Geology Reference
In-Depth Information
cooling histories, stratigraphic data from bounding
basins, the topographic character of entire ranges,
and the geographic character of large river
catchments can each provide useful clues to the
interpretation of the tectonic-geomorphic history
(Allen, 2008). We will see, for example, that
increasingly available digital data sets, such as
digital elevation models (DEMs), allow rapid
statistical characterization of landscapes at these
scales, that catchment geometries can serve to
delineate long-term interactions among faults, and
that patterns of bedrock cooling ages and hillslope
angles can sometimes be used to assess orogenic
steady state.
energy. What can tectonic geomorphology tell us
about how these plateaus grow and decay? Do
they grow steadily or in pulses? What controls
their upward and outward growth? We introduce
several of these “long-time-scale” problems here
and then delve into them in more depth later.
Steady-state topography
The concept of a dynamic equilibrium was
developed many decades ago (Hack, 1975) and
is now commonly described as a “steady state”
when applied to mountain ranges (Fig. 1.2).
Steady state is an appealing concept because it
suggests a dynamic balance whereby rock uplift
is counterbalanced by erosion, such that the
topographic character of a range remains steady
through time despite ongoing deformation. For
example, numerical models of the topography
of a range subjected to sustained deformation
predict that the range will evolve toward a
persistent topographic shape. The resulting
topographic shape is commonly modeled as a
function of three variables: the ratio of the
horizontal to vertical velocity (lateral versus
vertical advection of rock, or frontal accretion
versus underplating); the erosional efficiency of
rivers; and the diffusivity of hillslopes (Willett
et al
., 2001). Although these models typically
have an accretionary flux from one side
(Fig. 10.1), the inbound material can be added
to the orogenic wedge in two different ways:
frontal accretion or underplating. In models
characterized by rapid lateral advection, the
predicted steady-state topography is typically
asymmetric (Fig. 10.1). Similarly, lateral gradients
in rainfall tend to push the drainage divide away
from the moisture source (Willett, 1999), whereas
either underplating, more efficient erosion, or
more rapid diffusion are predicted to engender
more symmetrical topography. The challenge for
tectonic geomorphologists is to examine actual
mountain ranges and to use their characteristics
along with model predictions to gain insights into
the controls on topographic characteristics and
patterns of erosion and deformation. Whereas
model predictions may seem straightforward,
many questions remain to be answered with
respect to actual landscapes: What characteristics
Hot topics and unresolved questions
Over the past decade, several “hot topics” have
motivated numerous tectonic-geomorphic studies
at long time scales. For example, the appealing
concept of a topographic steady state has been
difficult to document with field data. Given the
many difficulties of reconstructing paleoeleva-
tions, particularly in mountain ranges experienc-
ing active erosion (see Chapter 7), we cannot
hope to be able to compare present and former
topographic characteristics in most mountain
ranges. Consequently, different approaches are
needed, such as the use of along-strike ergodic
substitutions in propagating ranges for which the
tectonic forcing through time is well known.
Similarly, the common correlation of higher
stream power with more rapid erosion rates
suggests that climate could affect tectonic defor-
mation: where erosion is more intense, the loss
in mass from an orogen might be expected
to induce a tectonic response whereby the influx
of rock (via faulting) into the eroded region is
enhanced (Whipple, 2009). But, does this inter-
action occur? How can we demonstrate it?
In the context of global topography, orogenic
plateaus, such as the Tibetan Plateau in the
Himalayan orogen or the Puna-Altiplano Plateau
in the Andes, represent some of the largest topo-
graphic features on Earth. These plateaus influ-
ence climate by redirecting the jet stream and
guiding storm tracks. Their steep topographic
margins represent huge gradients in potential
