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The large-wavelength deformations of the lithosphere: Materials for a history of the evolution of thought from the earliest times to plate tectonics

By
A.M.C Şengör
A.M.C Şengör
İ TÜ Maden Fakültesi, Jeoloji Bölümü, and Avrasya Yerbilimleri Enstitüsü, Ayazağa 34469, İstanbul, Turkey
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Published:
April 01, 2009

Today, the geodynamic deformations of the lithosphere manifest themselves in two main categories: structures of small wavelength and structures of large wavelength—“wavelength of structure” being defined as the distance between two amplitude crests of cogenetic structures belonging to the characteristic size category within a field of deformation. I call structures of small wavelength copeogenic (because they cut the lithosphere) and the structures of long wavelength falcogenic (because they bend the lithosphere). This book traces the rise of the awareness of long-wavelength structures with the objective of understanding their essential features.

The subdued expression and enormous size of long-wavelength structures have been joint impediments to the recognition and the understanding of their nature, yet many have known of their existence from the earliest times—mainly on the basis of observations of sea-level change. Change of level has been inferred so early that the origin of this inference is lost among mythic speculations. Vertical motions of the rocky surface with respect to a reference fixed to the earth have been much harder to recognize because of the difficulty of finding an appropriate point of reference and the selection of gauges showing distance to that point of reference in the past. The earliest explanatory models were based on observations that, in some areas, land was actively gaining on the sea and that in others in the past, some of the present land areas had been covered by marine waters, as shown by fossils.

These early models involve now long-abandoned mechanisms invented from few and disconnected observations, but they helped to make a clear distinction between structures of small wavelength and structures of large wavelength. It was already implicitly understood that the former could be investigated on a scale ranging from single outcrops to individual mountains, whereas the study of the latter necessitated a regional approach. Small-wavelength structures were thought to form quickly, even catastrophically. Large-wavelength structures seemed to evolve slowly, but belief in such legends as Atlantis, the continent that allegedly had become submerged in one day and night, blurred the picture for a long time. Distinctions based on size, geometry, and timing of evolution remained disputed as long as means of observation of large-wavelength structures remained inadequate. Only with the development of bio-stratigraphy in the late eighteenth century and of geomorphological methods of slope investigation in the early twentieth century was the presence of large-wavelength structures eventually recognized beyond doubt. These methods have also helped us understand their evolution. In particular, the detailed topographic investigations carried out in the United States west of the Mississippi River since the beginning of the nineteenth century made the presence of large-wavelength structures indisputable. When those topographic data became combined with geological investigations from the middle of the nineteenth century, it became obvious that older inferences concerning the relative rate at which such immense structures grow were correct.

However, that understanding was complete only after the period of development of geophysical methods to investigate what underlies long-wavelength structures. The latter part of that developmental period included the recognition of the mantle-plume generated uplifts. Between 1800 and 1960, geologists tried to accommodate the large-wavelength structures within the framework of all-encompassing global tectonic theories that were not nearly detailed enough for the purpose. Plate tectonics provided for the first time a comprehensive and detailed theory into the framework of which J. Tuzo Wilson placed his hypothesis of mantle plumes. It is now clear that mechanical loading, thermal changes in the mantle, and intracrustal flow events dominate the origin and evolution of long wavelength structures. Mantle plumes are the most significant non-plate-boundary generators of long-wavelength structures, and it is these structures and the fills of the associated lithogenetic environments that constitute their most faithful record.

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Contents

GSA Memoirs

The Large-Wavelength Deformations of the Lithosphere: Materials for a History of the Evolution of Thought from the Earliest Times to Plate Tectonics

Geological Society of America
Volume
196
ISBN print:
9780813711966
Publication date:
April 01, 2009

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