Geology Reference
In-Depth Information
thereby localizing deformation (Beaumont
et al
.,
2001, 2004). Several modeling studies of climate-
orogen coupling (Meade and Conrad, 2008;
Whipple and Meade, 2004) suggest that the
width and cross-sectional area of an orogen
should vary in concert with the effective erosivity
of a given climate (Fig. 10.2). Such studies
suggest diverse observations that could serve to
test such models. For example, a switch to a
more erosive climate should cause an increased
erosional efflux. If the convergence rate is held
steady across an orogen that can be represented
as a critically tapered wedge, the increased rate
of erosion should be balanced by a narrowing of
the width and cross-sectional area of the defor-
ming orogen. Hence, we should observe coeval
abandonment of distal faults, acceleration of
deformation in the hinterland, an increase in the
exhumation rate concomitant with decreased
cooling ages in the orogen's core, and a slowing
of the rate of subsidence in adjacent forelands
as the size of the orogenic load decreases
(Whipple, 2009). Because collection of such a
diverse array of data is uncommon and requires
a truly interdisciplinary suite of observations,
testing a proposed climate-tectonic coupling
provides a formidable challenge.
grow upward and outward. Do plateaus grow
steadily upward as the lithosphere systemati-
cally thickens beneath them, or do they rise in
more abrupt bursts, such as those hypothesized
to occur when part of a thickened and densified
mantle detaches, sinks, and is replaced by
warmer, buoyant asthenospheric material
(Molnar
et al
., 1993)? If surface uplift is pulsed,
is there a geomorphic signature that records the
change from slow to more rapid uplift?
Plateaus also grow outward, as well as upward.
Again, we commonly do not know whether that
growth is steady or pulsed, and what tectonic
style modulates outward growth (Fig. 10.3). The
traditional model for outward growth posits
propagation of thrust faults toward the foreland
that gradually extend the margin of the plateau.
In contrast, some recent observations suggest
that the lower crust beneath at least part of the
Tibetan Plateau is partially melted (Nelson
et al
.,
1996). If so, then gravitational gradients might be
expected to drive this flow outward and could
extend the plateau laterally, a phenomenon that
could occur with little or no faulting at the surface
(Fig. 10.3B). A third mode of proposed plateau
growth is a variant of outward thrust propagation.
In this model (Fig. 10.3C), newly formed thrust
faults at the leading edge of a plateau create
piggyback basins that fill with sediment as the
fault accumulates slip. Over time, the increasing
load of the basin fill and the resultant isostatic
subsidence make it more difficult to continue slip
on the existing fault and, thereby, promote
formation of a new thrust closer to the foreland
(Hilley
et al
., 2005). A corollary of this model is
Growth of orogenic plateaus
Thousands of kilometers of interplate conver-
gence over millions of years can create orogenic
plateaus, such as those associated with the
Himalaya and the Andes. Significant unresolved
questions remain about how these plateaus
Modes of Plateau Growth
A
B
C
Outward Thrust Propagation
Lower Crustal Outward Flow
Basin Filling & Propagation
Fig. 10.3
Modes of growth of orogenic plateaus.
A. Traditional model of progressive outward propagation of bedrock thrust faulting. B. Partial melting of the lower
crust beneath a thickened plateau and outward flow in response to gravitational gradients across the plateau margin
drive expansion without significant surface faulting. C. Sediment fills in piggyback basins create loads and subsidence
that favor formation of more distal thrust faults.
