Arkell.1956.Jurassic..

CHAPTER 21
NORTHERN ASIA
THE CONTINENT OF ASIA AND THE ANGARA SERIES
During the whole of Jurassic time a continuous continent stretched
from the Urals to the coast of China. Before the shallow Callovo-
Oxfordian flooding of the Russian platform in eastern Europe, continental
land extended also west of the Urals to join the Scandinavian shield, so
that the Asian continent reached from the Pacific to the North Atlantic,
as at the present day. Those who see any advantage in doing so follow
Grabau and call this continent Pal-Asia.
Its boundaries, it is true, were different from those of the present day;
but the shapes of all the continents have repeatedly changed throughout
geological time. Much of the south of the present Asia was submerged
beneath the Tethys, or detached by it (Peninsular India). The northern
boundary of Tethys ran approximately along a line drawn from the Aral
Lake to Hong Kong. From the north and NE. the continent was invaded
by arms of the Arctic Ocean, which extended up the present valleys of the
Ob and Lena to about latitude 60° N. but avoided the intervening Siberian
Platform and Taimyr. Another arm extended from the east up the Amur
valley into Transbaikalia. Sea also covered most of the extreme north-east
of Siberia, beyond the Lena.
Unlike Africa, Asia does not, and did not in Jurassic times, consist of
one immense pre-Cambrian shield, but of several smaller ones welded
together. The largest is the Siberian Platform, lying roughly between the
Yenesei and the Lena and between the Arctic coast and Lake Baikal.
It is much more encumbered by later sediments than the Scandinavian
shield. This is the Angara Land of Suess, but Suess's concept has been
enlarged and modified by subsequent discoveries. The ranges of
mountains which lap like a letter U round the south and south-east of the
'amphitheatre of Irkutsk' have turned out to be no 'ancient vertex' but the
front ranges of a late Jurassic or early Cretaceous orogenic belt that was of
great importance in eastern Asia.
Farther south several smaller shields or ancient nuclei occur, notably
under the Gobi and Tarim deserts, the Ordos and northern Tibet. They
are swathed around and cemented together by fold ranges, as yet little
known in detail, but at least in large part of Jurassic date. According to
Vialov( 1939) the nuclei and orogenshave a linear arrangement that stretches
right across Asia from west to east; but a less regular pattern such as was
sketched by Grabau (1928, p. 292) seems more likely. In such a vast
orogen these minor nuclei dwindle to the rank of median masses, analogous
with those in Europe and Anatolia, and with the Iranian Plateau.
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