Jökull


Jökull - 01.12.1980, Page 62

Jökull - 01.12.1980, Page 62
neither necessary nor the most simple explan- ation. Whether or not the model is the true explanation, it is nevertheless interesting to discuss the various objections that could be offered against it. Some of these will be dis- cussed below. (1) It has often been stated that a condition for the formation of an horizontal intrusion is that the minimum principal stress must be vertical. According to focal mechanism solutions, the minimum principal stress is ex- pected to be horizontal and not vertical in the crust of the Reykjanes Peninsula (Klein et al. 1977). The answer to this objection is simple: the statement is not true. In the first place, it does not take into account the importance of horizontal weakness in the strata. Such weakness, with essentially no tensile stress across it, is very important in deciding the form of an intrusion (see e.g. Pollard 1973, Mudge 1968). Secondly, the magma density may be higher than that of the rock, and the statement ignores the possible effects of vapor pressure — both of these have been discussed above. Thirdly, during downwarp and burial of undeformed sediments the vertical stress is higher than the horizontal stresses at all depths (Price 1974). But sills are common in sedimentary basins, and it is therefore likely that some of them were intruded while the maximum principal stress was vertical. (2) In the model, the magma applies a constant pressure perpendicular to the con- tact, and the shear stress is assumed to be zero. It could be pointed out, that we are dealing with a flowing magrna, and the above assumptions are therefore not true. Admit- tedly, in this case shear stress will exist and the normal pressure will decrease in direction of flow; due to viscous drag along the contact. However, Pollard (1973) has examined these factors and concludes that they have negli- gible effects on both form and stresses around the intrusion. (3) The throw is up to 20 m and the maximum subsidence is 50 m in the Vogar fissure swarm. This seems to be difficult to explain by a 9.5 m thick intrusion. But the model is only believed to have originated the fractures, and not being responsible for the subsequent subsidence. The subsidence, which is a common feature in the neovolcanic zone in Iceland, is probably due to various factors. Some of these factors might be: (i) Accumulation of high density lavas above the low density magma source. The crust, and the upper mantle above the source, would have a tendency to sink into it; owing to their higher density and also because of occasional press- ure relief in the magma source — during and after big eruptions and/or intrusions. (ii) Continuous melting of the rock next to (above) the magma source. In both the above cases the subsidence would be easiest where big fractures already existed; namely inside the fissure swarms. (iii) It should be noted that the measured subsidence is only relative, i.e. only referred to the flanks of the fissure swarms. But recent data indicate, that at least part of the measured subsidence in such swarms in Iceland is due to uplift of the flanks and not an absolute sinkage of the area bet- ween the flanks (Björnsson et al. 1978). (4) As said before, the faults are usually vertical or slightly inclined in the reverse trend. This is difficult to explain by a single sill. However, the process outlined by Bradley (1965), in which normal faults, with the above inclination, are formed ahead of the intruding sill, could explain at least part of the observed fracture pattern. But some of the fractures, near the west end of the Vogar fissure swarm, do cut rocks of different age, which makes a single intrusion unlikely (although not im- possible, as the sill could be younger than the youngest lavas at the surface). Without wishing to go into detail, I suggest the real situation could be similar to that outlined in Fig. 16. In this model each sill is supposed to give rise to a horst; and the reverse and vertical normal faults are initiated near the ends of the sill and propagate upwards to the surface. As before, the sills are not sup- posed to be responsible for the subsequent subsidence on the fissure swarm, but only to originate the fractures. Although I do not at 60 JÖKULL 30. ÁR

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