Geology Reference
In-Depth Information
A
Flexural Slip Faulting of Terraces
displaced terraces
present river
gradient
shortening
flexural slip faults
B
Terrace Displacement and Ages
terrace displacements
along faults
terrace ages
80 ka
30 ka
80 m
50 m
40 m
20 m
Fig. 9.7
Terraces displaced across
flexural slip faults.
A. Geometry of terrace displacement
defines greatest displacement near
the core of the fold. B. Terrace
ages and displacements of terraces
across different faults. Note that
offsets are not symmetrical across
the synclinal axis and that younger
terraces display lesser offsets
compared to the older terraces.
C. Rates of fault displacement based
on offset of dated terraces. Note that
rates are not necessarily constant
along the same fault through time.
D. Displacement of terraces by
flexural slip faults along the Ventura
River, California. Ages assigned to
the terraces are 92 ka (T5), 54 ka
(T4), 38 ka (T3), 30 ka (T2), and 17 ka
(T1). Modified after Rockwell
et al.
(1984).
C
Rates of Faulting
0.5 mm/yr
1.3 m
m/yr
0.5 mm/yr
1 mm/yr
80 ka
30 ka
1 mm/yr
0.7 mm/yr
1.7 mm/yr
D
Example from Southern California
0
1
100
50
0
terraces
km
T5
T4
T3
T2
T1
Ventura River
interval (Fig. 2.5), such that, with the exception
of rapidly rising coasts, marine terraces younger
than 80 ka are still commonly submerged.
Where rivers cross active folds or dip-slip
faults, fluvial terraces can also record progressive
displacement. Where faults rupture the surface,
it is expected that the age and height of
the terrace will generally correlate with the
magnitude of cumulative displacement (Plate 5).
Flights of fluvial terraces can overcome some of
the limitations imposed on paleoseismological
studies by the fact that trench walls in alluvium
are often unstable, so that deeply excavated
trenches are uncommon. Trenches in any of the
terrace risers may reveal paleoseismological
data on the last few ruptures that display
perhaps a few meters of displacement in each
rupture. In contrast, the entire vertical suite
of terraces can serve to define the long-term
displacement history, such that several tens
to hundreds of meters of displacement can be
recorded by the higher terraces (Plate 5). As
