Jökull


Jökull - 01.12.1980, Side 58

Jökull - 01.12.1980, Side 58
clear that there has been dilation across the fissure swarms. But, by the very nature of the fissures grouping into swarms, this dilation is local and best explained by intrusions. And this is the model proposed in this paper, i.e. a magmatic intrusion, to which we will turn now. A proposed model Three possibilities regarding a magmatic intrusion come into consideration: (1) A vertical dyke (or dykes), (2) a horizontal sill (or sills), and (3) a combination of both. The last, i.e. a complex of dykes and sills, is very difficult to handle mathematically and test the results. I therefore decided to examine only models one and two, in order to find out whether a simple intrusive model could explain the Vogar fissure swarm. A vertical dyke (or dykes) First we consider the possibility that each fracture represents a dyke in which the magma failed to attain the surface. This model was proposed by Walker (1965) to ex- plain non-eruptive, postglacial fissures and faults in Iceland; it has also been proposed by e.g. Duffield (1975) for the fissure swarms in Hawaii. It is therefore worth while to examine this model in some detail. The main question is: Can the dykes give rise to the observed fissures and faults on the surface? Without wishing to maintain dog- matically that they cannot, I see some diffi- culties in this explanation. First: we have the condition for dyke formation: p><rH + T, (1) where p is the magma pressure, <rH is the horizontal stress (perpendicular to the dyke), and T is the tensile strength of the rock (per- pendicular to the dyke). If the dyke stops pro- pagating, as assumed in this hypothesis, then p<CH+T. The term T is of the order 100 bars, and it is difficult to see how it could be overcome by the non-propagating dyke. Tak- ing also into account that dykes do not create any stress on the free surface immediately above their upper ends (assuming a uniform pressure distribution) (Pollard and Holzhausen 1979). Second: it is by no means clear how the dykes are supposed to give rise to the faults; in particular the vertical, closed faults. Usually, one would expect a dyke to form a right angles to <r3, i.e. in a principal plane of stress. The distinctive feature of such a plane is that the shear stress upon it is zero. Hence, it cannot possibly become a fault plane. This conclusion is indeed confirmed by field observations; as a rule, dykes do not occupy faults, but fissures, “the walls of which have been merely prised apart” (Richey 1939). It would therefore be very surprising if all the faults in the Vogar fissure swarm were caused, hence occupied, by dykes. Next we consider the single dyke model. Such a model has e.g. been proposed by Koide and Bhattacharji (1975) for rift valleys. In this case, the tension fractures, faults and grabens are supposed to be the result of a general ten- sion, caused by a single dyke at depth com- parable to the width of the main graben. As for the Vogar fissure swarm, this model does not appear to be very promising. In the first place, it would be difficult to explain why many of the faults h'ave reverse inclination, and why the majority is closed. Both these factors indicate a more complex stress system than simple tension. Secondly, the irregularity of the fissure swarm, both in dilation and ver- tical displacement, is difficult to correlate to a single dyke. Thirdly, if the formation of the fractures has been going on for thousands of years — which is not clear in the case of the Vogar fissure swarm, but appears to be the case in some other swarms in Iceland (Björnsson et ai. 1977) — then a single dyke is of course ruled out as an explanation. The last dyke model we consider, is the one proposed by Pollard and Holzhausen (1979). In this model a dyke swarm is again the cause of the fissure swarm, but the dykes are not sup- posed to occupy the faults and fissures at the surface. On the contrary, “just over the dike, the ground is usually undisturbed, but to 56 JÖKULL 30. ÁR

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