The accreting plate boundary Ardoukoˆba Rift (northeast Africa) and the oceanic Rift Valley
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Rift zone
Rift valley
Seafloor Spreading
Plate tectonics caused a revolution within earth sciences which then was transposed into science textbooks. The main objective of this paper is to explore how plate tectonics influenced Portuguese and Spanish science textbooks published from the 1960s through the 1980s. For this purpose, a qualitative method based on the concept of didactic transposition is used. The didactic transposition of seafloor spreading evidence such as ridges, rifts and trenches, transform faults, seafloor sediments, the age of seafloor basaltic rocks, the magnetic anomalies on the seafloor, the Benioff zones and the subduction process, and also the didactic transposition of the formation of mountains ranges and island arcs, convection currents, plate tectonics concepts, boundaries and motion, and plate tectonics acceptance are studied in a comprehensive sample of science textbooks. The analysis of textbooks shows that the didactic transposition of seafloor spreading, and plate tectonics started mainly in 1970s Portuguese and Spanish textbooks and had a strong development in 1980s textbooks. No major differences were found between the approaches to plate tectonics in similar age Portuguese and Spanish textbooks. At the beginning of the 1970s, textbooks presented partial evidence for seafloor spreading, such as magnetic anomalies and the characteristics of ridges, rifts and trenches. They also addressed convection currents but only those that were related to geosynclines. In the mid 1970s and in the 1980s, textbooks presented more comprehensive evidence of seafloor spreading, by adding didactical transpositions of transform faults, seafloor sediments and the age of seafloor rocks. They also presented in more detail topics such as magnetic anomalies, the Benioff zones, orogenic processes and the tectonic significance of ridges, rifts and trenches. Plate tectonic theory was presented in major textbooks as widely accepted, and discussions about speculative facts or processes were rare.
Seafloor Spreading
Rift valley
Continental drift
Transform fault
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There existed some blocks (micro-plates) in the oceans between Australia and Asia in the Cenozoic, when some blocks were separated from the Australian plate and moved northward and collided and sutured with some blocks that were separated from the Eurasian plate. In this period small ocean basins such as the South China Sea, Sulu Sea, Celebes Sea and Andaman Sea formed as a result of block separation and seafloor spreading, and finally the present tectonic framework formed in the Great South China Sea area. After a study of the Cenozoic tectonic history of the Great South China Sea area, the authors believe that Cenozoic tectonic activities in the Great South China Sea were not only related to collision between the Indian Plate and Eurasian Plate but also to subduction of the Pacific Plate beneath the Eurasian Plate and were also affected by the northward movement of the Australian. Plate. In the South China Sea Basin there occurred two events of seafloor spreading in the Cenozoic. The first seafloor spreading, which was oriented in a NW-SE direction, occurred before 42-35 Ma BP under the influence of the southeastward mantle flow beneath the Eurasian continent caused by India-Eurasia collision. The first seafloor spreading gave rise to the Southwest Basin of the South China Sea. The second seafloor spreading took place before 32-17 Ma BP. As the Pacific plate was subducted beneath the Eurasian plate to 700 km depth, the SE-directed flow of the upper mantle of the Eurasian continent was blocked and then turned toward the south, thus causing N-S-trending seafloor spreading in the South China Sea area, i.e. the second seafloor spreading. The second seafloor spreading resulted in the formation of the Central Basin of the South China Sea. After the Cenozoic South China Sea Basin was produced, collision between the blocks and seafloor spreading continued in the Great South China Sea area, and under the compression of these northward blocks the south margin of the South China Sea, sediments in the area were deformed, thus producing the Wanan movement (at about 10 Ma BP) on the south margin of the South China Sea.
Seafloor Spreading
Eurasian Plate
Pacific Plate
North American Plate
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Seafloor Spreading
Continental drift
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The age of the ocean floor and its time-dependent age distribution control fundamental features of the Earth, such as bathymetry, sea level and mantle heat loss. Recently, the development of increasingly sophisticated reconstructions of past plate motions has provided models for plate kinematics and plate boundary evolution back in geological time. These models implicitly include the information necessary to determine the age of ocean floor that has since been lost to subduction. However, due to the lack of an automated and efficient method for generating global seafloor age grids, many tectonic models, most notably those extending back into the Paleozoic, are published without an accompanying set of age models for oceanic lithosphere. Here we present an automatic, tracer-based algorithm that generates seafloor age grids from global plate tectonic reconstructions with defined plate boundaries. Our method enables us to produce the first seafloor age models for the Paleozoic's lost ocean basins. Estimated changes in sea level based on bathymetry inferred from our new age grids show good agreement with sea level record estimations from proxies, providing a possible explanation for the peak in sea level during the assembly phase of Pangea. This demonstrates how our seafloor age models can be directly compared with observables from the geologic record that extend further back in time than the constraints from preserved seafloor. Thus, our new algorithm may also aid the further development of plate tectonic reconstructions by strengthening the links between geological observations and tectonic reconstructions of deeper time.
Seafloor Spreading
Tectonophysics
Oceanic basin
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Seafloor Spreading
Rift valley
Ridge push
Transform fault
Mid-Atlantic Ridge
Classification of discontinuities
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The advent of the plate tectonics theory ~ 50 years ago has revolutionized Earth Science thinking and provided a solid framework for understanding how the Earth works. The observations and statistics by Forsyth & Uyeda (1975) showed the subducting slab pull to be the primary force driving seafloor spreading. This driving force readily explains the Pacific type seafloor spreading connected to subduction zones but is not straightforward to explain the Atlantic type seafloor spreading and continental drift. This has led to the general perception that “we still don’t know what drives plate tectonics
Seafloor Spreading
Continental drift
Solid earth
Slab
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Abstract The history of seafloor spreading in the ocean basins provides a detailed record of relative motions between Earth's tectonic plates since Pangea breakup. Determining how tectonic plates have moved relative to the Earth's deep interior is more challenging. Recent studies of contemporary plate motions have demonstrated links between relative plate motion and absolute plate motion (APM), and with seismic anisotropy in the upper mantle. Here we explore the link between spreading directions and APM since the Early Cretaceous. We find a significant alignment between APM and spreading directions at mid‐ocean ridges; however, the degree of alignment is influenced by geodynamic setting, and is strongest for mid‐Atlantic spreading ridges between plates that are not directly influenced by time‐varying slab pull. In the Pacific, significant mismatches between spreading and APM direction may relate to a major plate‐mantle reorganization. We conclude that spreading fabric can be used to improve models of APM.
Seafloor Spreading
Pacific Plate
Slab
North American Plate
Relative Motion
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The segmentation pattern of 750 km long Central Indian Ridge (CIR) between 3 o S and 11 o S latitudes in the Indian Ocean has been studied using multibeam bathymetry and magnetic data. Twelve ridge segments were identified that are separated by well defined transform faults and non-transform discontinuities. Magnetic model studies qualify the ridge as a slow spreading ridge with average full spreading rates varying from 26 to 38 mm/yr. The disposition of the magnetic anomalies suggests that the plate opening direction has not changed during the last 0-4 Ma period. Along axis variations in the magnetic anomalies, presence of axial volcanic ridges on the inner valley floor, variations in the depth and geometry of the rift valley, suggest distinct variations in the accretionary processes along the ridge. Based on these characteristics and the segmentation pattern, we suggest that ten ridge segments are undergoing less magmatic phase of extension, while two segments have shown characteristics of magmatic accretion. The linear segments with narrow and shallow rift valley floor and near symmetric magnetic anomalies are identified as segments with magmatic accretion. The influence of diffuse plate boundary zone on the young seafloor fabric generated by the CIR has been examined. The seafloor topography different from the normal ridge parallel fabric observed at few places over the NE flank of the CIR is suggested to be the consequence of gradual and progressive influence of the distributed diffuse plate boundary (between the Indian-Capricorn plates), on the newly generated oceanic lithosphere. Further, we documented distinct ridge-transform intersection (RTI) highs, three of these RTI highs are identified as oceanic core complexes / megamullion structures. Megamullion structures are found to be associated with less magmatic sections. Increased seismicity has been observed at the less magmatic segment ends suggesting the predominance of tectonic extension prevalent at the sparsely magmatic sections.
Seafloor Spreading
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Ridge push
Mid-Atlantic Ridge
Transform fault
Classification of discontinuities
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In a communication to the Geological Magazine , published in July, 1936, I gave a brief description of the rocks that build the Kedong scarp. I propose now to amplify that description. It is not always realized that the geological structure of the Rift Valley has been deduced almost wholly from physiographic evidence, and that proof of the alleged displacements must depend upon exact petrographic study of the rocks involved in the structures.
Rift valley
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Seafloor Spreading
Rift valley
Mid-Atlantic Ridge
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