Greinar (Vísindafélag Íslendinga) - 01.01.1977, Qupperneq 126

Greinar (Vísindafélag Íslendinga) - 01.01.1977, Qupperneq 126
124 mantle strength at depth, cases 1) and 3) most likely combine in explaining the undulations of the geoid. By the application of plasticity theory, it was concluded (93) that (when the Icelandic positive undulation was, for lack of the now available oceanic data, taken to be not much larger the country itself) under Iceland, the strength must be less than 10 kg/cm2. This would apply to the upper mantle and/or the Low velocity layer. The very extensive undula- tion of which Iceland is a part, could not exist by such a low strength. This seems to mean that causes of the undulations cannot be in the upper part of the mantle, but may in part be due to generally existing strength in the lower part, which would allow intermediate earthquakes. Deep sea trenches. These are in zones, where there certainly is strength down to 700 km. Referring to the picture given in (95) for the Tonga treneh, and considered to be typical for other such trenches, we have a high-Q bent tongue of thick- ness 50—100 km, which goes through a low-Q zone, reaching a depth of 200—300 km, to enter again high-Q material, ac- cording to present general results. The Benioff seismic zone is the upper surface of this high-Q tongue. The trench is at the upper end of the tongue, where the bending from the horizontal high-Q continuation of the tongue, i.e. from the lithosphere, is sharpest. As already pointed out, it is quite meaningless to try to tell the origin and history of this tongue on the basis of tempo- rary seismic data. On the other hand, the concurrence of the trench and its negative isostatic anomaly, with the sharp bend of a high-Q layer, is what first strikes the eye. It only depends on the strength of this high-Q layer, and the age of the trenches and the present stress in the bend, whether these quantitatively explain the trench, the gravity anomaly, and the deep earthquakes. As to the age of the trenches, it is just an ad hoc hypothesis in Plate Tectonics, that it is as high as some 150 My. We may not be far off the truth, if we assume that the
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Greinar (Vísindafélag Íslendinga)

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