Iceland rollover structures and gravityglide

Iceland sits astride the N Atlantic spreading-ridge. Western Iceland belongs to the N American plate while E Iceland is part of the Eurasian plate. It must at once be admitted that this island is not completely typical of the submarine spreading-ridge. Nevertheless, because it has a significant, sub-areal outcrop, it provides important data regarding ridge-spreading. This island, which is considered to be situated above a plume, dominantly consists of lava flows, the oldest of which, found at the surface, date back to 14-5 Ma and, as will be seen from Figure 3.7, the 'Atlantic ridge' continues through the island, albeit along an offset (transform) track.

It was during a traverse of the recently opened ring road, in 1982, with Agust Gudmundsson, that we saw evidence that convinced us of the mode and mechanism by which plates moved and generated high-magnitude compressive stresses directed away from the spreading-ridge. Lava flows in Iceland are intermittently, but, on the geological timescale, frequently generated along the active volcanic zone shown in Figure 3.7. In this central area, the lava flows dip gently away to east or west, from the fissures that permit their development. However, as one travels eastward from the volcanic zone, and encounters older flows, one notes that the dip of the lavas becomes reversed and progressively increases, until, at the east coast, they dip westward by as much as 10°. A similar, but mirror image, change is to be seen in a traverse westward from the central zone. A schematic E-W section through the island is shown in Figure 3.8a.

We reached the conclusion that one could most readily explain the inward dip of the lava flows on either side of the volcanic zone as part of two roll-over structures, back to back. Roll-over structures are well known, and are often found in the soft sediments of a major delta, which dip sea-ward at about 1-3°. Instability occurs along particularly favoured bedding surfaces, or narrow weak zones, so that the upper units slide down-slope towards the bottom of the delta, where they may form folds and/or thrusts. Up-slope, the glide-mass becomes detached by movement along listric faults to form the attendant anticlinal roll-over structures (Figure 3.8b). On what plane could the Icelandic lavas have slipped? We suggested that, in the case of Iceland, slip took place on the LVZ at the base of the oceanic lithosphere. Beneath the volcanic zone in central Iceland the asthenosphere approaches within a few kilometres of the surface, but deepens relatively rapidly away from either side of the volcanic zone (Figure 3.8b).

The development of such a roll-over structure requires the stretc.hing and shearing deformation of the uppermost rocks in the structure. It may be suggested that such deformation would not easily be accommodated in basalt, a rock known to be strong and, therefore, not easily deformed. However, such a suggestion does not take into account the cooling fractures which develop in lava flows. These lava flows are often extruded onto earlier lava flows, the upper surface of which is usually slightly weathered. The extruded lava of the fresh flow cools rapidly to develop mainly vertical, open fractures which intersect, or abut, to form, in plan, a polygonal pattern. Such fractures will, of course, easily open further to accommodate stretching.

In such a decidedly wet environment as is afforded by Iceland's climate (and as would be present in any submarine environment) the open fractures soon permit the surfaces of the polygon to become weathered

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