain occurred at 43 Ma
compatible with the mixing of juvenile mantle material (Clague & Dalrymple, 1989). The Hawaiian–Emperor

and depleted asthenosphere (Schilling et al., 1976) chain parallels other chains on the Pacific Plate, along
(Section 6.8). They are underlain by a thickened crust which volcanism has progressed at a similar rate
but thinned lithosphere, and represent a type of anom- (Fig. 5.7).