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topography were explored. Many of the salient
characteristics of such mountain ranges are cap-
tured well in the model, including both triangu-
lar facets at the mountain front and benches
in spur ridges. Exploration of the relative
importance of various surface processes and
comparisons with actual topography demon-
strated that bedrock-involved landsliding was
likely the dominant process in this setting.
on the notion that a strong coupling could exist
between (i) the surface processes that actively
rearrange the load on the crust and (ii) the defor-
mation field at depth. This coupling is exciting in
that it allows the geomorphology and those who
study it into the tectonic game. This proposed
coupling is illustrated in the work of Willett
(1999) (Figs 10.1 and 10.22), who has constructed
a model of collisional orogens with examples
from Taiwan and the Himalaya in mind. These
models examine cross-sections oriented normal
to the orogenic collision, and the rheology of
the crustal materials involved in the collision is
assessed explicitly (Fig. 11.19). The surface-
process component of this model is simple, as it
must be one dimension. Reflecting the assump-
tion that the dominant surface process is bedrock
fluvial incision and that the hillslopes will essen-
tially come along for the ride, Willett uses an inci-
sion rule based on stream power. The spatial
distribution of power varies as a function of the
precipitation falling upstream of the node being
assessed. Orographic forcing of the precipitation
is allowed. Importantly, these models illustrate
the strong coupling of the spatial pattern of ero-
sion rates, themselves governed by patterns of
precipitation, with the deep crustal strain-rate
field. This class of models quantifies the concep-
tual ideas of Koons (1989) that show the potential
influence of the atmospheric conditions (primar-
ily the wind direction) on the asymmetry of the
strain field in collisional settings (Fig. 11.19). That
this degree of coupling can be simulated in appar-
ently realistic models has acted as a catalyst for
a wide range of earth scientists to collect the sort
of field information (topography, meteorology,
exhumation patterns) that will act both as tests of
the present models, and as constraints on further
models at the mountain range scale. For example,
recent work in New Guinea reveals that this cou-
pling has been operating to localize the strain in
the collisional orogen in Iryan Jaya (e.g., Weiland
and Cloos, 1996).
In their review of geomorphic modeling,
Tucker and Hancock (2010) illustrate the utility
of the recent version of the CHILD (Channel-
Hillslope Integrated Landscape Development)
model (Tucker
et al
., 2001). In this and several
of the other existing landscape evolution models,
Orogen-scale models
In a series of models that extended the early
work and ideas of Koons (1989) on the Southern
Alps, Beaumont and coworkers (e.g., Beaumont
et al
., 1992, 1996; Willett
et al
., 1993) have added
interactive channels and an explicit treatment
of both the geometry and the rheology of the
crust in sometimes quite complicated collisional
settings. The crust is allowed to deform by two
mechanisms: shallow crust behaves as a cohe-
sionless frictional-plastic (Coulomb) material
with a specified yield strength; at depth, the
deformation mechanism alters to one of ther-
mally activated power-law creep. The switch-over
(loosely speaking, the brittle-ductile transition)
between these two mechanisms is determined
dynamically within the model and depends on
the thermal and mineralogical structure. In the
surface-process components of these models, the
code is simplified considerably by treating both
hillslope and channel processes in any particular
node (see Kooi and Beaumont, 1994, 1996).
In an important contribution, Kooi and
Beaumont (1996) have explored more generic
mountain range evolution (Fig. 11.18). They
illustrate that the older conceptual models of
many major geomorphologists as diverse as
King, Gilbert, and Davis (Fig. 1.2), each of whom
developed their conceptual models with parti-
cular real-world landscapes in mind, can be
illustrated with such a generic model by paying
attention to one or another phase of the
development, or by setting the tectonic and
surface-process rates differently.
Considerable attention has been devoted to the
two-sided orogen (Koons, 1990; Willett
et al
.,
1993; Willett, 1999). Much of this work followed
