Page 20 - Global Tectonics

HISTORICAL PERSPECTIVE 7

history of reversals of the Earth’s magnetic fi eld.
A further precursor to the development of the
theory of plate tectonics came with the recognition, by
Mantle J.T. Wilson in 1965, of a new class of faults termed
transform faults, which connect linear belts of tectonic
activity (Section 4.2). The Earth was then viewed as a
mosaic of six major and several smaller plates in relative
motion. The theory was put on a stringent geometric
basis by the work of D.P. McKenzie, R.L. Parker, and
W.J. Morgan in the period 1967–68 (Chapter 5), and

confirmed by earthquake seismology through the work
Figure 1.5 The concept of sea floor spreading (after of B. Isacks, J. Oliver, and L.R. Sykes.

Hess, 1962). The theory has been considerably amplifi ed by
intensive studies of the geologic and geophysical pro-
cesses affecting plate margins. Probably the aspect
volcanic submarine swell or rise which occupies a about which there is currently the most contention is
median position in many of the world’s oceans (Fig. the nature of the mechanism that causes plate motions
1.5). Oceanic crust is much thinner than continental (Chapter 12).
crust, having a mean thickness of about 7 km, com- Although the basic theory of plate tectonics is well
pared with the average continental thickness of about established, understanding is by no means complete.
40 km; is chemically different; and is structurally far less Investigating the implications of plate tectonics will
complex. The lateral motion of the oceanic crust was fully occupy Earth scientists for many decades to
believed to be driven by convection currents in the come.
upper mantle in the fashion of a conveyer belt. In order
to keep the surface area of the Earth constant, it was
further proposed that the oceanic crust is thrust back
down into the mantle and resorbed at oceanic trenches. 1.3 GEOSYNCLINAL
These are vast bathymetric depressions, situated at
certain ocean margins and associated with intense vol-
canic and earthquake activity. Within this framework THEORY
the continents are quite passive elements – rafts of less
dense material which are drifted apart and together by

ephemeral ocean floors. The continents themselves are Prior to the acceptance of plate tectonics, the static
a scum of generally much older material that was model of the Earth encompassed the formation of tec-
derived or separated from the Earth’s interior either at tonically active belts, which formed essentially by verti-
a very early stage in its history or, at least in part, steadily cal movements, on the site of geosynclines. A review of
throughout geologic time. Instead of blocks of crust, the development of the geosyncline hypothesis and its
we now think in terms of “plates” of comparatively explanation in terms of plate tectonics is provided by
rigid upper mantle and crust, perhaps 50–100 km thick Mitchell & Reading (1986).
and which we term lithosphere (a term originally coined Geosynclinal theory envisaged elongate, geographi-

by R.A. Daly many years ago and meaning “rock layer”). cally fixed belts of deep subsidence and thick sediments
Lithospheric plates can have both continental and as the precursors of mountain ranges in which the strata
oceanic crust embedded in them. were exposed by folding and uplift of the geosynclinal
The theory of sea fl oor spreading was confi rmed in sediments (Dickinson, 1971). A plethora of specifi c
the period 1963–66 following the suggestion of F.J. Vine nomenclature evolved to describe the lithological asso-
and D.H. Matthews that the magnetic lineations of the ciations of the sedimentary fill and the relative locations

sea floor might be explained in terms of sea fl oor of the geosynclines.
spreading and reversals of the Earth’s magnetic fi eld The greatest failing of geosynclinal theory was that

(Section 4.1). On this model the conveyor belt of oceanic tectonic features were classified without there being
crust is viewed as a tape recorder which registers the an understanding of their origin. Geosynclinal