Geology Reference
In-Depth Information
almost exclusively on the displacement along
faults. Although more subtle offsets may occur
in folds, the deformation represented by them
can constitute an important fraction of the total
strain. Much of the landscape that is preserved
today in active fold-and-thrust belts results from
the initiation and amplification of growing folds
(Fig. 4.39). The shape of those folds determines
the uneroded geometry of the land surface. In
such cases, the dip of the forelimb and backlimb
determines the surface slope and the position of
the drainage divide. The shape of the fold can,
therefore, strongly influence both the nature
and efficiency of surface processes that erode
and redistribute mass. If we want to understand
the modern landscape as a product of the
long-lived interactions of surface processes with
deforming structures, we need to know how
those structures evolved through time.
Geomorphology provides one key to this
understanding. In cross-section, most folds have
a steeper forelimb than backlimb (Figs 4.36 and
4.37). Even when the uplifted surfaces have
been dissected by erosion, contrasts in the dip
of the limbs of the folds are commonly discern-
ible in the modern topography and indicate the
“facing direction” or orientation of the underly-
ing blind thrust. For example, stream lengths are
often asymmetrical across a fold. Shorter, steeper
streams occur on the forelimb, causing the
drainage divide to be initially displaced toward
the forelimb. Information about how folds have
grown laterally can also be revealed by stream
patterns. As a plunging fold begins to grow
laterally, rivers that flow across the axis of the
fold must either incise into the uplifted area or
be deflected out of their present courses and
around the nose of the fold. Initially, rivers
usually tend to maintain their course across a
fold. To do so, they need sufficient stream power
to erode enough of the newly uplifted material
so that they can maintain a downstream gradient
across the fold axis (Burbank
et al.
, 1996c; Amos
and Burbank, 2007). Through the process of
erosion, they create
water gaps
across the fold
axis (Fig. 4.39). If at some time the rate of lower-
ing of the stream bed by erosion is insufficient
to keep pace with the rate of structural uplift of
the fold, then the stream will be “defeated,” a
wind gap
will develop along the abandoned
river course, and the river will flow subparallel
to the limb of the fold until it finds a low point
where it can traverse the fold (Fig. 4.39). The
pattern of rivers adjacent to folds and the
presence of wind and water gaps can be inter-
preted to indicate the direction of propagation
of a fold ( Jackson
et al.
, 1996). If the time of
abandonment of the wind gaps or the age of the
uplifted surfaces along the fold's flanks is
known, then the rate of propagation can also be
defined (Medwedeff, 1992).
It is becoming apparent in many areas that
blind thrusts pose major seismic hazards for
urban areas. Most of the recent destructive
earthquakes in the Los Angeles basin, for
example, occurred along blind thrusts. Even
when enlightened city planners have prohibited
building adjacent to known faults, the failure to
recognize the potential for strong accelerations
due to earthquakes on buried structures has led
to inadequate building standards and huge loss
of property and lives during recent earthquakes.
To the extent that the geometry of folds can be
used to infer the shape of underlying faults and
to the extent that unravelling the history of
folding provides insight into rates of deforma-
tion, analysis of folds can provide fundamental
constraints on tectonic rates, patterns of past
and probable future deformation, and seismic
hazards.
Summary
A diverse array of faults and folds has formed at
the Earth's surface in response to variations in
stress fields, inhomogeneities in rocks, contrasts
in crustal strength, and interactions among mul-
tiple structures. It seems clear that the build-up
of stresses across and along rupture surfaces
causes earthquakes. Unfortunately, direct meas-
urements of stress are difficult to make, and
understanding of changes in the distribution of
stresses before, during, and after faulting is still
incomplete. Nonetheless, earthquakes can be
usefully thought of as part of a cycle in which
strain that accumulated during interseismic
intervals is fully or partly recovered during
