Geology Reference
In-Depth Information
t 5
310-470 cal yr B.P.
Wellington
r 4
t 4
Fault
r 2
t 3
t 2
t 2
t 2
r 1
m
r m
l
m'
k
l'
offset
k'
r 4
j
i
r 3
i'
t 5
r 2
j'
t 4
t 3
6
4
2
0
t 2
t 2
t 2
t 1
overbank silt
and sand
scale (meters )
?
?
?
Offset Fluvial Terraces, New Zealand
01020 30 40 m
Fig. 9.6 Displaced fluvial terraces along the Wellington Fault, New Zealand.
The lowest terrace, dated at < 250 yr, is not displaced by this strike-slip fault, whereas the next highest terrace (t 2 ) and
the channels cut across its surface show a displacement of 4 m attributable to the last major earthquake. Note that
any fault displacement of the riser (r 1 ) between t 2 and t 1 was beveled off during creation of t 1 . The amount of offset of
the risers increases systematically with each older terrace. Important controls on terrace width are exerted by the
geometry of the river channel prior to abandonment (note the triangular shape of t 3 ). The vertical throw across the
fault is small ( 10%) compared to the horizontal displacement. Modified after Van Dissen et al. (1992).
most reliable correlation will usually result from
consideration of the entire suite of treads and
risers and any relative or absolute dating of
their  surfaces. Second, the offset of formerly
continuous risers is measured across the fault
for each terrace level. In addition, any other
linear features, such as channels or gullies,
located on the terrace treads and trending across
the fault are also measured (Fig. 9.6). As
described earlier (Box 6.1), careful attention
should be paid to whether risers have been
modified since the upper tread was abandoned
and whether slip rates should be based on ages
for the upper or lower treads that bracket the
riser. Finally, to the extent permitted by available
dates, a history of cumulative offset and rates of
offset through time is developed.
Although a similar methodology could be
applied to marine terraces that have been cut
by  strike-slip or dip-slip faults, fluvial terraces
have  a considerable advantage in terms of
reconstructing the record of the past 10-50 kyr
because the available terrestrial record in this
interval is commonly more complete. Whereas
climatically driven changes in water and
sediment discharge between 10 and 50 ka have
generated multiple fluvial terraces in many sites,
sea level was below present for this entire
Fluvial terraces
When a river course flanked by flights of fluvial
terraces is oriented at a high angle to a strike-slip
or dip-slip fault, the terraces displaced by the
fault can provide an excellent record of
progressive offsets. If some or all of the terraces
can also be dated, then the rates of fault
displacement over the duration of the dated
sequence can be derived. In regions where
climatically controlled terraces are widespread,
it may not be necessary to date the terraces
directly adjacent to the fault, because sequences
of soils, loess, or volcanic ashes that overlie the
terrace treads may permit correlation with other,
better dated, terraces in the same region.
The assessment of offset fluvial terraces
requires several steps. First, the correlation of
terrace treads and risers across the fault must be
determined. Because of changes in the river
course through time, the height of risers between
terraces along strike-slip faults is generally a
better guide to correlation than is the width of
the terrace tread, which is sensitive to patterns
of lateral migration by the former channel.
If,  however, significant vertical (dip-slip)
displacement has occurred along the fault, then
riser height will vary as well. In any case, the
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