Geology Reference
In-Depth Information
along a footwall flat (Fig. 6.9). Deposition that
occurs following an increment of fault slip will
fill the space above the newly tilted forelimb
and will tend to restore a subhorizontal upper
depositional surface (Dolan
et al.
, 2003). In the
process, this deposition creates an irregular
trapezoid of new growth strata that thins over
the forelimb. Subsequent deposition in the
absence of fault slip will cause simple vertical
aggradation with no tapering of bed thickness
across the forelimb. Thus, the magnitude of bed
taper across the forelimb is indicative of the
amount of relative uplift of the fold crest. If the
dip of the underlying fault is known or can be
deduced, then the amount of fault slip required
to produce the observed differential uplift can
be calculated. Dating of the base of each unit of
growth strata will place bounds on the timing
of the faulting events that caused each event
of differential uplift. Because fold forelimbs are
commonly hundreds of meters long, these
geometries have commonly been investigated
using boreholes, rather than trenches. This
strategy, therefore, requires correlation of strata
between boreholes, which introduces another
source of uncertainty.
can be made. One obvious complication for
such studies arises from changes in sea level.
Whereas sea level has been quite steady since
about 8 ka, sea level rose about 120 m as ice
sheets melted during the preceding 10 kyr
(Fleming
et al.
, 1998). Thus, a 20-ka terrace
exposed at today's shoreline would actually
represent 120 m of rock uplift since its formation.
For most coastal records, data from a reliable
sea-level curve must, therefore, be incorporated
into any analysis attempting to determine the
total rock uplift.
Along many uplifting coasts, wave-cut
abrasion platforms (Fig. 2.1 and Plate 1A)
provide the most abundant records of local rock
uplift. Coseismic uplift causes a relative sea-level
fall and exposes the newly emergent shoreline
to wave attack (Figs. 6.2 and 6.10). The shape of
the resulting platform depends on local rock
strength, coastal orientation with respect to
wave energy (Adams
et al.
, 2002, 2005), the time
between successive uplift events, and the rate of
eustatic sea-level rise. Broader platforms
result from weaker rocks, higher wave energy,
and longer interseismic intervals, because these
factors prolong the exposure interval and
increase the effectiveness of wave attack on
coastal bedrock. If the platform-carving pro-
cesses are sufficiently efficient, then steadily
rising sea level also can generate wide abrasion
platforms as previously formed platform is
inundated and wave attack is focused pro-
gressively higher on the sea cliff. Except during
intervals when sea level is constant, the change
in sea level between seismic events must be
added to the observed coseismic offset (usually
measured with respect to sea level at the time of
the earthquake) in order to determine the total
rock uplift.
A staircase of raised abrasion platforms on
the Peruvian coast (Bourgois
et al.
, 2007) reveals
the importance of accounting for sea-level
changes (Fig. 6.10B and C). Fifteen terraces
span
∼
90 m of elevation and were created over
the past 20 kyr during which sea level rose
120 m. The total rock uplift (
>
200 m) is, there-
fore, more than twice as large as indicated by
the height (90 m) of the oldest terrace above
current sea level. If ages and current elevations
Buried faults in coastal settings
The largest earthquakes in the world in the past
60 years (Chile in 1960 and 2010, Alaska in 1964,
Sumatra in 2004, and Japan in 2011) have
occurred on subduction zones. Given the human
population density in many coastal areas, and
the fact that roughly half of the world's popula-
tion lives along tectonically active plate margins,
such earthquakes pose a major hazard. Most of
these ruptures occur both at significant depths
(commonly not breaking the surface) and far
beneath the ocean surface. Hence, they are
largely inaccessible. Given these facts, how can
a record of past subduction-zone earthquakes
be generated?
Displaced shoreline features typically provide
the best coastal evidence of coseismic subduction-
zone offsets. One appealing aspect of studying
coastal features, in addition to the ocean
vistas, is that detailed reconstructions of
vertical deformation with respect to sea level
