Geology Reference
In-Depth Information
Fig. 10.23.
Axial surface vs. fault, cross-section view.
Full arrows
indicate stratigraphic facing (up) di-
rection.
a
Dip change used to infer location of a fault,
half arrow
indicates possible displacement direc-
tion.
b
Extrapolated markers meet, showing an axial surface is present, not a fault
The defining feature of a fault is the separation of a marker surface. It is tempting
to map a fault anywhere the dip changes rapidly (Fig. 10.23a) in either outcrops or on
seismic profiles. At such locations, the continuity of beds or reflectors may be difficult
to establish, leading to the inference of a fault. If extrapolation of the markers towards
each other shows that they can meet without offset (Fig. 10.23b), then the probability
that the feature is an axial surface must be considered. Dip-domain fold hinges can be
very tight. Faults with no stratigraphic separation are probably axial surfaces with large
dip change. Of course strike-slip and oblique-slip faults cutting folds may have loca-
tions where the stratigraphic separation is zero even though the slip may be signifi-
cant. In such situations it is expected that fault separation will appear elsewhere along
the fault surface, demonstrating that displacement has, in fact, occurred.
10.5.3
Fault Cutoff Geometry
The geometry of the cutoff lines of marker surfaces against the fault provides a test of
the quality of the interpretation. The relationships are nicely shown on an Allan dia-
gram (Sect. 8.4). The Allan diagram of the normal fault shown in Fig. 10.24 represents
a reasonable throw distribution along a fault. The throw increases from SE to NW and
is approximately constant down the dip of the fault at any one location.
The Allan diagram from Fig. 10.24 has been modified in Fig. 10.25 to show typical
fault-separation problems. At point 1 the throw on marker 5 is significantly more than
on the markers above and below it. At point 3 the throw is significantly less than on
markers above and below. The lack of consistency along the fault and up and down the
fault, makes the interpretation of marker 5 suspect. Lesser throw on one horizon, as at
point 3, might be explained by a change in lithology to a rock type that favors folding.
Nevertheless the interpretation should be checked. The reversal of separation at point 2
implies a significant problem. Only a strike-slip fault would be expected to show rever-
sals of separation along strike. But the strike slip would not be confined to a single
marker horizon, suggesting an error in the interpretation.
A stratigraphic separation diagram shows the fault separation in terms of the units
juxtaposed across the fault along a single line, for example along the map trace of the
fault (Sect. 7.7.3). The curves for the hangingwall and footwall are not expected to cross
(Fig. 10.26), because this implies that either (1) the fault changes from a thrust to a
