Geology Reference
In-Depth Information
Fig. 8.25.
Effect of normal-fault displace-
ment on the thickness of a
growth unit. a Cross section,
T u = fault throw at upper sur-
face, T l = fault throw at the
lower surface of the unit. b Iso-
pach map
ture contour map of the top or base of the unit. The affected zone extends from the fault
cutoff of the upper surface of the affected bed on one side of the fault to the fault cutoff
on the lower surface of the affected bed on the other side of the fault (Fig. 8.24). The zone
of thinning (normal fault) or thickening (reverse fault) is centered on the trace of the fault
through the middle of the unit. Such elongate zones of thinning or thickening can indi-
cate the presence of faults having displacements that are too small to completely separate
the map unit and that might otherwise go unrecognized.
The throw on the hangingwall or the footwall of a fault that does not completely
separate the unit can be determined from the maximum and minimum thicknesses of
the unit as
T = t max - t min ,
(8.4)
where T = fault throw, t max = maximum thickness inside (reverse fault) or outside (nor-
mal fault) the zone of thickness change, and t min = minimum thickness inside (normal
fault) or outside (reverse fault) the zone of thickness change.
For a growth unit (Fig. 8.25) the thickness change is different on opposite sides of
the fault. Less thickness change occurs on the upthrown side of the fault because the
displacement is partly erased by sedimentation. The asymmetry can be used to infer
the dip direction of the fault. A normal fault dips toward the thicker side and a reverse
fault dips away from the thicker side.
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