Past plate motions - The Solid Earth - page 87

the seafloor, the lineations, palaeomagnetic measurements and dating of drilled
and dredged basement samples will slowly refine and improve the picture of the
history of the Pacific back to Jurassic time.

3.3.5 The continents
Figure 3.30,aseries of snapshots of the continents, shows how they have moved
relative to each other through the Phanerozoic. By the late Carboniferous to
earliest Permian, the continents were all joined together and formed one super-
continent, which we call Pangaea (Greek, 'all lands'). The northern part of
Pangaea, comprising today's North America, Greenland and Eurasia, has been
named Laurasia or Laurentia , and the southern part of the continent, comprising
South America, Africa, India, Antarctica and Australia, is called Gondwanaland .
Laurasia and Gondwanaland became distinct during the Jurassic when Pangaea
rifted in two. The wedge-shaped ocean between Laurasia and Gondwanaland is
the Tethys Sea. (Tethys was the wife of Oceanus in Greek mythology.) It is this
sea which has presumably been subducted beneath Laurasia as India and Africa
have moved northwards. The Mediterranean, Caspian and Black Seas are the last
vestiges of this ancient ocean, the completion of whose subduction resulted in
the building of the Alpine, Carpathian and Himalayan mountain chains. All these
regions contain scattered outcrops of ophiolites (a suite of rocks with chemical
and lithological similarities to the oceanic crust, which may be examples of crust
from ancient back-arc basins, Section 9.2.1). There is considerable current debate
about the reconstruction of continental fragments prior to the Pangaea supercon-
tinent (uncertainty increases with age) and there are many differing views on
the arrangements of the current continental pieces into pre-Pangaean superconti-
nents. Rodonia was a Mid-to-Late Proterozoic (750-

1000 Ma) supercontinent.
It is proposed that, in the Late Proterozoic, it broke up into Pannotia, Siberia and
North China. Pannotia then further split into Laurentia (the Precambrian core
of North America), Gondwanaland and Baltica. It is proposed that, at about the
same time, there was a major global glaciation ('snowball' Earth). The oldest
reconstructions shown in Fig. 3.30,which are largely based on Dalziel (1997),
are presented only as one example of what Precambrian geography may have
been like, rather than as absolute fact.

∼

−→
Figure 3.30. Palaeocontinental maps showing the present-day continents in their
previous positions. ANT, Antarctica; GR, Greenland; IND, India; M, Madagascar;
NOAM, North America; and SOAM, South America. Solid lines, fracture zones and
magnetic lineations. Dark grey shading, large igneous provinces (LIPs) volcanics
produced at hot spots. (Reconstructions provided by Kylara Martin of the PLATES
Project, Institute for Geophysics, University of Texas at Austin, Lawver et al. 2003.)