Page 430 - Global Tectonics

412 CHAPTER 13

because, although this represents just 5% of the Earth’s transition to an “Icehouse Earth,” and eventually the

land area, 23% of the global flux of dissolved material triggering of the Ice Ages of the past 3 Ma.
in rivers is derived from rivers with a source in the
Tibetan/Himalayan region. There is some indication

from the fauna and flora, and the sedimentary record
of northern India, that there was a major intensifi cation 13.2 ECONOMIC
of the southwest monsoon about 8 Ma ago (An et al.,
2001). The question remains, however, whether this GEOLOGY
correlates with the uplift of the Tibetan Plateau
(Section 10.4.3). Current models for this uplift, which
may involve the convective removal of thickened litho- 13.2.1 Introduction
sphere beneath Tibet (Section 10.4.6), imply that the

final uplift phase may have been relatively sudden, in The application of plate tectonic theory to the explora-
geologic terms. Attempts to obtain an independent tion of economically viable mineral and hydrocarbon
estimate of the timing of this uplift, using paleobo- deposits is a common approach in the field of economic

tanical evidence or the dating of fault systems, have geology. Plate tectonics has provided exploration geolo-
proved to be inconclusive, with some results confi rming gists with a framework to which they can relate the
the 8 Ma date, but others indicating a date of 14–15 Ma specific environments and spatial relationships of

for the final uplift of Tibet (Spicer et al., 2003). Spicer economic deposits (Rona, 1977; Bierlein et al., 2002;

et al. suggest that a possible explanation for this is that Richards, 2003). Studies of this kind have increased
the uplift occurred progressively from south to north as the search for small and covered deposits becomes
over a period of 6–7 million years. progressively more important. This approach has led to
Greatly enhanced weathering of silicate rocks in the a classification of economic deposits according to plate

late Miocene, would have removed CO 2 from the Earth’s tectonic processes. Many of the observations that
atmosphere and might well account for the pronounced support this classification (listed below) are discussed by

global cooling revealed by oxygen isotope studies at or Mitchell & Garson (1976, 1981), Rona (1977), Tarling
near the Miocene–Pliocene boundary, i.e. at about 6 Ma (1981), Hutchinson (1983), Sawkins (1984), and Evans
ago (Fig. 13.8). As indicated above this may have pro- (1987).
duced effects that led, ultimately, to the initiation of the
1 autochthonous deposits directly related to
Ice Ages approximately 3 Ma ago.

Thus plate tectonic processes influence all the major magmatism at plate margins and interiors;
factors that are currently thought to determine the 2 allochthonous deposits related to plate margin
Earth’s long-term changes in climate. The concentra- magmatism;
tion of CO 2 in the atmosphere, at any particular point 3 deposits related to sedimentary basins formed
in time, is thought to be determined largely by the by plate motions;
amount of volcanism at that time. Thus the exception-
4 deposits related to climate and to changes in
ally high levels of CO 2 associated with the “Greenhouse
paleolatitude resulting from plate motions.
Earth” of the Cretaceous period are related to super-
plume activity, and high rates of sea floor spreading and Whereas plate tectonic theory has been useful for

subduction, all three giving rise to enhanced volcanic understanding the origin and evolution of economi-
activity. Conversely, systematic decreases in plume cally viable deposits, alternative approaches also have
activity, and plate accretion and destruction, would been employed, especially with regard to mineral
cause global cooling. However, the periods of pro- deposits. One area of current research involves investi-
nounced global cooling during the past 50 Ma are not gating the potential links among the formation of ore
associated with decreases in volcanism (Fig. 5.13). It deposits, the evolution of large igneous provinces (LIPs,
seems probable therefore that one needs to invoke the Section 7.4.1), and the effects of deep mantle plumes
other potential impacts of plate tectonic processes on (Ernst et al., 2005). The formation of LIPs and the rise
the Earth’s climate, notably changes in oceanic circula- of deep mantle plumes may involve tectonic and mag-
tion and the consequences of mountain building, and matic activities that operate independently of plate
enhanced weathering, to explain the mid-Cenozoic motions. In addition, the understanding of mineral