Arkell.1956.Jurassic..

CHAPTER 29
SHIELDS, SHELVES, MOBILE BELTS
AND GEOSYNCLINES
For many geological purposes it is useful to classify the land areas of the
earth into (1) shields, or ancient continental nuclei, composed of pre-
Cambrian crystalline complexes, which have been relatively stable and
received little sediment all through post-Archaean time; (2) shelves, or
moderately rigid platforms, surrounding the shields, often with the
crystalline basement not far below or protruding as large horsts, but the
surface beneath the Mesozoic cover composed of either flat-lying or
(farther from the shields) often highly folded, consolidated and eroded
Palaeozoic sediments which are relics of older mountain chains; and
(3) mobile belts, in which stand the youthful Tertiary mountain chains of
the present day and which display in their Mesozoic and Tertiary sedimentation
a high degree of mobility, often with geosynclinal deposition,
volcanic extrusions and batholithic intrusions. The shields and stable
parts of the shelves together form the rigid elements of the earth's surface,
the kratogens; the mobile belts and mobile parts of the shelves form the
unstable elements or orogens.
In general the principal shields conspicuous at the present day (North
America, South America, Antarctica, Scandinavia, Africa with western
Arabia, peninsular India, Angaria, Sinia, Western Australia) were already
in existence in Jurassic times. So also were the shelves; indeed, our
concept of the shelves is conditioned largely by the development of the
Mesozoic epicontinental deposits left upon them. The mobile belts,
however, were by no means always the same in the Jurassic as in the
Tertiary and at present, although for the most part distributed along the
same general lines, namely: in a ring round the Pacific, and as a connecting
band east-west across Asia and the Mediterranean and Alpine region.
In this chapter will be brought together salient facts bearing on the
distribution and condition of the shields, shelf-seas, mobile belts and
geosynclines during the Jurassic. Finally, in the last two chapters (30, 31)
are critically summarized the volcanic activity and diastrophic movements
that occurred during the Jurassic in the mobile belts.
It must be emphasized how arbitrary is the division between shields,
shelves, and mobile belts: all grade into one another. Like all attempts
to classify natural phenomena, this is to some extent an artificial scheme;
but it has its uses.
SHIELDS
The absence of marine fossils from the greater part of the shields makes
dating of their deposits extremely uncertain, and it is consequently
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