Geology Reference
In-Depth Information
Joint spectrum of
hybrid joints
σ
1
σ
3
(extensional)
σ
2
North
North
σ
3
σ
1
α
σ
3
Bedding tilted through
αº
towards the southeast
stereographic representation. The centre crosses represent a horizontal set of discontinuities (say
bedding in a sediment or
flow banding in an igneous rock). The hybrid system plots from the
circumference (vertical, extensional) inwards, up to dips of about 60 degrees to horizontal. The
lower right
figure shows how this pattern might appear if the whole rock mass was tilted through
α
degrees (after Hancock, 1991).
there is a possibility for hybrid fractures to form with dihedral angles
between 0 and 60 degrees (sub vertical), and a potential joint spectrum,
as illustrated in
Figure 3.29.
Details are given in Hancock (1985), who
notes that the regular arrangement of structures such as joints within
large areas (>1,000 km
2
) of weakly deformed rocks, gives con
dence
that they are indeed linked to tectonic processes, as per theory.
Engelder (1999), however, questions the predictive validity of the
Mohr-Coulomb approach in detail.
The concept of fracture formation in a large mass of rock governed
by a Mohr-Coulomb strength law and under uniform stress condi-
tions, is helpful in explaining joint formation at a site but generally
geological history and local stress conditions and constraints means
that the situation is more complex. The soil or rock mass is unlikely to
be uniform and will include intrinsic
flaws, pre-existing discontinuities
