Geoscience Reference
In-Depth Information
is defined as uniform; if not,the state of stress is no
uniform. For the uniform state of stress, the values of
the stresses are calculated by the values of the applied
strains. For the no uniform state of stress the values of
the stresses can not be calculate directly, but using the
effect of their impact. Exactly, this approach is used
for qualitative evaluation of the stress by the different
Earth's sciences. The type of the stress field in Tec-
tonophysics is determined by the space orientation of
the principal stress axes and their relative rates.
Practically, this approach has lead to the definition of
the extensional stress (in spite the impossibility to
accept this type of mechanical deformation inside the
Earth's
crust),
the
compression
stress,
the
strike
Fig. 2.2
Ellipsoid of the acting strain (ABC) and ellipsoid of
the deformations (XYZ). A—maximum compression; B—inter-
mediate compression; C—minimum compression; X—maxi-
mum deformation (lengthening); Y—intermediate deformation;
Z—minimum deformation (shortening) (after Aubouin et al.
1988
)
stress, etc.
Angelier (
1994
) comments that always when a
reconstruction of the stress field is done, one of the
reconstructed axes is subvertical. He explains this
effect with the gravitational field and the free Earth's
surface. The vertical stress is directly dependent from
the weight of the rocks and the pore strain. Hence, the
tectonic stress is acting, in the most of the cases, on
subhorizontal plane. The reconstruction of the pa-
leostresses from the analyses of the fault systems has
shown that the deviation of the stress axis determined
as a vertical one is surprising small—normally about
10 (Angelier
1994
).
The strains creating the deformations in the rocks
can be defined in the space by the ellipsoid, the three
axes A, B, and C of which determine the maximum,
the intermediate, and the minimum strain (Fig.
2.2
).
Generally, this ellipsoid is randomly space-oriented
relatively to the geometric characteristics of the rocks
before their deformation. The relationship between
the strain ellipsoid and the stress ellipsoid (axes X,
Y and Z in Fig.
2.2
) is not direct, nor simple. The
example in Fig.
2.2
(according to Aubouin et al.
1988
) represents the ideal case of isotropic material.
Both the two ellipsoids have the same orientation of
the axes. The maximum deformation is corresponding
to the maximum strain. The general case is the ran-
dom orientation of the ellipsoids to each other. The
field studies on the rock deformations define the ori-
entation of the axes X, Y, and Z. It is not easy, nor
simply to determine the principal stresses r
1
, r
2
, and
r
3
. But in some conditions regarding the orientation
of the principal strains before the deformation,
the relationship between the two ellipsoids can be
simplified. For the structural studies, a simplified
acceptance for the relationship tectonic strain
()
resulting deformations in the rocks is applied.
It is well known that the strain and the stress
relationship does not directly mean fractures forma-
tion, but however strain and stress are causes of
fracturing. Hunt et al. (
2011
) described the principal
causes of fracture-density variations (Table
2.1
), but
concluding immediately that this list of fracture-
causing variables is much more extensive, and often
unique to a particular fracture system.
As a result of the rock fracturing, different types of
single or complex structures appear. The ranging and
mutual relationship can be simplified to the scheme
presented
in
Fig.
2.3
,
following
Norsk
Geologisk
Tidsskrift 69 Supplement (Nystuen
1989
).
A fracture is used as a more general term for all
kinds of fracturing caused by mechanical stress in the
rocks. The term is irrespective of whether dislocation
has taken place along the fracture.
A joint is a fracture surface in the rock along which
no displacement has occurred. A joint system is
composed of intersecting joints assumed to have been
formed during the same deformational event.
A fault is a fracture separating two bodies of rock,
which have moved relatively to one another and it is
the fundamental term for a fracture surface in the
Earth's crust on which displacement has occurred.
