Geology Reference
In-Depth Information
Madison Arm of
H
ebgen Lake
1 km
N
Obsidian
Sand Plain
terrace edges
asymmetric
meander belt
direction of tilting
and
river migration
Progressive Meander Migration
Fig. 8.27
Pattern of meander scars of the Madison River near the 1959 Hebgen Lake rupture.
Normal faulting during the Hebgen Lake
M
=
7.3 earthquake caused large displacement and tilting that was down to the
north-northeast (see Fig. 4.26). Note the position of the river channel on the northern and eastern margin of its
meander belt. Partial drowning of the meander belt due to coseismic damming of the lake is obvious from the
truncated meanders along the southwest shore of the lake. In the region depicted here, the coseismic offset was only
0-1 m down to the north. The series of meander scars facing toward the present river and the scale of those abandoned
meanders suggest that the river maintained its size, but migrated laterally in response to tilting, probably as the
cumulative result of repeated earthquakes on the Hebgen Lake fault. Modified after Alexander
et al.
(1994).
Under conditions of steady tilt, but without major
avulsions that completely relocate the meander
belt, one might expect to see a suite of meander
scars all facing in the same direction and
indicating the generally unidirectional migration
of the river across its floodplain toward a low
point (Alexander
et al
., 1994) (Fig. 8.26). If tilting
was abrupt and caused a major avulsion and
relocation of the meander belt, the channel
pattern should reflect that jump in average
position (Fig. 8.26C). In the area that has been
abandoned, meander scars should face in both
directions, as is typical of a modern meandering
river, rather than facing one direction - which
indicates progressive, unidirectional migration.
Measurements of the relative position of the old
channel with respect to the width of the meander
belt can indicate whether the channel position
was symmetrical within the meander belt and
untilted prior to avulsion, whether it had begun
to migrate in the direction of tilt prior to avulsion,
or whether it had been off-center in the opposite
direction prior to avulsion, which would suggest
that the direction of tilt was suddenly reversed
(Reid, 1992). In the Owens River setting (Fig.
8.25), for example, this last condition would be
analogous to having the dome deflate prior to
inflation, whereas, with a folded structure, it
could be analogous to deformation during a
seismic cycle in which the interseismic and
coseismic displacements are roughly equal in
magnitude, but in opposing directions.
In contrast to such oscillating uplift and
subsidence, regions with persistent coseismic
faulting and interseismic sediment loading com-
monly produce a unidirectional and sustained tilt
(e.g., Figs 4.25 and 4.30). In the area affected by
the 1959 Hebgen Lake normal-fault earthquake
(Fig. 4.26), such tilt is manifested by meander
patterns displaying a consistent migration
direction toward both the fault trace and the
zone of maximum displacement (Fig. 8.27).
Summary
At Holocene time scales, deformation tends to
occur in one of two modes: episodic coseismic
