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Jökull - 01.12.1980, Qupperneq 57

Jökull - 01.12.1980, Qupperneq 57
1970, Tchalenko and Ambraseys 1970). In the case of the Vogar swarm, the angle between the underlying fault and the second order fractures is about 10°, according to T. Ein- arsson. A large horizontal component of movement is therefore to be expected. But no evidence of horizontal movement is found, although the vertical throw is up to 10 m (omitting fracture number 105) and the width up to 7.5 m. Horizontal movement should easily be demonstrated, if it existed, because of matching jags and notches on the fracture walls. Second: there is no reason to suppose that the crust beneath the peninsula meets the requirements for Riedel shears. In Riedel shears the different behaviour of the upper and lower part of' the strata is due to difference in ductility; the upper layers being more duc- tile than those below. These conditions are met where sediments overlie crystalline base- ment. But certainly not where brittle lavas alternate with hyaloclastites, down to at least 1800 m depth — as is the case on the Reykja- nes Peninsula. In the much quoted clay ex- periments, the ductile material is on the top, but in the present case it is vice versa; due to an extremely high heat gradient, the crust be- comes ductile at shallow depths (a íew km). Third: the depth to the assumed principal fault is only half the width of the proposed shear zone. On the surface, these shear zones are often less than 100 m wide. This would mean that the principal fault is only about 50 m below the surface, hence the surface frac- tures should not be deeper than this. Such a situation is, in my view, extremely unlikely as all the evidence indicates that the surface fractures are in fact much deeper than this. A transform fault. Many authors have pro- posed a left-handed transform fault running along the Reykjanes Peninsula (e.g. Ward et al. 1969, Scháfer 1972). Movements on this trans- form fault are supposed to give rise to the fissure swarms on the surface. The mechanism is therefore essentially analogous to the Riedel shears, discussed above, except that here only one deep-seated fault (the transform fault) is supposed to give rise to all the fissure swarms. Although Ward (1971) has modified the above picture in that he proposes many transform faults on the peninsula, most of the objections below also apply to his model. Excepting the last one, all the above arguments against the Riedel shear hypothesis also apply here. But there are also other ob- jections that especially apply to the single transform fault hypothesis. First: the great variety in mean orientation of the fractures, in different parts of the peninsula, does not agree well with a single transform fault as the cause. This variation amounts to 27° (Guðmundsson 1979). Second: the fractures are clearly grouped into definite fissure swarms, and es- sentially no fractures occur between them. If a single transform fault (or a wrench fault) caused the fracture formation, one would ex- pect thenr to be roughly evenly distributed along the deformation zone. This is, indeed, observed where sediments overlie an active wrench fault (Tchalenko and Ambraseys 1970), but is clearly not the case on the Reykjanes Peninsula. Third: clay experiments, set up to test this hypothesis, show it to be very doubt- ful. All the faults formed in these experiments “show a major strike-slip offset” (Courtillot et al. 1974). As said before, no evidence of horizon- tal movement has been found. An oblique spreading ridge. This hypothesis has been proposed by e.g. Nakamura (1970). The term “an oblique spreading ridge” is certainly not a very clear cut one, but the main points seem to be the follwing: “A N75E — trending sinistral strike-slip movement is implied below the surface”, and there is also “an opening component”. “The faulting, trending N75E, has the same amount of strike-slip and dilation components.” Regarding the proposed strike-slip move- ment below the surface, the before mentioned objections are repeated; the geology of the peninsula does not give any positive evidence of such a fault. Nor do I know of any evidence that the whole peninsula is dilating or spreading at a constant rate, e.g. 1 cm/year as is often quoted in the literature. It is of course JÖKULL 30. ÁR 55
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