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The Development of Plate Tectonic Theory
Prior to the advent of Plate Tectonics, geological phenomena were explained in terms of what was known as 'Geosynclinal Theory'. It was held that sedimentary rocks were deposited in linear intra-continental depressions, known as 'Geosynclines', and that mountains were formed by subsequent inversion of the buried sedimentary material to form 'Orogens'. In North America, the Appalachian and Cordilleran geosynclines are located along the margins of the North American continent, and are separated from one another by an area of largely undeformed sedimentary rock laid down in epeiric seas that episodically flooded the tectonically stable continental interior. The latter, in contradistinction to the marginal geosynclines, is referred to as the North American 'craton'.
Alpine and Appalachian geosynclines.
In contrast to the Appalachian
and Cordilleran geosynclines of North America, the European Alpine Geosyncline
appears to be located within rather than at the margins of a continental
area. Nevertheless, in both cases, the geosynclines were postulated
to be formed of two contiguous but contrasting belts of rock - 1)
a miogeosynclinal belt composed of folded and thrust shallow water sediments
(cross bedded sandstones, limestones) of essentially the same kind,
but thicker, as those deposited in the epeiric seas of the cratonic interior;
and 2) an eugeosynclinal belt characterised by the presence of deep
water sediments (graded bedding), basaltic volcanics rocks, gabbros, serpentinites
(altered peridotites), and granites. The basaltic rocks, gabbros, and serpentinites
were collectively referred to as 'OPHIOLITES',
and the deep water sediments as 'FLYSCH'.
In the Appalachian and Cordilleran systems, the
mio- eu-geosynclinal 'couple' were arranged such that the eugeosynclinal
belt was located marginal to the ocean, and separated from the cratonic
INTERIOR by the miogeosyncline. However, in the Alpine system, the geosyncline
was composed of two mio-eu-geosynclinal 'couples', arranged such that each
couple was a mirror image of the other. The line of symmetry separating
the two couples was termed the 'zwischengberge'
(German for 'between the mountains'), and the miogeosyncline separated
the eugeosyncline from the continental EXTERIOR.
The progressive sequence of events involved in the
evolution of the geosyncline involved:
formation of the linear miogeosynclinal and eugeosynclinal
depressions;
infilling of the depressions with 'flysch' sediments
contemporary with the effusion of ophiolitic rocks along the internal margins
of the eugeosyncline;
deformation of the eugosyncline and progressive migration
of the deformation from the eugeosyncline to the miogeosyncline;
exhumation of the deformed geosyncline to form a
mountain system;
formation of late stage mollasse basins around the
edges and within the rising mountain system.
This concept of orogenic development based on studies of the Alpine system evolved during the 1950's. However at roughly the same time a different concept that came later to be known as Plate Tectonic Theory began its development with the ideas of H.H. (Harry) Hess concerning the nature and origin of oceanic crust. Before the second world war Hess was a geophysicist whose interest was in the oceanic gravity anomalies associated with island arcs. As a submarine commander during the war however, he was able to make many observations concerning the morphology and composition of the ocean floor. Two of his most important observations were also the simplest: he noted that all rocks underlying the relatively thin veneer of recent sediments forming the floors of oceans were invariably mafic in composition, that is, there were no granites or continental schists and gneisses; and secondly, that there appeared to be no very old rocks in the oceans. Following the further demonstration that mid-ocean ridges were topographically elevated because they were the locii of high heat flow and volcanism, rather than because they were formed of relatively low density crustal rocks, Hess proposed that oceanic crust was formed at mid-ocean ridges, and that consequently modern oceans grew by a process of sea-floor spreading. Hess's proposal was entirely compatible with the idea of Continental Drift proposed by Alfred Wegener. In fact Hess's proposal was essentially a restatement of Wegener`s much earlier claim that "the Mid Atlantic Ridge should be regarded as the zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima (rising) from depth." (Wegener, 1912, p.305). The concept was immediately 'marketed' by Dietz (1963) in explanation of the formation of continental geosynclines, where miogeosynclines were held to represent the shallow-marine environment of continental margins, and the eugeosynclines the deformed and metamorphosed equivalents (flysch) of the deep-water continental slope, rise and abyssal plane. The geosynclinal ophiolites were considered to be pieces of oceanic crust physically incorporated into the slope and rise sequences during deformation provoked by sea-floor spreading, and granites the result of melting of the slope and rise sediments as a result of the same actions.
Dietz's theory of geosynclinal development.
The concept of sea-floor spreading found spectacular support in the discovery of:
ocean floor magnetic striping (Vine and Mathews)- the record of the Earth's magnetic field 'frozen' into magnetite crystallizing in quasi steady-state basalt effusions at mid-ocean ridges;
transform faults (Wilson; Sykes) - seismic activity only in the segment of the transform fault located between the oceanic ridges;
sea- floor subduction (Oliver and Isacks) - the concept of high and low Q materials, allowing the transmission of higher P wave and lower P wave velocities, respectively, and the seismic detection of subducted high Q oceanic crust beneath the Tongan volcanic arc;
and the Deep Sea Drilling Project - proof that the world's oceans were all composed of relatively young oceanic crust.
The Deep-Sea Drilling Program - Pacific region.
Acceptance of the concept led to the postulate that the Earth's surface could be divided into a set of plates with boundaries marked by ocean ridges, subduction zones and tranform faults, and also to the general law that the rate of formation of oceanic crust at oceanic ridges must balance the rate of consumption of oceanic crust at subduction zones, and that most if not all tectonic phenomena on the surface of the Earth is the result of `plate interaction`.
Geosynclines' were therefore reinterpreted to represent amalgamated remnants of continental margin sedimentary/volcanic sequences and oceanic crust (with the latter represented by the ophiolites of geosynclines) whereas 'orogens' were considered to result from collision of continents following ocean closure. But if orogens are the result of collision, what collisions caused the formation of the Appalachian and Cordilleran 'marginal' orogenic systems?
The Plate Tectonics Creed:
The basement rocks of the oceans are relatively
young, and invariably of mafic composition.
Sea floor spreading is proven by the magnetic
characteristics of the oceans, and the special character of transform
faults.
Oceanic crust is formed at mid-ocean ridges and
consumed at subduction zones; the average rate of consumption equals the
average rate of production.
The Earth's surface is made up of a limited number
of 'plates', and the boundaries to the plates are ocean ridges, subduction
zones, and transform faults.
Most geologic phenomena are related to processes
operating along plate boundaries.
Overhead sequence:
Alpine and Appalachian geosynclines (13alps.gif)
Divergent bicouple (06geosyn.gif)
Dietz - miogeoclines (06Dietz.gif)
Vine and Mathews - paleomagnetism (06magsei.gif)
Wilson - transform faults (06magsei.gif)
Oliver and Isacks - subduction (06magsei.gif)
DSDP (06dsdp.gif)
Morgan - plates; active and passive margins (06wldmap.gif)
Dewey - conservation of surface area; bilateral symmetry
of collision zones. (06creed.gif)
Essay Questions:
1) Explain how Geosynclinal Theory evolved into Plate
Tectonic Theory.
2) How would you explain the concept of 'geosynclines'
in terms of Plate Tectonic Theory?
3) Describe the steps in the confirmation of Plate
Tectonic Theory.
FIGURES:
Structural Provinces of North America.
Alpine and Appalachian geosynclines.
Development of an Alpine geosynclines.
Sea-floor spreading and orogenic models of Dietz.
The Deep-Sea Drilling Program - Pacific region.
Figure 17. Conservation of surface area.
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