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Asymmetric Orogenic
Topography
Orogen Area versus Climate Erosivity
shift in divide
lag time a
lag time b
1.25
2
Erosive (wet)
orogen
area
1
1.00
climate
erosivity
Time 1
Time 2
Time 3
A
Non-Erosive (dry)
Erosion Flux
0.75
0
B
0
123456789
0
Time (My)
height
U vertical
U horiz
Accretionary
Flux
Fig. 10.2 Interplay between climate erosivity and
orogenic cross-sectional area.
In this numerical model of a critically tapered wedge, the
erosivity of climate regularly varies on a million-year time
scale and the orogenic cross-sectional area responds to
changes in erosion rate by shrinking when erosion rates
are high during wet phases and expanding during dry
phases. Note the lag time for the orogenic area to respond
to the climate change. Subscript “0” denotes initial
conditions. Modified after Meade and Conrad (2008).
ic
Two-Sided
Orogen
Model
Deforming Orogen
S
Undeforming Plate
Fig. 10.1 Predicted topographic evolution in a
collisional orogen.
A. Orogenic topography in which fluvial incision and
hillslope diffusion drive erosion that competes with
vertical uplift and horizontal advection. The progressive
lateral shift in the drainage divide results from a high
ratio of horizontal to vertical velocity. The “Time 3”
panel represents a topographic steady state that has
become independent of the duration of accretion or
erosion. B. Key geometric elements of a numerical
model dominated by frontal accretion (little to no
underplating). At steady state, the accretionary influx is
balanced by the erosional efflux. Note the predicted
particle pathways through the orogen and the spatial
variation in the ratio of horizontal to vertical velocity.
Point S is a fixed point where incoming continental
crust detaches from the incoming mantle and obducts
on to the “backstop.” Modified after Willett et al . (2001).
2009; Roe, 2005; Smith, 1979). For many decades,
the prevalence of young cooling ages in many
of  the world's large mountain belts suggested
to  geologists that late Cenozoic uplift of the
mountains had helped trigger widespread alpine
glaciation. This idea was turned on its head by
Molnar and England (1990), who argued that
accelerated erosion by glaciers caused young
cooling ages and drove isostatic uplift of ranges.
Since that time, numerous studies have sought
linkages among climate change and the tectonic
evolution of orogens. The conceptual rationale
behind such linkages is generally straightforward,
especially in the context of critically tapered
orogenic wedges (Dahlen and Suppe, 1988):
spatial variations in precipitation should drive
correlative variations in erosion; and the
resulting removal of mass from localized regions
of an orogen should induce a restorative tectonic
influx (a return to critical taper), commonly by
spatially focused strain. This rationale has been
used to predict that the zones of most active
thrusting in an orogen will depend on the side
from which storms approach the orogen (Willett,
1999). At even larger scales, surface erosion has
been predicted to “draw” channelized flow of
lower crust toward the surface (see Fig. 1.9),
serve to define a steady-state landscape? At what
temporal and spatial scales is it relevant to assess
steady state? How do we tease out the effects of
asymmetric climate forcing versus asymmetric
tectonic forcing? And how do we account for all
the fluxes into and out of orogens?
Climate-tectonic interactions
High topography resulting from tectonic
deformation can be observed to influence
climate. Orographic precipitation tends to focus
rainfall on the windward side of ranges and to
create rain shadows on their lee sides (Galewsky,
 
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