Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 132

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a) y
Fig. 6: Illustration of the theoretical rules in a bi-axial stress field with oi and 02
as the principal compressive stresses. These stresses can always be considered as
a superposition of the principal stresses s and -s on a hydro- (i.e. litho-) static
pressure 00. Case a) 01 = 00 -|- s; 02 = 00 -s. The maximum shear stress is x
rightlateral on the sides AB and CD of the considered element, left-lateral on
AD and BC. In case of yield, the conjugate shear fractures (which are the very
type of yield in ductile elastic material, such as may be expected below an upper-
most skin of the earth’s crust) make an acute a = 70-90°, bisected by the larger
compressive principal stress, such as is illustrated by the rhomb AB1CD1. Case
b) 01 = 00 -s, 02 = 00 + s. We only point out the fact that the shear on respec-
tive lines has changed from right-lateral to left-lateral, and vice versa. For 00 =
5 kbar, or a crustal depth of about 15 km, say, we need about s < 1 kbar to
cause shear fractures. For tensional fractures at this depth, about s = 4 kbar would
be necessary, but even in that case a tensional fracture would not form at all, as
we are dealing with ductile material. Crustal fractures reaching down to or being
formed at depths of over 5-10 km, say, cannot possibly be tensional by the very
nature of the material, to leave out other obstacles. Only in brittle surface- and
shallow layers, overlying the deep shear fractures (cf. the shallowness of earth-
quake foci in submarine ridge system), there results a compromise between the
causative shear movement at depth and the tendency of brittle material to break
under tension, that is: echelon fractures, which also tell the sense of the deeper
shear movement. The fractures so formed do not necessarily reach magma depth.
Instead, vertical movement in the deeper ductile material may become tlie main
manifestation on shear planes. We seem to see an expression of this in the
Reykjanes peninsula type of uplifts, in dip-slip shallow earthquakes and in the
characteristic vertical steplike profiles of axially oriented elements of submarine
ridges. In (1) strike-slip focal mechanism is inferred on such elements, but that
component seems usually to be too small, in comparison with the dip-slip one,
to have been recognized in seismic work.