Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 160

Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 160
156 sediments of the shelf SW of Reykjanes — these lack natural ther- mo-remanent magnetization — and why the elongate anomalies on the Reykjanes peninsula are quite different from those on the sub- marine ridge (26). Two further points of general natin-e should be mentioned in this connection. First, the magnetic picture of the Reykjanes penin- sula (26) strongly suggests that the late volcanic activity plays directly a role, whether or not tectonic activity per se also enters the picture. Volcanism may, therefore, play a role on submarine ridges also. How large a role, is a more difficult question. First, it very much depends on the accuracy of the picture of the submarine anomalies to what degree the magnetization of primary volcanic bodies can be separated from the picture due to alteration or fault- ing or structural boundaries. Secondly, where anomalies are very broad, the magnetic body may be relatively deep or thick, and the faulting beneath the younger sediments must also be considered. The distal anomalies of the ridge (21, Fig. 2a) are much broader than those of the narrow axial zone. Fractures in the former region would almost certainly be much older than those of the narrow axial zone and the lines of alteration might be correspondingly broader. But the difference has probably also a deeper root: the two types of anomaly are of different categories. The broad and very long ones could be intimately connected with such a phase of areal faulting as is evidenced by the Second Main Tectonic Phase in Iceland (11, 9), during which most of the country was uplifted in blocks, while the Tertiary plateau basalts subsided around the country, especially clearly south of eastem Iceland, i.e. in the oceanic lane in which the broad magnetic anomalies are so clear. The subsidence here seems to be indicated by the sinking of the seis- mic Layer 3 in Iceland from 4 km to 10 km depth (27, Fig. 37). In other words, the narrow ridge-anomalies are a category belong- ing to a late phase of the zonal activity which set in after the Second Main Tectonic Phase, whereas the distal anomalies are a category belonging to an areal tectonic phase, perhaps that of the Second Tectonic Phase. In that category the anomalies are most likely to be due to faults in the basalts, hidden morphologically under a veneer of marine sediments. And when we look at the mosaic of anomalies in the intermediate field between the axial
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Greinar (Vísindafélag Íslendinga)

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