Geology Reference
In-Depth Information
Fig. 8.15.
Fault cutoff map of fault from Deerlick Creek coalbed methane field (data from Groshong
et al. 2003b).
a
3-D oblique view of marker surface truncated at fault.
b
3-D oblique view of fault sur-
face showing hangingwall and footwall cutoff lines (
heavy lines
)
Fig. 8.16.
Cross section in the dip direc-
tion of the fault, showing pro-
jection of fault cutoffs onto
horizontal and vertical fault
cutoff map planes
(Fig. 8.16). If the fault is curved in cross section, a horizontal projection foreshortens
the width of the fault. Three-dimensional computer graphics techniques allow maps
to be made directly on the fault surface (Fig. 8.15b).
The construction of a fault cutoff map is illustrated for a fault seen in an under-
ground mine along two coal seams in the Black Warrior basin of Alabama. The fault
dips about 60° to the northeast and forms one side of a full graben (Fig. 8.17). To the
southeast it dies out by transferring its displacement to a parallel fault. To the north-
west the fault zone continues along multiple parallel branches. The first step in con-
structing the cutoff map is to choose the plane of the map. In this example the fault
dips steeply and so projection onto a vertical plane is suitable. The trace of the map
plane is taken parallel to the trend of the fault and with a view direction to the south-
west, from the hangingwall to the footwall (Fig. 8.18). The vertical scale is exaggerated
by a factor of three because the maximum displacement on the fault is so small relative
to its length. The elevations of the cutoffs of the top of both coal seams against the fault
are transferred to the cutoff map along lines perpendicular to the trace of the map,
which is the method of illustrative section construction (Sect. 6.3). The splays at the
northwest end of the fault necessitate a choice of which elevations to map. The entire
zone is included here so that the cutoff map (Fig. 8.18) shows the total displacement
across the fault zone.
