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Fig. 2.6 Solution (lower hemisphere) on 6 data from the karst
region of Lakatnik (Western Balkan Mountain—Bulgaria).
a Input data; b Reconstruction of the principal stress axes.
Close to the symbols for r 1 , r 2 and r 3 on the right of the
stereonet the azimuth, the dip angles and the Last Square Error
(in brackets) are plotted
of the conjugated joints and faults and the stress field,
defined with its principal axes r 1 , r 2 , and r 3 (Angelier
1994 ). Nikolaev ( 1977 ) developed a method for sta-
tistical processing and interpretation of joints with
good opportunities at various practical structural
studies. After analyzing important quantity of joint
diagrams, he concluded that the joints' maximums are
always distinguished by more or less expressed
asymmetry and eccentricity. Accepting that the joints
grouped into maximums have been formed at a con-
stant regional tectonic stress field, he concluded that
the angle between the conjugated joints changes in a
particular way (Fig. 2.7 ) during the joints' formation.
This change is explained with the fact that the
appearance of every new joint causes some change in
the stress conditions in its close vicinity. The uni-
versal pressure changes and this leads to change in the
shear angle h, at which the next joints originate.
Explanation can be easily found if we consider the
relationship ( 2.1.4 ) from Sect. 2.1 . The creation of a
new joint leads to decreasing of l (the coefficient of
friction), and consequently the angle h for every next
discontinuity will grow, i.e., the dispersion of the
joints will be oriented toward the minimum tectonic
stress r 3 .
Similar conclusions were also made by Engelder
( 1994 ). According to him, when a pair of shear joints
is developed, a torsion of the joint plane appears due
to local changes in the stress field, which under
particular conditions might lead to the forming of en-
echelon segments. As a whole, an asymmetric distri-
bution of frequencies is obtained in the joint sets
(Fig. 2.7 ), which is expressed through the forming of
eccentric fields of density on the density diagrams. As
Fig. 2.7
Schematic 3D model of the shear joints' formation
and
their
asymmetric
dispersion
(following
the
idea
of
Nikolaev 1977 )
a rule, the extension of iso-areas disperses toward the
axis of minimum stress r 3 .
From the Griffith theory (Griffith 1924 ), it is
known that the joints are not distributed in their own
plane, but concentrated toward a direction closer to
r 1 . This tendency of deviation of the joints' own
planes toward the direction of maximum stress is
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