Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 80

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ture of the chain, Fig. 11. A gradual thickening towards the
axis, of the 6.7 to 6.9 km/s layer above the Moho, demon-
strates that the layer was not bent downwards in the axis
just by depression under load. All these boundaries, from the
uppermost one at 10 km, to the Moho at 50 km depth, must
have been formed by some process after the orogeny, possibly
a long time after it. How can we believe that this smooth and
symmetrical Moho, sinking from a depth of 25 km at the
flanks to 50 km in the middle, is a chemical jump, especially
when the higher seismic boundaries cannot be? It must be a
jump to high-pressure polymorphs, and there are possibilities by
way of shear to support that conclusion. We shall discuss this
topic extensively in Chapter 6, but anticipate here that the Moho
means a change to dense, “high pressure” nanocrystals, which
automatically create a far higher local pressure around them-
selves than corresponds to the lithostatic pressure at their
depth. The Moho and its up and down movements during geo-
logical time, causing at least some known regressions and trans-
gressions, in full agreement with the theory of isostasy, will
be considered further in Chapters 6 to 9.
Having been led to the insight that many of the deeper seis-
mic layers form by physical changes, we can also get some
understanding of the fact that boundaries do, at least in some
known cases, move vertically during geological time, while we
leave for Chapter 9 the discussion of the changes of the Moho
at certain times:
Following the Hercynian in middle Europe, the Danish area
began to subside south of a fault running from the Norwegian
Trench, across Scania, and south of Bornholm. The subsidence
amounts to 5 km in Jutland, where sediments of Permian and
lower age fill up the depression. Here the “seismic” sial is as
thin as 4 km, whereas north of the fault it is about 14 km (55).
Seismically expressed, the Conrad boundary has moved up-
wards by 10 km under Jutland, whereas the Moho has a normal
depth. Here we can reasonably assume slow physical changes
under a load of 5 km of sediments and at least 4 km of “seis-
mic” sial, in an area that was earlier influenced by Hercynian