Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 78

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ELEVATION (km)
DISTANCE FROM SHORELINE (km)
Fig. 10. Seismic boundaries in Sierra Nevada. (After (56)).
The result of such an expulsion would be strengthening of
the respective mineral, a slight densification, but especially an
increase in rigidity and the bulk modulus, which means increase
in the seismic velocities. But such a process of expulsion of
dislocations would in general not be identical with a change
to a denser polymorph.
Other effects on seismic velocity may be considered, such
as decrease in porosity and possible changes of mineral equi-
librium (53, 54). But it now seems to the present author that
expulsion of dislocations may probably give the sharpest seis-
mic boundaries before polymorphism comes into consideration.
These remarks may then open the way to the understanding
of more seismic boundaries than that of Layer 3.
Such boundaries are, in significant cases, mainly a function
of depth, rather than of rock type. This is shown by horizontal
boundaries right through the roots of folded mountains like
the Alps (55). A careful study of the Sierra Nevada (56)
shows remarkably regular symmetric boundaries, Fig. 10, the
uppermost one (6.0 to 6.4 km/s) going horizontally through
the vertically layered granitic axial zone of the mountain
chain. And the boundaries ignore the otherwise complex struc-
Fig. 11. Reconstruction of the structural stages of the Sierra Nevada.
(After (56)).