Page 120 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 107

Superplume Fig. 5.13c), which may constitute about 50% of the
High-latitude surface world’s supply. Also of economic significance is the

(a) 30
temperature placement of a large percentage of the world’s diamond
20
°C supply at this time, probably as a result of the dia-
10
monds’ having been translated to the surface by the
0
0 150 rising plumes. During the plume episode the rate of
geomagnetic reversals (Section 4.1.4) was very low (Fig.
(b)

5.13e), with the field remaining in normal polarity for
300
Sea level some 35 Ma. This indicates that activity in the core,
200

m where the geomagnetic field originates (Section 3.6.4),
100
was low, perhaps related to the transfer of considerable
0
0 150 quantities of heat to the mantle.
Acceptance of a mid-Cretaceous superplume episode
(c)
5 is not universal. For example, Anderson (1994) suggests
Oil resources
that the phenomena of this period were caused by a
4 general reorganization of plates on a global scale associ-
ated with the break-up of Pangea and reorganization of
10 9 Mg Ma –1 3 2 the Pacific plate. The mantle upwelling in the latter may

then have been a passive reaction to plates being pulled
apart by their attached slabs. The episode would thus
be viewed as a period when mantle ascended passively
1 Black Shales as a result of changing plate motions.
0
0 150
(d) 35
Oceanic crust 5.8 DIRECT
30 production
10 6 km 3 Ma –1 25 MEASUREMENT OF
RELATIVE PLATE
20
15
MOTIONS
0
0 150
(e) 6 5 Reversal rate It is now possible to measure the relative motion
Reversals Ma –1 4 3 2 between plates using methods of space geodesy
(Gordon & Stein, 1992). Before about 1980 the only
methods available for this type of investigation were the
0 1 standard terrestrial geodetic methods of baseline mea-
surement using optical techniques or laser ranging
0 50 100 150
Age (Ma) instruments such as the geodolite (Thatcher, 1979).
These methods are certainly sufficiently precise to

Cenozoic Cretaceous
measure relative plate motions of a few tens of milli-
Figure 5.13 Phenomena associated with the mid- meters a year. However, as noted in Section 5.3, in some
Cretaceous superplume (after Larson, 1991a, 1991b, with regions the strain between plates is not all dissipated
permission from the Geological Society of America). across a narrow plate boundary, but may extend into
the adjacent plates for great distances, particularly in
continental areas (Fig. 5.5). In order to study these large-