Geology Reference
In-Depth Information
9.6
Analysis of Faults
The drag geometry (Sect. 7.2.7) provides the basis for fault recognition by SCAT analy-
sis. The presence of a fault is recognized from the distinctive cusp pattern on the trans-
verse dip component plot (Figs. 9.17-9.20). A cusp is also present on the dip vs. depth
plot but may not be as clearly formed. The cusp is caused by the dips in the drag fold
adjacent to the fault and is expected to occur within a distance of meters to tens of
meters from the fault cut. A traverse perpendicular to the fault plane will show the
minimum affected width, whereas a traverse at a low angle to the fault plane, such as
a vertical well drilled through a normal fault, will show the maximum width. The fault
cut is at the depth indicated by the point of the cusp. The azimuth vs. depth plots
distinguish between steepening drag that occurs where the faults dip in the direction
of the regional dip of bedding (Fig. 9.17) and flattening drag that occurs where the
fault dip is opposite to the regional dip of bedding (Fig. 9.18). Steepening drag main-
tains a constant dip direction whereas flattening drag may produce a reversal in the
dip direction. A drag-fold axis that is oblique to the regional fold axis produces mul-
tiple fold axes on the dip-azimuth diagram (Figs. 9.19, 9.20). Both the regional dip and
the drag-fold axis appear on the dip-azimuth diagram and the tangent diagram, allow-
ing both directions to be determined.
If either the dip direction of the fault or its sense of slip is known, the other property
of the fault can be determined from the direction the cusp points on the
T
-component
Fig. 9.17.
Structure contour map, cross section, and SCAT plots for a normal fault with drag that steep-
ens the regional dip. Fault strike is parallel to the regional strike.
L:
regional strike;
T:
regional down-
dip direction;
T':
regional up-dip direction. (After Bengtson 1981a)
