Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 124

122
geny, the depression is filled with more or less consolidated
rocks, which are still lighter than the bordering crust, so that
the lateral thrust continues, but the effects of the thrust are
now wholly different.
For sake of simplification, we can first consider the geo-
synclinal rocks as a unity, keeping in mind, however, that at
depths of more than 5—10 km also these have some plasticity.
The result of the lateral thrust is now a general kneading of
the deeper parts, which causes both uplift, to form a mountain,
and the creation of a compensating root. There is no general
state of hydrostatic pressure in the uppermost mass, but ulti-
mately its weight must counterbalance the lateral overpressure
at depth, and stop the process of uplift entirely, when erosion
is left out of consideration.
Instead of this simplified case, the mountain building may
fall into a number of stages in space and time. To mention
an example, the border parts of a geosyncline might yield
easier than the central parts, so that the mountain chain be-
came double. There will, in reality, be a great number of varia-
tions, i.a. depending on the stage of consolidation or final
petrological and strength properties within the geosynclinal
mass.
Deep earthquákes and deep trenches. We come now to pheno-
mena which, as to distribution on the earth’s surface, are con-
nected with geosynclines and orogenic mountain chains.
First, there is the fact that medium deep earthquake foci,
down to about 250—300 km, are limited to such mountain
chains. When we consider that neither seismically nor iso-
statically do we see any reason to assume that these orogenic
bodies reach deeper than about 50 km, it is really strange that
breaking stress reaches depths of 300 km below them. Did
isostatic readjustment for these bodies ever reach such depths,
and can it still do so, tens of millions of years after orogeny,
even though we take erosion into account? The clue to this
problem may lie in the fact that seismically, shields retain
their characteristics down to 300—400 km (88), see below.