Geoscience Reference
In-Depth Information
N
60°
N
F
F
8
60°
(a)
(b)
N
N
Figure 8.7
(a) This outcrop
pattern of simple offset strata
suggests a simple sinistral strike-slip
fault, as shown in (b), but the
pattern could also be produced by
normal (c) or reverse (d) faulting.
erosion
level
erosion
level
(c)
(d)
Once the direction of motion on a fault has been established,
the next question is: what is its sense of motion? There are
numerous observations that can help answer this question,
but fi rst, a note of caution. Observations of shear sense
should always
be made parallel to the direction of motion
(i.e. parallel to the slip lineations you have already recorded).
If you cannot determine the direction of motion, then you
cannot be sure of the signifi cance of any shear-sense
indicators you fi nd. Simple offset features illustrate this point
well (Figure 8.7); it is easy to misinterpret offsets of dipping
beds if you haven't fi rst determined the direction of fault
motion.
The best indication of fault motion (giving both the sense
and magnitude of displacement) is the offset of a linear
feature intersecting the fault plane, visible as a 'piercing
point' on both sides of the fault (Figure 8.8, p. 172). Imagine a
buried pipe cut and offset by an active fault: you could match
the two broken pipe-ends up on either side of the fault to see
how far it had moved. However, linear features such as the
pipe are rarely accessible, so evidence for relative fault motion
should be gleaned from other kinematic indicators in the
vicinity of the fault (Figure 8.9, p. 172).
