Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 158

tip

(a)

d

b

TL tip

sc

dc

TL

r

FW

TL

DV

A

FW

B

HW

(b)

HW

Fig. 4.96 (A) Faults have a limited extent and can cut through the
surface (A) or not (B), in which case they are regarded as blind
faults. Fault terminations (tip and tip lines: TL) are marked in both
cases. FW marks the footwall and HW the hanging wall of the fault
surfaces.

H

T

DV

line (Fig. 4.96). In the case of faults that reach the Earth's
surface, the intersection line between the fault plane and
the topographic surface is called the fault trace and the point
where the fault trace ends is called the tip point or tip . Blind
faults are those which terminate before reaching the
Earth's surface and although they can cause surface defor-
mation, like monocline folds, there is no corresponding
surface fault trace (Fig. 4.96) to the fault bounded at the
front and upper ends by termination or tip lines.
Fault planes can have different forms. At the surface
most faults appear as fairly flat surfaces (Fig. 4.97a) but at
depth they can show changes in inclination. Some faults
show several steps: in high angle faults, stepped segments
showing a decrease in dip are called flats (Fig. 4.97b),
whereas in low angle faults, segments showing a sudden
increase in dip are called ramps (Fig. 4.97c). Flats and
ramps give way to characteristic deformation at the topo-
graphic surface; in normal faulting, for instance, bending
of rocks in the part of the hanging wall block located over
a ramp results in a synclinal fold , whereas the resulting
deformation over a flat is an anticlinal fold. Ramps can be
also present in faults with vertical surfaces as in strike-slip
faults, which are called bends , or orientated normal (side-
wall ramp) or oblique (oblique ramps) to the fault strike.
Listric faults are those having a cylindrical or rounded sur-
face, showing a steady dip decrease with depth and ending
in a low-angle or horizontal detachment (Fig. 4.97c).
Detachment faults can be described as low-angle faults,
generally joining a listric fault in the surface that separates
a faulted hanging wall (with a set of imbricate listric or flat-
surface faults) from a nondeformed footwall. Detachments
form at mechanical or lithological contacts where rocks
show different mechanical properties, a decrease in friction

Fig. 4.94 (a) Total displacement vector (DV) in a fault (general
case). If the movement is oblique, a dip component (dc) and a slip
component (sc) can be defined. DV can be orientated by the rake ( r )
over the fault surface, whose orientation is given by the strike ( )

and slip ( ) angles. (b) Other components can be separated from

DV: the vertical offset or throw ( T ) and the horizontal offset or
heave (H).

(a) (b)

HWB

UTB

HWB

DTB

UTB

FWB

DTB

FWB

Fig. 4.95 Relative position of blocks in a fault: hanging wall block
(HWB); footwall block (FWB); upthrow block (UTB); and
downthrow block (DTB) in (a) a reverse fault and (b) a normal
fault.

possible, and the displacement has to be guessed by the
observation of offset layers. In this case the separation can
be defined as the distance between two homologous
planes or features at either side of the fault, that can be
measured in some specific direction (like the strike and dip
directions of the layer).
Faults initially form to a limited extent and progressively
expand laterally; the offset between blocks increasing with
time. The limit of the fault or fault termination, where
there is no appreciable displacement of blocks is called tip