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indistinguishable from that of a single growing
fault (Fig. 10.13A). Alternatively, the footwall
topography could show smooth displacement
gradients, whereas intrabasinal high points could
mark the former segment boundaries (Fig. 10.13C)
(Anders and Schlische, 1994).
Next, the character of the landscape in
question should be examined. Does the range
display a smooth topographic profile or do
saddles separate major crests? Does the bounding
fault display a clear segmentation with discrete
regions where faults step over, overlap, or break
into numerous smaller faults? Do regional gravity
anomalies show a gravity low above a hanging-
wall basin that extends the full length of the
range and is systematically deeper toward its
center, or do the gravity anomalies indicate the
presence of intrabasinal basement ridges? If
there are intrabasinal ridges, do they correlate
spatially with segment boundaries or with
topographic saddles in the footwall?
Using digital elevation data, it may be possible
to test whether segment boundaries are associ-
ated with slip deficits (Simpson and Anders,
1992). For example, consider some range fronts
that have been interpreted as being bounded
by segmented normal faults and in which
some visual correspondence exists between the
segment boundaries and topographic lows in
the footwall. If topography is controlled by
segment boundaries and characteristic offsets
on each segment, then the topography should
slope downwards from the mid-points to the
end-points in each segment. If, on the other
hand, the entire range can be treated as being
controlled by a single fault, then, for half of each
segment, the topography should increase toward
the end-point, rather than decrease.
In the Beaverhead Mountains in Idaho
(Fig. 10.13D), segment boundaries have been
identified based on map patterns of faults (e.g.,
Fig. 4.16), but only some of these appear to
represent zones of linkage between older, smaller
faults. For example, at the Nicholia-Blue Dome
boundary, no slip deficit is apparent in the footwall
topography, and mapping across the basement
high shows that six smaller faults have together
accommodated nearly as much displacement as
the single bounding fault does in the middle of
Coupled Footwall Topography and Basin Subsidence
unsegmented
o g
A
late
early
m
B
segmented
late
early
segment A
segment C
segment B
composite
C
late
early
intra-
basinal
high
intra-
basinal
high
D
4
10 km
Beaverhead Mtns.
2
125
segment
boundary
zones of
distributed faulting
Mollie
Gulch
300
Baldy
Mtn.
Blue
Dome
Lemhi
Leadore
Nicholia
Fig. 10.13
Development of segmented and
unsegmented normal faults and basins.
Models for displacement showing topography of the crest
of the uplifted footwall and of the downthrown hanging
wall. Darker shaded areas show early displacement on
fault(s). A. Unsegmented fault with monotonic decrease
in displacement toward the tips of the fault. B. Segmented
fault with more displacement in the center of each
segment than at the tips. Older faults have joined, but
there is a persistent displacement deficit at the segment
boundaries. Both the footwall topography and basement
ridges reflect the fault segmentation. C. Following linkage
of segmented early faults, displacement deficits at the
boundaries are erased in the footwall topography.
Intrabasinal high-standing basement ridges are sites
where deformation is distributed among several smaller
normal faults. D. Footwall topography and gravity
anomalies of the Beaverhead Range, Idaho. The Mollie
Gulch-Leadore segment boundary is associated with
a gravity high, complexly faulted basement ridge, and
a topographic saddle. The Nicholia segment is bounded
on each end by modest gravity highs. In each case,
displacement due to distributed faulting nearly matches
that in the center of the segment. The Beaverhead Range
appears to show aspects of models B and C. Modified
after Anders and Schlische (1994).
