Geology Reference
In-Depth Information
A
Ostler Fault Zone
n
10 km
North Central
southern fault tip
2.0
B
Haybarn Section
<10 ka
17±1 ka
23±2 ka
1.5
23 ka
70±15 ka
1.0
70 ka
140±15 ka
0.5
0.0
32
34
36
38
40
42
44
46
48
50
2.0
C
<10 ka
17±1 ka
23±2 ka
70±15 ka
140±15 ka
Central Sections
1.5
17 ka
23 ka
1.0
0.5
70 ka
0.0
0
5
10
15
20
25
30
35
40
Along-Strike Distance from Southern Tip (km)
Fig. 9.30
Fixed-tip faults in the Ostler Fault zone.
A. Map of the surface trace of the Ostler Fault, an east-vergent thrust fault. Normalized displacement versus length on
the Haybarn (B) and Central (C) sections. Similarities of the normalized profiles at 17, 23, and 70 ka argue for fixed
tips on each of these segments. Modified after Amos
et al.
(2010).
propagating folds (Fig. 4.10). The use of
normalized displacement gradients and of ages
of geomorphic surfaces can permit unambigu-
ous discrimination among these fault-growth
models. For a fold whose tip is propagating, ages
along the upper surface of the fold should get
progressively younger toward the tip (Fig. 9.29A).
In contrast, two diagnostic features help distin-
guish fixed-tip folds. First, deformed geomorphic
surfaces should have the same age along their
length (Fig. 9.29B). Second, normalized length-
displacement profiles should remain similar irre-
spective of age, i.e., ongoing differential uplift
amplifies the fold in a self-similar way.
Given the progression of ages along its crest,
Wheeler Ridge anticline provides an excellent
example of a propagating fold (Fig. 9.25). In
contrast, a succession of deformed and dated
terraces along the Ostler Fault in New Zealand
suggests that the tips of current fault segments
have been fixed for many millennia (Fig. 9.30).
A suite of fluvial terraces ranging in age
from Holocene to the penultimate glaciation
(140 ka) have been deformed along the
length of the Ostler Fault (Amos
et al.
, 2010).
Although older terraces have been significantly
more deformed than younger terraces, the pat-
terns of normalized displacement versus length
appear indistinguishable from one terrace to
the next, despite a seven-fold range of ages
(Fig. 9.30B and C). This similarity in shape
and the absence of lengthening of the fault
