A study of the palaeomagnetism of subglacial basalts, SW Iceland: a comparison with oceanic crust
Subglacial volcanic sequences in Iceland were used as analogues of oceanic crust to get an indication of the degree of primary heterogeneity of magnetic directions that can be expected within subaqueous volcanic sequences erupted in both deep and shallow water. In the Icelandic sequences the degree...
| Published in: | Geophysical Journal International |
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| Main Authors: | , , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1990
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| Subjects: | |
| Online Access: | http://gji.oxfordjournals.org/cgi/content/short/103/1/13 https://doi.org/10.1111/j.1365-246X.1990.tb01748.x |
| Summary: | Subglacial volcanic sequences in Iceland were used as analogues of oceanic crust to get an indication of the degree of primary heterogeneity of magnetic directions that can be expected within subaqueous volcanic sequences erupted in both deep and shallow water. In the Icelandic sequences the degree of tilt, brecciation and the amount of hyaloclastite are known so it is possible to test for primary magnetic heterogeneity. We measured magnetic directions of 119 oriented core samples (2.5 cm diameter) collected from five sites in SW Iceland where there is good exposure in Quaternary subglacial volcanic sequences. Sample sites vary from compact pillowed flows (most analogous to deep water volcanic sequences) to pillow breccia with up to 40 per cent interpillow hyaloclastite (more analogous to shallow water sequences). Magnetic directions of multiple samples from 16 individual pillows have angular differences of 2°–66° with most being less than 10°. Of the five sites sampled, two sites have inclinations that approximate that of the centred axial dipole, one site has steeper inclinations and two sites have shallower inclinations. When the site averages of magnetic direction with their circles of α 95 are plotted, there is little overlap. There is little correlation between either the scatter of magnetic measurements or the percentage of non-dipolar inclinations and the occurrence of pillow breccias, hyaloclastite or depositional dips. We doubt the tectonic significance of small amounts of tilt (<30°) estimated from drill cores on the basis of palaeomagnetic inclination as it is not uncommon for rocks to rotate by as much as 30° after they cool through the Curie temperature or for such a scatter to be the result of secular variation. With this in mind, the same caution must be applied when orienting seafloor basalts for palaeomagnetic or structural studies using geopetal features such as drain-away ledges or segregation vesicles, because the rocks may rotate after these features form (at 1200°-1000°C) but before the ... |
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