Geology Reference
In-Depth Information
Fig. 1.29.
General terminology for a
surface (
patterned
) offset by a
fault.
Heavy lines
are hanging-
wall and footwall cutoff lines
Fig. 1.30.
Fault slip is the displacement
of points (
open circles
) that
were originally in contact
across the fault. Here the cor-
related points represent the
intersection line of a dike and
a bed surface at the fault plane
across a fault. It may be difficult to distinguish between a shear zone and a fault zone
on the basis of observations at the map scale, and so here the term fault will be under-
stood to include both faults and shear zones.
The term hangingwall refers to the strata originally above the fault and the term
footwall to strata originally below the fault (Fig. 1.29). Because of the frequent repeti-
tion of the terms, hangingwall and footwall will often be abbreviated as HW and FW,
respectively. A cutoff line is the line of intersection of a fault and a displaced horizon
(Fig. 1.29). The HW and FW cutoff lines of a single horizon were in contact across a
fault plane prior to displacement. Across a fault zone of finite thickness or across a
shear zone, the HW and FW cutoffs were originally separated by some width of the
offset horizon that is now in the zone.
1.6.1
Slip
Fault slip is the relative displacement of formerly adjacent points on opposite sides of
the fault, measured along the fault surface (Fig. 1.30; Dennis 1967). Slip can be subdi-
vided into horizontal and vertical components, the strike slip and dip slip components,
respectively. A fault in which the slip direction is parallel to the trace of the cutoff line
of bedding can be called a trace-slip fault. In horizontal beds a trace-slip fault is a
strike-slip fault. Measurement of the slip requires the identification of the piercing
points of displaced linear features on opposite sides of the fault. Suitable linear fea-
tures at the map scale might be a channel sand, a facies boundary line, a fold hinge line,
