Page 278 - Physical Principles of Sedimentary Basin Analysis

8

Rheology: fracture and flow

Faulting and folding are important processes that shape the Earth. These different types of
deformations reflect two basic types of rheological behavior of rocks – brittle and ductile.
Rheology is the study of the deformation and flow of rock under the influence of stress. In
the brittle regime the deformation is by fracture and by sliding along fault planes, and in
the ductile regime the rock yields. Whether a rock is brittle or ductile depends on temper-
ature and stress. The rock behaves as brittle when the stress needed for brittle deformation
(fracture or frictional sliding) is less than for yielding, and vice versa. This chapter presents
simple empirical laws that quantify the brittle and the ductile behavior of rocks.

8.1 Faults
A fault is a planar fracture where the two sides of the fracture have been displaced relative
to each other. The direction of the fault block movement defines three different types of
faults, as shown by Figure 8.1.A strike-slip fault is a steeply dipping (near vertical) fault
where a horizontal slip has occurred along the strike, as illustrated in Figure 8.1a. Most
large strike-slip faults in continental settings are plate boundaries. The two other fault types
are dominated by translation directly up or down the dip of the fault plane. The two sides of
a non-vertical fault are called the footwall and the hanging wall, where the block above the
fault is the hanging wall, and the block below is the footwall.A normal fault results from
extension, where the hanging wall moves down relative to the footwall (see Figure 8.1b).
The opposite case, compression, creates a reverse (thrust) fault, where the hanging wall
moves up relative to the footwall, as shown in Figure 8.1c. These three types of faults can
also be understood in terms of the orientation of the principal stress.

8.2 Friction
Fault blocks move by sliding along fault planes, where the sliding is resisted by friction,
see Figure 8.2. The friction is proportional to the normal force pressing the fault planes
together. The normal stress σ n therefore gives the shear stress τ f in the fault plane as

τ f = μσ n (8.1)

260