Possible layering of mantle convection at the top of the Iceland Hotspot: a crosscheck between 3-D numerical models and gravimetric, seismic and petrological data
3-D numerical models describing convection cells that may develop in a buoyant layer over an upper-mantle hotspot are developed. Results are compared with those of an alternative model, which assumes an upper partially molten layer having the same density as the underlying mantle, but of much higher...
| Published in: | Geophysical Journal International |
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| Main Authors: | , , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
2012
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| Subjects: | |
| Online Access: | http://gji.oxfordjournals.org/cgi/content/short/188/1/35 https://doi.org/10.1111/j.1365-246X.2011.05250.x |
| Summary: | 3-D numerical models describing convection cells that may develop in a buoyant layer over an upper-mantle hotspot are developed. Results are compared with those of an alternative model, which assumes an upper partially molten layer having the same density as the underlying mantle, but of much higher viscosity. Geophysical and geochemical data over Iceland are used to assess the models. For the case of a buoyant upper layer, a major finding of our modelling is the prediction of a cold downward moving sheet in the upper layer situated just above the axis of the hot underlying mantle hotspot. When compared to geophysical data from Iceland, this modelled feature may explain why: (i) along the hotspot axis, seismic tomography reveals a mantle that remains cold down to ~80 km depth; (ii) the ridge crest systematically avoids the axis of the hotspot and (iii) the ridge axis is displaced by ~60 km to the east or the west of the hotspot axis, these ridge axes corresponding to the modelled upwellings of hot material in the uppermost layer driven by the cold downwelling axial sheet. Finally, the model also predicts development of gravity swells a few hundred kilometres long and several tens of kilometres wide, with a peak to valley amplitude of 5-10 mGal, lying over ascending sheets of upwelling material that float over the hotspot. Comparison with the residual gravity map over Iceland does indeed show the presence of a long gravity high along the active eastern section of the ridge. Over the AskjaVolcano, the long gravity high intersects an off-axis gravity high oriented northwest, which joins the Kolbeinsey Ridge. It is proposed that this branching may be the cause of the eastward drift of the western section of the Iceland ridge that took place 3 Ma. Further south, parallel to the same ridge-associated positive gravity anomaly, a more recent branching is found with a very long gravity high, oriented in an easterly direction, but that eventually contours the whole eastern coast of Iceland. This latter feature may be the ... |
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