Page 388 - Global Tectonics

370 CHAPTER 11

11.5 THE the plate are re-projected using the same Eulerian rota-
tion. The continents are then successively shifted along

SUPERCONTINENT fixed latitudes, that is, rotated about the magnetic pole,
until the overlap of continental margins is minimized.
Although the paleomagnetic data do not provide a
CYCLE unique sequence of reconstructions, they clearly indi-
cate the gross trends of plate movements during ancient
times. More detailed inferences on the evolution of
11.5.1 Introduction particular regions are then made from their geology
viewed in terms of plate tectonic mechanisms.
Geologic evidence for the repeated occurrence of con- The application of paleomagnetic methods for the
tinental collision and rifting since the Archean has led Precambrian is less straightforward than for Phanero-
to the hypothesis that the continents periodically zoic times for three main reasons (Dunlop, 1981). First,
coalesced into large landmasses called supercontinents. the error limits of isotopic ages typically are larger.
The best known of the supercontinents include Gond- Second, isotopic and magnetic records may be partially
wana (Fig. 3.4) and Pangea (Fig. 11.27), which formed reset during metamorphism to different degrees, and
in the latest Proterozoic and late Paleozoic times, the distinction between pre- and post-orogenic isotopic

respectively. Other supercontinents, such as Rodinia and and magnetic overprints can be difficult. Third, over-
Laurussia, also have been proposed for Late Proterozoic prints occur during post-orogenic cooling and uplift,
and late Paleozoic times, respectively. Processes in the and the temperatures at which isotopic systems close
mantle that may have led to their assembly and and magnetizations stabilize are different, so that the
dispersal are discussed in Section 12.11. dates may be younger or older than the magnetizations
by intervals of tens of millions of years. However, even
given these uncertainties and the gaps in the paleomag-
11.5.2 Pre-Mesozoic netic record arising from the lack of suitable samples of
certain ages, the data allow investigators to test the valid-
reconstructions ity of paleogeographic reconstructions for pre-Mesozoic
times based on the geologic record on the continents.
Paleogeographic maps for the Mesozoic and Cenozoic
can be computed by the fitting together of continental

margins or oceanic lineations of the same age on either 11.5.3 A Late Proterozoic
side of an ocean ridge (Chapters 3, 4). The location of
the paleopoles can be determined from paleomagnetic supercontinent
measurements (Section 3.6) and so the only unknown
in these reconstructions is the zero meridian of longi- Similarities between the Late Proterozoic geologic
tude. These combined techniques cannot be used for record in western Canada and eastern Australia (Bell &
reconstructions prior to the Mesozoic because in situ Jefferson, 1987; Young, 1992) and between the south-
oceanic crust is lacking. western USA and East Antarctica suggest that these
Methods of quantifying plate motions in pre- areas were juxtaposed during Late Proterozoic times
Mesozoic times involve the use of paleomagnetic data (Dalziel, 1991, 1995; Moores, 1991; Hoffman, 1991) (Fig.
coupled with high-precision geochronology. Ancient 11.19a). This seemingly radical suggestion was referred
plate edges, although somewhat distorted, are marked to as the SWEAT (South West US and East AnTarctica)
by orogenic belts and ophiolite assemblages (Sections hypothesis. The widespread Grenville orogenic belts,
2.5, 11.4.3), which indicate ancient sutures between that immediately pre-date the Late Proterozoic, suggest
welded continents and accreted terranes. Evidence pro- that many other continental fragments can be added to

vided from the past distributions of flora and fauna and this reconstruction to form a Late Proterozoic super-
indicators of paleoclimate also aid these plate recon- continent called Rodinia (Fig. 11.19a). Laurentia (North
structions (Sections 3.4, 3.5). For a particular time the America and Greenland) forms the core of the

paleomagnetic pole for each ancient plate is rotated to supercontinent and is flanked to the north by East
an arbitrary single magnetic pole and the continents on Antarctica. The reconstruction shows that the North