Wilson cycle

Wilson cycle

1. W
WILSON CYCLE ilar faunas that are adjacent to one another. He
argued that Wegenerian continental drift could
account for the separation, between North Amer
ica and Europe, of tracts having the similar
faunas. These fauna-bearing tracts can be refitted
to a logical distribution by a simple reconstruc-
tion of the pre-Atlantic Ocean continent assem-
blage (Bullard et al., 1965). However, such a re-
construction does not account for the dissimilar
fauna-bearing adjacent tracts that remain in the
The discovery of the paired matching of mag-
netic reversal anomalies of the ocean floors of the
world (Vine and Matthews, 1963) has established
the geometric basis for lithosphere plate evolu-
tion. Lithosphere is generated at accreting plate
margins such as the Mid-Atlantic Ridge and is
subducted into the mesosphere at consuming plate
margins such as the Peru-Chile Trench. Conti-
nents are passive passengers of evolving litho- assembly. Wilson argued from geologic relation-
sphere; because of buoyancy, they are not con-
sumed but are separated and reassembled into
complex mosaics as a consequence of plate evo-
ution over the course of geologic time. During
approximately 400 my intervals the entire area of
the floors of the oceans of the world is generated
and consumed; the present seafloors are no older
than approximately 200 my. During these inter-
vals, megacontinents such as Pangea are sepa-
rated and carried away as smaller continents to
configurations such as the current one.
Current theory holds that lithosphere plate
ev-
olution, the mechanism by which the internal heat
of the Earth is dissipated, has been the principal marked by extensive faulting, thrusting and crushing;
geotectonic process responsible for the evolution
of the continental geologic record of
approxi
mately 3.5 by and that several earlier Pangaeas
had been assembled and separated in Precam-
brian to Paleozoic time as a consequence of global
lithosphere plate evolution. Detailed study of the
Oceanic magnetic anomaly patterns, paleomag-
netic data, and the distribution of Phanerozoic
fauna and flora and of mountain belts and their
ships that this anomaly could be accounted for by
invoking Paleozoic continental drift. He pro-
posed that the Appalachian/Caledonian Orogenic
Belt was the result of the opening-closing evolu-
tion of a Proto-Atlantic Ocean, during Paleozoic
time. Following several lines of evidence, he pro-
posed that such a model of geologic history pro-
vided a unified explanation for the following:
IC]hanges in rock types, fossils, mountain building ep-
isodes and paleoclimates represented by the rocks of the
Atlantic region;. . wherever the junction between
contiguous parts of different realms is exposed, it is
.. there is evidence that the junction is everywhere
along the eastern side of a series of ancient island ares;
the fit appears to meet the geometric requirement
that during a single cycle of closing and reopening of
an ocean, and in any latitudinal belt of the ocean, only
one of the pair of opposing coasts can change sides.
This perceptive model has been verified by
more recent studies of the Appalachian/Caledon-
jan System, and the rapidly developing theories of
associated rocks has firmly established that litho- lithosphere plate tectonics (Dewey, 1969). The
sphere plate evolution is the driving mechanism cycle of ocean opening and closing with respect
to adjacent
continental margins is now known as of Wegenerian continental drift, with the breakup
of Pangaea in early Mesozoic times and subse-
quent transport of continents resulting in their
present geographic distribution.
In 1966, Professor J. Tuzo Wilson of the Uni-
versity of Toronto published a now classic paper
in which he argued that not only had Wegenerian
continental drift occurred since the early Meso-
zoic breakup of Pangaea, but also that continen-
the Wilson cycle.
In the past decade researchers have learned that
continental
drift is not a random movement of
continents as implied by the term drift. lnstead,
moving continents have rather precise geographic
tracks because their motion is a consequence of
plate evolution as revealed by the distribution of
the oceanic magnetic anomaly signatures. How-
tal drift had occurred in pre-Pangaea times. Wil- ever, Dewey (1975b) has pointed out that al-
son pointed
out that for the Appalachian/ though the Wilson cycle has now been applied to
numerous plate margin environments and the
ing similar Lower Paleozoic faunas are separated broad geologic corollaries have become wel
by the present Atlantic Ocean whereas other re-known, detailed reconstructions ofspecific cycles
remains a difficult or impossible task, for several
Caledonian Orogenic System, some regions hav
gions within the separated segments have dissim-
836

2. WILSON CYCLE
A
NCIPIENT RUPTURE OF CONTINENT
T
1
i
Continental crust 1141V
Mantl
B
Accreting plate margin
RIft valley
EV LZSZ1M
LIthosphere
C
Oceanic crust
D
Oceanic ridge
Sea lev
ALLI
Moho A
Lithosphere
Continental shelf E
Consuming plate margin
Arcean-type orogen Irencn
G
CONTINENT-CONTINENT cOLLISION
H
Suture
Himalayan-type orogen
FIGURE 1. Schematie illustration of the Wilson cycle, the evolution of oceans, in the framework of lithosphere
plate evolution. (Adapted from Bird and Dewey, 1970, Fig. 9)
837

3. WILSON CYCLE
reasons. First, critical geologic evidence may be
destroyed during plate evolution; also, there is no
oceanic magnetic anomaly record for pre-Meso-
zoic cycles. Second, evolution of plate margins
interacting with fragments of continents and
microcontinents produces exceedingly complex
geometries and timing that, although perhaps
present in the rock record, may be too intricate
to solve. Third, second-order effects at consum-
of the tectonic and volcanic
processes attendant
with the consuming plate margin. If the uncou-
pling takes place
at a considerable distance ocean-
ward from the continental margin, an island arc
like
Japan forms. Consumption of the oceanic
lithosphere requires a change in the plate geom-
etry and eventual consumption of the
accreting
plate margin (Fig. 1F). During this time the
op-
posing continental shelf may continue to enlarge,
as an Atlantic-type half-ocean. As the ocean con-
tinues to contract, the opposing continental mar-
gin eventually arrives into the regime of the con-
suming place margin and becomes involved in the
evolving Andean-type orogen. This is the early
stage of continent-continent collision. Because of
buoyancy constraints of the continental litho-
sphere, plate consumption ceases in the later
stages of continent-continent collision. The litho
sphere slab detaches beneath the growing Hima
layan-type orogen, and the final stage of the Wil-
son cycle orogenic evolution wanes, with the fully
evolved mountain belt remaining as a suture be-
tween the newly joined continental masses. A
model of orogenic evolution based on the Wilson
cycle and lithosphere plate tectonics was proposed
by Dewey and Bird (1970). A later episode of con-
lithosphere mantleto form such a margin constitute tinental drift may separate the suture. As pointed
ing plate margins, such as body forces, may be
important but produce effects that are indistin-
guishable from effects directly related to relative
plate displacements (see Dewey, 1975a). Also, as
pointed out by Wilson (1966), paleomagnetic data
for Paleozoic and older rocks are likely to remain
of little use in
quantitative reconstructions of con-
tinent positions.
Figure 1 is a schematic illustration of the basic
concept of the Wilson cycle in the framework of
lithosphere plate evolution; it is adapted from Bird
and Dewey (1970, Fig. 9). Figure 1A
represents
the beginning of the Wilson cycle, the initial rup-
ture of a continent and formation of an accreting
plate margin. The tectonic framework of litho-
sphere rupture has been recently discussed by Tur-
cotte et al.(1977). Themechanisms of thesub-
a significant andunsolved problem of mantle dy-
namics and petrogenetic process (see Ringwood,
1975). It is known, however, that the rigid lith0-
out by Wilson (1966), this accounts for the
pres-
ent separation of the Appalachian/Caledonian
Orogenic Belt about the north Atlantic Ocean.
sphere overlies a weaker, partly melted zone of
the asthenosphere, the so-called low velocity zone
(LVZ, Fig. 1B). As continent separation proceeds,
a
rift valley (Fig. 1B) and small ocean (Fig. 1C)
evolve, constituting the early ocean-opening stages
of the Wilson cycle. The African Rift and Red Sea
are good examples. As drift of the newly sepa-
rated continents continues by the symmetric ac-
JOHN M. BIRD
References
Bird,
J.
M., and Dewey,
J. F., 1970, Lithosphere plate
continental margin tectonics and the evolution of the
Appalachian orogen, Geol. Soc. America Bull. 81,
1031-1060.
cretion of new lithosphere about the plate margin,
continental shelves of continent-derived sediment
accumulate at the ocean-continent interfaces. A
fully developed ocean (Fig. 1D), with a central
ridge along the plate margin and evolved conti-
nental shelves, is called an Atlantic-type ocean.
By observation
of trenches, their associated
seismicity, and reconstructions of oceanic mag-
netic anomaly patterns about them, it is known
that oceanic lithosphere uncouples and descends
into the mesosphere. Lithosphere behaves as a
rigid body and accretion and consumption of
lithosphere proceeds on a global net balance; on
any great circle the amount of new lithosphere
generated at accreting plate margins is consumed
at consuming plate margins. Figure 1E illustrates
a simple accreting-consuming plate margin ocean,
the initial stages of ocean closing in the Wilson
cycle. The result of the uncoupling adjacent to the
continental margin is the conversion of that mar-
gin to an Andean-type orogen, as a consequence
Bullard, E.; Everett,
J. E.; and Smith, A. G., 1965, The
fit of the continents around the Atlantic, Royal Soc.
London Philos. Trans. 258A, 41-51.
Dewey, J. F., 1969, Evolution of the Appalachian/
Caledonian orogen,Nature222, 124-129.
Dewey,J. F., 1975a, Finite plate evolution implications
for the evolution of transforms, triple junctions, and
orogenic belts, Am. Jour. Sci. 275A, 260-284.
Dewey, J. F., 1975b, The Wilson Cycle, Geol. Soc.
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Dewey, J. F., and Bird, J. M., 1970, Mountain belts and
the new global tectonics, Jour. Geophys. Research 75,
2625-2647.
Ringwood,A. E., 1975, Composition and Petrolog)y of
the Earth's Mantle. New York: McGraw-Hill, 618p.
Turcotte, D. L.; Ahern, J. D.; and Bird, J. M., 1977,
The state of stress at continental margins, Tectono-
physics.
Vine, F. J., and Matthews, D. H., 1963, Magnetic
anomalies over oceanic ridges, Nature 199, 947-949.
Wilson, J. T., 1966, Did the Atlantic close and then re-
open? Nature 211, 676-681.
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