Rates of erosion and uplift - Tectonic Geomorphology - page 227

between about 60 and 110 ° C, with faster anneal-
ing at higher temperatures. This thermal window
is termed the “partial annealing zone.” Similarly,
with (U-Th)/He dating of apatite, a partial reten-
tion zone exists between about 40 and 80 ° C.
When the tops and bottoms of these partial
retention or annealing zones can be captured in
thermochronological sampling transects, they
can help to delineate former geothermal gradi-
ents and define the magnitude of rock uplift (Fig.
7.21). A recent study from the White Mountains
of California used apatite from each sample for
both fission-track and (U-Th)/He dating and suc-
ceeded in delineating exhumed partial retention
and/or annealing zones for each dating tech-
nique (Stockli et al. , 2000). The ages for samples
beneath the base of these zones define the onset
of rock uplift and accelerated erosion at 12 Ma.
Both techniques capture the penultimate episode
of major cooling around 55 Ma, and because the
full partial annealing and/or retention zone is
encompassed by each, a paleogeothermal gradi-
ent and the magnitude of rock uplift since 12 Ma
can be estimated for each data set (Fig. 7.21).

of new material at its toe (or anywhere else)
in order to maintain its taper. If any of the
controlling conditions or properties change, the
wedge should adjust to a new equilibrium taper.
Geologically, this model implies that an
orogenic wedge should undergo constant defor-
mation throughout its mass to accommodate the
irregular addition of new material to the wedge,
losses of material through erosion, and redistri-
bution of mass through deposition or faulting.
Geological materials, however, are not truly
Coulomb-like in their behavior. Rather than
deforming everywhere, they break and deform
along discrete planes (faults). Thus, raising of
the surface of the wedge in order to counteract
erosion typically occurs through thrust faulting
or underplating. When conditions change such
that the taper of the wedge becomes too
great (supercritical), it can adjust itself by three
different mechanisms: propagation of the toe
(the leading edge) of the wedge, erosion of the
elevated surface, or normal faulting (extension).
Normal faulting causes a rapid thinning of the
wedge and can promote nearly isothermal uplift
and decompression in the underlying rocks.
Large-scale normal faulting under regimes
of contraction has been identified in many
collisional orogens. Many Cenozoic collisional
ranges, including the Himalaya, Alps, and
Taiwan's Central Range, have experienced
significant extension (Burchfiel et al. , 1992;
Crespi et al. , 1996; Platt et al. , 1998; Ratschbacher
et al. , 1989). In most of these ranges, contraction
and extension were coeval, rather than sequen-
tial. Commonly, the rate and amount of displace-
ment are difficult to establish, so that tectonic
denudation rates are not well constrained. When
pressure-temperature-time studies of the rocks
beneath a normal fault show rapid depressuriza-
tion, however, the magnitude, timing, and rate
of denudation can be estimated more precisely.
Most tectonicists have traditionally ascribed
rapid decompression and rapid cooling to
tectonic denudation, that is, normal faulting and
extension. Pressure changes on the order of
1 kbar/Myr ( ∼ 3 km/Myr) were commonly thought
only to be possible via normal faulting. Indeed,
this interpretation may be correct. However,
recent measurements of surface erosion rates by

Tectonic denudation versus
geomorphic erosion

Tectonic denudation occurs when faulting thins
the crust and brings formerly buried rocks closer
to the land surface. Such thinning can abruptly
redistribute large loads from an elevated region
on the surface to a topographically lower area,
and it promotes an isostatic response that can
locally cause bedrock uplift.
It has been suggested that many compressional
tectonic regimes, such as those found at conver-
gent plate margins, can be modeled as critically
tapered wedges of a Coulomb material (Davis
et al. , 1983) that is everywhere at the point of
failure. Like a wedge of snow driven before a
plow, the taper of the wedge is defined as the
angle between its upper and lower surfaces and
is a function of the frictional coupling at its base,
the dip of the basal detachment surface, and the
material properties of the wedge itself (Fig. 7.22).
Under conditions of constant shortening and
consistent material properties and geometries,
the wedge will adjust internally to the addition