Geology Reference
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mean topography or with large-scale geological
structures along a survey transect reveal the
extent to which they co-vary (Fig. 5.7). If
the pattern of vertical geodetic uplift mimics
the smoothed topography, such similarity could
suggest that: (i
)
the present landscape results
from the persistence of the present deformation
field over a long interval of time; and/or
(ii
)
short-term deformation includes some
permanent, anelastic strain. On the other hand,
a mismatch between vertical geodetic rates and
topography may indicate that decadal rates are
unrepresentative of the long-term pattern of
deformation. Such a mismatch is not unexpected,
because the interval between large earthquakes
is commonly long when compared to the
duration of geodetic surveys. Hence, these
surveys typically span only some part of
the interseismic period and do not include
coseismic deformation (Fig. 5.1). During this
interseismic interval, elastic (and sometimes
permanent) strain slowly accumulates, whereas
deformation during an earthquake may rapidly
release the elastic energy stored in the crust and
produce deformation in the opposite sense
(Natawidjaja
et al
., 2004).
A mismatch between topography and uplift
patterns along a leveling line may indicate that
surface processes successfully reshape and
modify any pristine topography. Under these cir-
cumstances, a differential vertical deformation
field could be well recorded by an underlying
geological structure, such as a fold, but the fold
itself may be uplifting strata that are readily
eroded as soon as they are raised above base
level. This scenario appears to apply to the
Himalayan foreland of Nepal ( Jackson and
Bilham, 1994b), where active folding is displayed
by shallow structures and by the surveyed uplift
rates associated with the Main Siwalik Thrust
(MST: Fig. 5.7A), but the deformation is not well
reflected by the topography (Lavé and Avouac,
2000). A topography-uplift mismatch is not very
surprising here, because the fluvial strata of the
Himalayan foreland commonly appear to be
eroded nearly as rapidly as they are uplifted
(Burbank and Beck, 1991). On the other hand,
the highest rates of uplift along this same
leveling-line transect coincide with the high
topography of the Greater Himalaya. In these
bedrock ranges, rates of erosion and of bedrock
uplift may be in a rough equilibrium (Burbank
et al
., 1996b; Lavé and Avouac, 2001), so that the
average surface topography is changing very
little despite the rapid bedrock uplift.
Patterns of vertical deformation can provide a
data set against which models for ongoing
deformation can be tested. The Himalayan survey,
for example, traverses both known structures,
such as the Main Boundary and Main Central
Thrusts, and inferred ones, such as the crustal
ramp beneath the Greater Himalaya (Fig. 5.7A).
The survey reveals broad regions of more rapid
uplift that are 20-50 km wide (Fig. 5.7B) and
that appear to be related to deformation on
these underlying structures. The overall pattern
of surveyed deformation can be imitated using
models that treat the crust as an elastic medium
that will deform in response to differential slip
along faults. The orientation of a fault, its slip
rate, and the thickness of its hanging wall
determine the deformation attributed to it in the
model. One such model ( Jackson and Bilham,
1994b) for this Himalayan transect suggests
that the uplift pattern of the Greater Himalaya
results from interseismic elastic strain due to a
large decrease in slip rate between the crustal
ramp and the main detachment farther south
(Fig. 5.7B). Although the results of such models
are not unique, they provide a possible explana-
tion for why and where interseismic strain is
occurring and can help guide thinking about
deformation within orogenic belts.
Leveling-line surveys of coseismic deformation
across structures underlain by thrust faults serve
to examine the vertical deformation attributable
to individual seismic events (Fig. 4.28A).
Changes in displacement along serial traverses
can be compared with theoretical models that
predict variations in offset along the length of a
fault from zero at fault tips to a maximum near
the center. If one were interested in whether a
particular earthquake represented a characteristic
quake for a specific fault, a comparison of the
pattern of coseismic uplift or subsidence along a
traverse with the topography along the same
traverse might provide useful insights. According
to leveling-line data, the 1983 Coalinga earthquake
