On crustal plates

1975 ◽  
Vol 65 (5) ◽  
pp. 1495-1500
Author(s):  
Don Tocher
Keyword(s):  
Plate Tectonics ◽  
Seismic Waves ◽  
Continental Drift ◽  
Active Regions ◽  
Continental Margins ◽  
Island Arcs ◽  
Low Velocity ◽  
Transform Faults ◽  
Sea Floor Spreading ◽  
Velocity Layer

Abstract During the decade just past, developments in Seismology have played an active and central role in the development of the concept of Plate Tectonics. Observational Seismology has provided support for and verification of a number of the dynamic aspects of the hypotheses of continental drift, sea-floor spreading, transform faults and the underthrusting of the lithosphere at island arcs and some continental margins. Those types of seismological evidence which bear on the question of the thickness of the lithosphere are either indirect or circumstantial, or both. As early as 1926, Gutenberg postulated the existence of a layer at a depth of 80 to 150 or 200 km, probably worldwide in extent, in which the velocities of seismic waves are slightly lower than in the immediately overlying layers. Some plate tectonics workers equate this low-velocity layer to the relatively-weak asthenosphere required by Plate Tectonics to underlie the stronger, more brittle lithosphere. In this review, several lines of evidence are marshalled in support of a plate model of the continental crust in seismically active regions in which a layer of decoupling of an upper, lithospheric layer from the weaker substrate may lie in the crust itself at a depth of perhaps 10 to 15 km.

Plate Tectonics Now and in the Past

2004 ◽  
Author(s):  
John J. W. Rogers ◽  
M. Santosh
Keyword(s):  
Plate Tectonics ◽  
Oceanic Lithosphere ◽  
Continental Margins ◽  
Island Arcs ◽  
Low Velocity ◽  
S Waves ◽  
Ocean Ridges ◽  
1960S And 1970S ◽  
Tectonic Contact ◽  
The 1960S

The concepts known as plate tectonics that began to develop in the 1960s built on a foundation of information that included: • The earth’s mantle is rigid enough to transmit seismic P and S waves, but it is mobile to long-term stresses. • The earth’s temperature gradient is so high that convective overturn must occur in the mantle. • The top of the mobile part of the mantle is a zone of relatively low velocity at depths of about 100 to 200 km. This zone separates an underlying asthenosphere from a rigid lithosphere, which includes rigid upper mantle and crust. • Seismic activity, commonly accompanied by volcanism, occurs along narrow, relatively linear, zones in oceans and along some continental margins. • The zones of instability surround large areas of comparative stability. • Ocean lithosphere is continually generated along mid-ocean ridges and destroyed where it descends under the margins of continents and island arcs. This causes oceans to become larger, but shrinkage of oceans can occur where lithosphere is destroyed around ocean margins faster than it is formed within the basin. • Some of the belts of instability are faults with lateral offsets of hundreds of kilometers. • Some continental margins are unstable (Pacific type), but others are attached to oceanic lithosphere without any apparent tectonic contact (Atlantic type). • Different areas containing continents and attached oceanic lithosphere move around the earth independently of each other. Most of this chapter consists of a summary of plate tectonics in the present earth, including processes along plate margins and the types of rocks formed there (readers who want more detailed information are referred to Rogers, 1993a; Kearey, 1996; and Condie, 1999). We also briefly discuss plumes and then finish with a word of caution about interpreting the history of the ancient and hotter earth with the principles of modern plate tectonics. Starting from the body of continually expanding information summarized above, numerous earth scientists in the 1960s and 1970s began to establish a conceptual framework that would organize scientific thinking about the earth’s tectonic processes. This required a new terminology, and it arrived rapidly (Oreskes, 2002). Geologists decided to call the stable areas “plates” and the unstable zones around them “plate margins.” Thus, the concept became known as “plate tectonics.” Plates are essentially broad regions of lithosphere, although the failure to detect low-velocity zones under many continents leaves unresolved questions.

scholarly journals A Geologist Reflects on a Long Career

2018 ◽  
Vol 46 (1) ◽  
pp. 1-20 ◽  
Author(s):  
Dan MKenzie
Keyword(s):  
Plate Tectonics ◽  
Continental Drift ◽  
Temperature Structure ◽  
Earth’S Gravity Field ◽  
Plate Motions ◽  
Sea Floor ◽  
Long Wavelength ◽  
Geological Processes ◽  
The Earth ◽  
Sea Floor Spreading

Fifty years ago Jason Morgan and I proposed what is now known as the theory of plate tectonics, which brought together the ideas of continental drift and sea floor spreading into what is probably their final form. I was twenty-five and had just finished my PhD. The success of the theory marked the beginning of a change of emphasis in the Earth sciences, which I have spent the rest of my career exploring. Previously geophysicists had principally been concerned with using ideas and techniques from physics to make measurements. But the success of plate tectonics showed that it could also be used to understand and model geological processes. This essay is concerned with a few such efforts in which I have been involved: determining the temperature structure and rheology of the oceanic and continental lithosphere, and with how mantle convection maintains the plate motions and the long-wavelength part of the Earth's gravity field. It is also concerned with how such research is supported.

Southeastern Atlantic Canada, Northwestern Africa, and Continental Drift

1971 ◽  
Vol 8 (10) ◽  
pp. 1218-1251 ◽  
Author(s):  
Paul E. Schenk
Keyword(s):  
North Atlantic ◽  
Plate Tectonics ◽  
Continental Drift ◽  
Continental Collision ◽  
Atlantic Canada ◽  
Island Arcs ◽  
Late Precambrian ◽  
The North ◽  
Northwestern Africa ◽  
Southeastern Atlantic

The model applies plate-tectonics to explain the geologic evolution of southeastern Atlantic Canada and northwestern Africa. The North Atlantic may have opened and closed several times from the middle Cryptozoic to the present. Closings of the ocean caused collisions between continents and also island arcs. Openings were ragged so that parts of one continent were transposed to the other, and sialic fragments became offshore micro-continents. Africa has progressively lost increments of continental crust to North America.Precambrian blocks of southeastern Atlantic Canada may be remnants of an African shelf. which was crumpled during a billion-year old continental collision (Grenville orogeny). After ragged rifting during the Late Precambrian these fragmentary blocks were carried eastward as micro-continents off Africa. Both early (Danakil Alps of the Red Sea) and late-stage (Canary Islands) recent analogues appear valid. The micro-continents ponded turbidites, which formed rise-complexes off Africa. Continental glaciations in the Late Precambrian and Late Ordovician not only make excellent inter-regional chronostratigraphic units in almost unfossiliferous strata. but also may confirm the African origin of Nova Scotia. Subducting plate-margins increased offshore volcanism and narrowed the Paleozoic Atlantic. Late Paleozoic continental collision again between Africa and North America sandwiched the micro-continent, telescoped the sedimentary/volcanic complexes, and flooded the sutured area with granodiorite. Middle Carboniferous carbonates and sulfates record vestiges of the Paleozoic Atlantic, and mixing of the Euro-African fauna with that of the western Paleozoic Atlantic of the northwestern Appalachians. The Atlantic was closed at least along the latitude of Atlantic Canada and Morocco. During the Mesozoic, an accreting margin uplifted this area, quickened redbed deposition and volcanism, initiated restricted marine sedimentation, and inaugurated the present North Atlantic east of the African remnant of southeastern Atlantic Canada.

All at Sea

2018 ◽  
Author(s):  
Roy Livermore
Keyword(s):  
Plate Tectonics ◽  
First Generation ◽  
Basaltic Magma ◽  
Ocean Floor ◽  
Tennis Ball ◽  
Magma Chambers ◽  
Transform Faults ◽  
Ocean Ridges ◽  
Sea Floor Spreading ◽  
Ocean Ridge

According to first-generation plate tectonics, sea-floor spreading was nice and simple. Plates were pulled apart at mid-ocean ridges, and weak mantle rocks rose to fill the gap and began to melt. The resulting basaltic magma ascended into the crust, where it ponded to form linear ‘infinite onion’ magma chambers beneath the mid-ocean tennis-ball seam. At frequent intervals, vertical sheets of magma rose from these chambers to the surface, where they erupted to form new ocean floor or solidified to form dykes, in the process acquiring a magnetization corresponding to the geomagnetic field at the time. Mid-ocean ridge axes were defined by rifted valleys and divided into segments by transform faults with offsets of tens to hundreds of kilometres, resulting in the staircase pattern seen on maps of the ocean floor. All mid-ocean ridges were thus essentially identical. Such a neat and elegant theory was bound to be undermined as new data were acquired in the oceans.

scholarly journals A harbinger of plate tectonics: a commentary on Bullard, Everett and Smith (1965) ‘The fit of the continents around the Atlantic’

2015 ◽  
Vol 373 (2039) ◽  
pp. 20140227 ◽  
Author(s):  
John F. Dewey
Keyword(s):  
Plate Tectonics ◽  
Torsional Rigidity ◽  
Continental Drift ◽  
Shear Zones ◽  
Continental Margins ◽  
Real Motion ◽  
Euler’S Theorem ◽  
The 1960S ◽  
Conjugate Margins ◽  
350Th Anniversary

In the 1960s, geology was transformed by the paradigm of plate tectonics. The 1965 paper of Bullard, Everett and Smith was a linking transition between the theories of continental drift and plate tectonics. They showed, conclusively, that the continents around the Atlantic were once contiguous and that the Atlantic Ocean had grown at rates of a few centimetres per year since the Early Jurassic, about 160 Ma. They achieved fits of the continental margins at the 500 fathom line (approx. 900 m), not the shorelines, by minimizing misfits between conjugate margins and finding axes, poles and angles of rotation, using Euler's theorem, that defined the unique single finite difference rotation that carried congruent continents from contiguity to their present positions, recognizing that the real motion may have been more complex around a number of finite motion poles. Critically, they were concerned only with kinematic reality and were not restricted by considerations of the mechanism by which continents split and oceans grow. Many of the defining features of plate tectonics were explicit or implicit in their reconstructions, such as the torsional rigidity of continents, Euler's theorem, closure of the Tethyan ocean(s), major continental margin shear zones, the rapid rotation of small continental blocks (Iberia) around nearby poles, the consequent opening of small wedge-shaped oceans (Bay of Biscay), and misfit overlaps (deltas and volcanic piles) and underlaps (stretched continental edges). This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society .

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