Geology Reference
In-Depth Information
to make this choice (Cowgill, 2007; Harkins and Kirby, 2008; Lensen, 1964). For example, if
preserved riser offsets are equal to observed offsets of features that act as “passive markers”
(such as small channels) on the upper terraces (see figure E), then the age of the upper tread
should be used. Similarly, if an age gradient can be demonstrated from (i) older ages in shel-
tered positions adjacent to the offset riser to (ii) younger ages farther from the riser, the age
progression argues for riser preservation despite continued occupation of part of the lower
tread (see figure F). In contrast, if offsets of passive markers on an upper tread are larger than
riser offsets (see figure B), or if the riser offset is equivalent to that of a passive marker on the
lower tread (see figure C), then the riser has been modified during occupation of the lower
tread and the age of abandonment of the lower tread should be used (Cowgill, 2007).
Once risers are formed due to channel incision to a lower level, risers begin to degrade. If left
undisturbed, diffusion is likely to dominate the degradation process, such that a predictable
topographic profile will develop (Hanks
et al.
, 1984). Subsequent erosion of the toe of the riser
will cause it to steepen and depart from the predictable diffusion profile, such that the signature
of episodic erosion by a channel on the lower tread could be embedded in the topographic
profile. In the context of regional studies, comparisons of riser profiles among dated and
undated terrace treads can serve as a guide to whether the treads are the same age (Harkins
and Kirby, 2008).
No single strategy for choosing between lower and upper terrace treads is likely to work,
because so much depends on what data can be extracted from the local geomorphology. The
recommended approach combines careful attention to detail and multiple criteria that are sup-
portive of one interpretation or the other. See Cowgill (2007) for a more detailed discussion.
bottoms, on to the fault plane. Each such
projection is commonly associated with some
uncertainty. Consider, for example, trying to
calculate the slip on a thrust fault based on a
survey of a displaced terrace surface that
has been projected on to a cross-section
perpendicular to the fault trace (Fig. 6.17A
and B). The slip calculation should include
uncertainties related to the slopes of the upper
and lower offset surfaces, the dip of the fault,
and the position of the fault tip beneath the
scarp. One of the easiest ways to incorporate
such uncertainties is via Monte Carlo simulations
in which a probability distribution is assigned
to each variable in the calculation (Fig. 6.17).
Many of these probabilities may have Gaussian
distributions, such as would be expected from a
regression on a surveyed topographic slope.
Some other probabilities may be harder to
quantify, such as the position of the buried fault
tip beneath the scarp. If its position seems likely
to be random along the scarp, a box-like
probability distribution could be assigned. Or, if
experience suggests that, 95% of the time, the
tip lies beneath the lower third of the scarp,
then a corresponding probability distribution
can be defined. In a Monte Carlo simulation for
fault slip, a value for each variable is randomly
drawn from each variable's associated probability
distribution and then is used to define a specific
geometry from which the fault slip is calculated.
This computerized process is repeated thousands
of times and rapidly produces a probability
distribution for the fault slip (Fig. 6.17C).
In theory, every large earthquake on a fault
could be represented by a suite of displaced
geomorphic features. In order for this to be true,
some new geomorphic markers would have to
form in the interseismic interval between
faulting events, and older markers that were
displaced previously would have to be preserved
until the time of measurement. Such conditions
are not uncommon, because each seismic
displacement commonly forces some aspects of
the geomorphic system to adjust, thereby
producing new markers. In many landscapes,
