DEFINITION OF CONTINENTAL DRIFT AND PLATE TECTONICS: HISTORICAL DEVELOPMENT OF THEORIES

The science of tectonics began with Niels

Stensens’s (alias Nicolaus Steno) publication in 1669 of strain

analyses and reconstructions of the pre-deformational configuration

of Tuscany, northern Italy. The 18th and early 19th

centuries were dominated by views that deformation and

uplift of mountain belts were caused by magmatic intrusions

and volcanic eruptions and/or the downhill sliding of rock

massifs, for example, James Hutton in 1795, Sir James Hall

in 1815, Baron Leopold von Buch in 1824, de Saussure in

1796, Gillet-Laumont in 1977, and Studer in 1851–53. In

1829 and 1852 Leonce Elie de Beaumont published an alternative

view in which the deformation and uplift of mountain

belts was attributed to thermal contraction of a cooling

Earth, and this view was popular until the mid-20th century.

Mountain belts were viewed as products of crustal shortening

by the Rogers brothers (1843) and by James Dwight Dana

(1866, 1873), but these scientists still attributed the shortening

to global thermal contraction and explosive volcanism (A.

M. Celal Sengör, 1982).

Classical theories of orogenesis were upheaved in 1875

with the publication of Eduard Suess’s Die Entstehung der

Alpen. Suess synthesized structural and stratigraphic relationships

in mountain belts of the world and emphasized the controlling

role of crustal shortening. Although still following

the scheme of global contraction, Suess noticed that the global

orientation of mountain belts did not obey any simple

geometry as expected from uniform global contraction but

seemed to reflect an interaction between older rigid massifs

and a progression of contractional events on the margins of

the massifs. All these models assumed local contraction and

did not consider any large-scale translations of rock massifs.

In 1884 Marcel Bertrand reinterpreted geological relations in

the Alpine Glaurus in terms of a “nappe,” which required

large horizontal movements, around 25 miles (40 km).

Although the nappe theory was not officially recognized until

1903 at the International Geological Congress in Vienna, the

publication of this paper marks the beginning of widespread

recognition of significant translations in rock massifs. In

Suess’s last volume of Das Antlitz der Erde (1909), he admitted

that the amount of shortening observed in mountain belts

was greater than could be explained by global cooling and

contraction, and he suggested that perhaps other translations

have occurred in response to tidal forces and the rotation of

the planet. Suess argued on geological grounds for a former

unity between Africa and India.

The idea of stable plate interiors can be traced back to

Leopold Kober’s Der Bau der Erde (1921), in which he

defined kratogens as areas of low or no mobility, and orogens

as areas of high mobility. About this same time (1908, 1916),

Emily Argand, clearly of the mobilist school, had documented

a continuous Mesozoic through Cenozoic kinematic evolution

of the Alpine Orogen. In 1915 and 1929 Alfred Wegener published

Die Entstehung der Kontinente and Die Entstehung der

Kontinente und Ozeane. Wegener argued strongly for large

horizontal motions between cratons made of sial, and using

such data as the match of restored coastlines and paleontological

data, he founded the theory of continental drift. The most

notable of these early mobilists were Alex du Toit, Reginald

A. Daly, Arthur Holmes, Solomon Calvi, and Ishan Ketin (A.

M. Celal Sengör, 1982). Alfred Wegener (1915) and Alex du

Toit (1927) matched positions of Precambrian cratons and

younger fold belts and sedimentary basins between Africa and

South America to support continental drift, and it was not

until much later, in the 1960s, that continental drift theory

developed into a true, multidisciplinary science. It was the

geophysical exploration of the seafloor and documentation of

rock paleomagnetism that provided the strongest evidence for

seafloor spreading and continental drift and led to the theory

of plate tectonics (J. Tuzo Wilson, 1965).

Relative plate motions are now corroborated by a plethora

of different types of data, including but not limited to the

qualitative assertions of the early pioneers. Geological similarities

between continents with similar strata and structures

continue to offer some of the strongest evidence supporting

continental drift. Precise computer-aided geological correlations

continue to this day to be one of the most powerful tools

for reconstructing past configurations of the continents.

Paleontological and biological data have supported the

continental drift hypothesis, with copious early arguments by

Arldt (1917), Jaworski (1921), and Alfred Wegener (1929).

Since then, paleontological research in tectonics has established

paleobiogeographic provinces and has led to the establishment

of biostratigraphic timescales (e.g., Truswell, 1981),

used for dating geological events. Paleontological data have

been directly used to infer plate movements in a few cases.

Nordeng (1959, 1963) and Vologdin (1961, 1963) described

heliotropic growth of stromatolite columns toward the equator

and noted that the apparently changing directions of the

equator through time could be interpreted in terms of a wandering

pole (or shifting continents). William McKerrow and

Cocks (1976) used paleobiogeography to infer the rate of closure

of the Iapetus Ocean; Runnegar (1977) used the districontinental

drift and plate tectonics: historical development of theories 91

bution of cold water circumpolar faunas to infer the drift of

Gondwana over the poles. Dwight Bradley and Timothy

Kusky (1986) used graptolite biostratigraphy to infer convergence

rates and directions for the Taconic orogeny.

Tectonic syntheses based on paleoclimatology have

exploited relationships between certain types of deposits,

which are most likely to form between specific latitudes. For

instance, tillites associated with glaciation tend to form in

high latitudes, whereas evaporites and carbonate reefs have a

preference for growing within 30°N/S of the equator (e.g.,

Briden and Irving, 1964). Thus, evaporites, tillites, red beds,

coal deposits, and phosphorites all give some indication of

paleolatitude and can be used with other data for plate reconstructions

(e.g., Frakes, 1981; Ziegler, 1981).

The transition zone between thick

buoyant continental crust and the thin dense submerged

oceanic crust. There are several different types, depending on

the tectonic setting. Passive, trailing, or Atlantic-type margins

form where an extensional boundary evolves into an

ocean basin, and new oceanic crust is added to the center of

the basin between originally facing continental margins.

These margins were heated and thermally elevated during

rifting and gradually cool and thermally subside for several

tens of millions of years, slowly accumulating thick

sequences of relatively flat sediments, forming continental

shelves. These shelves are succeeded seaward by continental

slopes and rises. The ocean/continent boundary is typically

drawn at the shelf/slope break on these Atlantic-type margins,

where water depths average a couple of hundred

meters. Passive margins do not mark plate boundaries but

rim most parts of many oceans, including the Atlantic and

Indian, and form around most of Antarctica and Australia.

Young immature passive margins are beginning to form

along the Red Sea.

Convergent, leading, or Pacific-type margins form at

convergent plate boundaries. They are characterized by active

deformation, seismicity, and volcanism, and some have thick

belts of rocks known as accretionary prisms that are scraped

off of a subducting plate and added to the overriding continental

plate. Convergent margins may have a deep-sea trench

up to seven miles (11 km) deep marking the boundary

between the continental and oceanic plates. These trenches

form where the oceanic plate is bending and plunging deep

into the mantle. Abundant folds and faults in the rocks characterize

convergent margins. Other convergent margins are

characterized by old eroded bedrock near the margin,

exposed by a process of sediment erosion where the edge of

the continent is eroded and drawn down into the trench.

A third type of continental margin forms along transform

or transcurrent plate boundaries. These are characterized

by abundant seismicity and deformation, and volcanism

is limited to certain restricted areas. Deformation along transform

margins tends to be divided into different types depending

on the orientation of bends in the main plate boundary

fault. Constraining bends form where the shape of the

boundary restricts motion on the fault and are characterized

by strong folding, faulting, and uplift. The Transverse Ranges

of southern California form a good example of a restraining

bend. Sedimentary basins and subsidence characterize bends

in the opposite direction, where the shape of the fault causes

extension in areas where parts of the fault diverge during

movement. Volcanic rocks form in some of these basins. The

Gulf of California and Salton trough have formed in areas of

extension along a transform margin in southern California.

See also CONVERGENT PLATE MARGIN PROCESSES; DIVERGENT

OR EXTENSIONAL BOUNDARIES; PLATE TECTONICS;

TRANSFORM PLATE MARGIN PROCESSES.

continental shield See SHIELD.

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