Geology Reference
In-Depth Information
the adjacent segment (Anders and Schlische,
1994). The gravity highs here result from the
distributed faulting in the absence of a significant
slip deficit. Whereas the topographic and basin
pattern appears analogous with model C (Fig.
10.13C), the total slip accumulation is consistent
with model A (Fig. 10.13A). In contrast, the Mollie
Gulch-Leadore segment boundary is associated
with an apparent slip deficit, as indicated by a
topographic saddle, such that the linkage between
segments appears to have happened more
recently. This geometry is consistent with model B,
in which segment boundaries represent zones of
persistent slip deficits (Fig. 10.13B).
Do all normal faults that link together tend
toward a state in which displacement gradients
change smoothly from end to end along the
entire composite fault, eventually eliminating
deficits associated with former segment
boundaries? This behavior seems to be clearly
true for some ranges, but we do not know
whether slip deficits observed at some present-
day segment boundaries will persist through
time or be smoothed and eliminated. The
position of any given range on this spectrum
from segmented to unsegmented can be evalu-
ated by comparing model predictions for these
end-members (Fig. 10.14) to surface and subsur-
face data on fault geometries and displacement,
footwall topography, stratal thicknesses, and
stratal tilt in the hanging-wall basins.
Topography of folds
Consider the growth of contractional folds under
these basic assumptions: both blind thrusts and
ones that cut the surface drive differential rock
uplift and folding; faults propagate laterally as
they accumulate strain; and erosive forces attack
hanging walls when they are uplifted above
local base level. What governs similarities or
differences in the geomorphic evolution of
individual folds? Certainly, the geometry of the
causative fault is important. For example, the
hanging wall of a purely fault-bend fold will
never be uplifted more than the height of the
footwall ramp, whereas uplift in a displacement-
gradient fold can greatly exceed the ramp height
(see Fig. 4.36). Variations in the stratigraphy and
erosional resistance of the hanging wall also
affect its evolution. During the development
of the fold-and-thrust belts that are commonly
found associated with collisional mountain
ranges, thrust faults propagate beneath and into
the foreland basins bounding the range. When
the hanging wall consists only of the sediments
that have previously filled the foreland, their
relatively homogeneous and typically low
resistance to erosion will yield a more spatially
uniform and predictable pattern of dissection
(see Fig. 9.26) than when resistant bedrock is
also uplifted in the hanging wall. When both
foreland-basin strata and bedrock are uplifted
and exposed to erosion, the large contrasts in
erodibility between the less indurated sediments
and the bedrock promote a distinct erosional
and topographic pattern. During initial stages of
uplift (which could amount to 1-5 km, depending
on the stratal thicknesses in the foreland),
relatively rapid dissection and stripping can occur
of the weakly cemented sediments. Thus, the fold
might develop limited surface topography with
only a few hundred meters of relief, representing
a small fraction of the total rock uplift (Fig. 10.15).
As resistant bedrock is elevated above base level,
rates of surface erosion can drop abruptly and a
much higher fraction of the rock uplift may begin
to be reflected in surface uplift. In semi-arid to
arid landscapes, these bedrock ridges may be
almost undissected despite several kilometers of
uplift. Such is the case in the Tien Shan of
Contractions, folds, and drainage networks
The Himalaya, Tibetan Plateau, Caucasus, Alps,
Zagros, Taiwan, and many of the major,
non-volcanic mountain ranges of the world have
developed over spans of millions of years in
response to compressional stresses exerted bet-
ween converging plates. Contrasting landscape
and tectonic characteristics emerge in this setting
at differing spatial and temporal scales. In the pre-
vious chapter, we described the growth of simple
folds, such as Wheeler Ridge or Dragon's Back,
over a span of 10-200kyr (Figs 9.20 and 9.26).
Here we examine what happens to such folds over
longer time spans, and how multiple growing
folds and faults may interact in the landscape.
