Plate Tectonics

The vertical and lateral movement of the Earth's crust has been inferred on the basis of the folded and fractured character of outcrops of the earths crust, and directly observed wherever and whenever large earthquakes have been experienced. For this reason one of the traditional explanations of the creation of space filled by sediments has been the vertical tectonic movement; an observation that as long ago as the 1550's Leonardo de Vinci had used to explain fossils on mountaintop. However the tie of this vertical movement to continental drift and plate tectonics was not recognized properly till the early 1970's. Prior to this time, the state of New York's geologist James Hall had erroneously ascribed the fill of the Appalachian geosynclines to vertical movement alone. In 1916 Schuchert used synchronous marine sediment packages that onlapped the continent to produce one of the first relative sea-level charts. This tracked the percentage of the North American continent covered by the sea through the Phanerozic. He ascribed these sea-level changes to continental diastrophism. Stille (1924) also felt that these synchronous changes in sea level could only be understood in terms of synchronous movements of the continent, and in conjunction with Lotze from Berlin also constructed sea-level charts. Later Umbgrove in 1939 ascribed similar rhythms in the sedimentary section as a response to tectonic movement. The vertical tectonic model was changed with the advent of the plate tectonic model and the recognition of continental drift.

The initial realization that continental drift may have occurred began with the early exploration of the Atlantic three centuries ago. At this time Spanish and Portuguese navigators, including Abraham Ortelius, recognized the match between the west coast of Africa and the east coast of South America. Later Eduard Suess an Austrian geologist proposed in "The Origin of the Alps" (1857) that the coasts of Africa and South America matched and also wrote that horizontal movement of the lithosphere, rather than vertical uplift formed mountain ranges by folding and thrust faulting. In 1912 Alfred Wegener, a 32-year-old German meteorologist, supported this contention with his book Continental Drift suggesting that some 200 million years ago the supercontinent Pangaea split into Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. In addition to the match Wegener cited geologic features, terrestrial fossils and glacial sediments common to both continental margins as evidence of continental drift. Geologists like DuToit, who worked in the Southern Hemisphere and the Indian subcontinent supported Wegener's position, however it was largely dismissed in Western Europe and the USA because Wegener's theory lacked a plausible mechanism to explain how the continents to moved (Dott and Batton 1976). However later paleo-magnetic data was found that also supported the drifting of the continents and the breakup of Pangaea (Dott and Batton 1976).

In 1928Arthur Holmes (Dott and Batton 1976), despite Harold Jeffrey's objections to the contrary, argued for Wegener suggesting that convection in the Earths mantle was the mechanism that caused the drift of the continents. Harry Hess (1962) of Princeton and Bob Dietz, of the U.S. Coast and Geodetic were supporters of Wegener and Holmes and suggested that seafloor spreading was responsible for the continental drift. Hess proposed that as oceanic crust spread away from the ridges with a conveyor belt-like motion it was simultaneously descending and being consumed at the oceanic trenches. Starting in the 1950s magnetometers were used in the Atlantic to map parallel patterns of magnetic on the ocean floor that had a zebra-like pattern. Vine and Matthews in 1963 proposed that the alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. On the Leg 3 of the Deep Sea Drilling Project Glomar Challenger, 1968, drilled 17 holes at 10 different sites along a oceanic ridge between South America and Africa established that the sediments of the Atlantic were youngest close to the mid Atlantic Ridge and were older closer to the continental margins proving that the seafloor spreading hypothesis was correct. In 1965, J. Tuzo Wilson described transform faults that connected areas of ocean spreading and subduction, so initiating the new geologic discipline of Plate Tectonics.

Subsequently it was established that there are four types of plate boundaries:

Divergent boundaries Where new crust is generated as the plates pull away from each other
Convergent boundaries Where crust is destroyed as one plate dives under another
Transform boundaries Where crust is neither produced nor destroyed as the plates slide horizontally past each other

The tectonic behavior of these plate boundaries is now being used to partially explain the cyclicity of the sedimentary record. Cloetingh (1988) believes that the major changes in base level that occurred through the Mesozoic and Tertiary were related to extensive movement along the transform faults that dissect the oceanic crust and whose movement is connected to oceanic ridges spreading.

Tuesday, April 07, 2015
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