Geology Reference
In-Depth Information
the mean surface (Fig. 7.2). Each of these types
of uplift is typically referred to some agreed-
upon reference frame, such as the geoid or mean
sea level. Relative uplift can also be useful to
define, particularly at a local scale. By relative
uplift, we refer to differential displacement of
some point or points relative to others. For
example, when an anticline grows, rocks in the
core of the fold are uplifted relative to its flanks.
Commonly, the scale of a fold is such that there
is no geophysical response to localized crustal
thickening. Even if the whole area is subsiding
and the mean surface is lowering, documenta-
tion of relative uplift within a fold remains
useful, because it carries implications for the
local tectonics and erosion. Within a local
reference frame, observations of relative uplift
serve to pinpoint patterns of deformation.
T
2
T
1
T1T2erosion/deposition
Tectonic
wedge
upper slope
T
2
wedge
T
1
wedge
basal slope
Fig. 7.22
Collisional mountain belt as an orogenic
wedge.
Simple model of a growing orogenic wedge at two
stages (T
1
, T
2
). As long as the material properties remain
constant, the wedge taper is unchanged, and the wedge
grows self-similarly through frontal or basal accretion.
Thrusting within the wedge and deposition on its
surface can temporarily increase the taper, which will
then decrease back to its critical taper via some
combination of erosion, extension, or forward
propagation of the toe of the wedge.
landsliding (Hovius
et al.
, 1997), glacial erosion
(Koppes and Hallet, 2006), and river incision
(Burbank
et al.
, 1996b; Finnegan
et al.
, 2008)
suggest that geomorphic erosion rates of up to
10 mm/yr can be sustained for perhaps millions
of years in rapidly deforming mountains. Thus, it
is no longer clear that rapid unloading or decom-
pression, such as that associated with some ultra-
high-pressure rocks (Hacker, 2007), is exclusively
caused by lithospheric processes. In fact, docu-
mented erosion rates may be fast enough to
accommodate nearly all of the estimated rates of
unloading. This erosional capability does not,
however, discount the likelihood that extension,
diapirism, or delamination is implicated in many
of the rapidly cooling orogens; it simply means
that alternative explanations involving high rates
of erosion should be investigated.
Marine terraces
One of the classic means for documenting uplift
of rocks comes from studies of marine terraces. As
described in Chapter 6 on paleoseismology, a
terrace of known age and elevation above present
sea level can be used to compute a rate of bed-
rock uplift (Fig. 6.10), if the position of sea level at
the time of its formation is known. Correlation of
multiple terraces in a single transect can define
changes in rates through time, whereas regional
studies of dated terraces can yield two- and three-
dimensional bedrock uplift patterns (Chappell,
1974). Where terraces are well preserved, dating
of these terraces to define their appropriate
correlation to the eustatic record often provides
the biggest challenge (Anderson and Menking,
1994; Perg
et al.
, 2001). Where dissection has
been extensive, however, simple identification of
former terraces within the landscape can be very
difficult. On the South Island of New Zealand,
Rates of uplift
A distinction has already been drawn between
rock uplift, surface uplift at a point, and uplift of
Fig. 7.21
(
cont'd
)
are still zero. Rock uplift and cooling at 12-Ma have raised the former PAZ or PRZ and are
recorded by a zone with nearly homogeneous 12-Ma ages. The previous cooling event (now at 60 Ma) is preserved
in the uppermost rocks. B. Structural geometry and sample locations in the White Mountains of California. Key to the
interpretation is the linear projection of the Miocene unconformity because the paleodepth of each sample in the
Miocene is measured beneath this projected surface. C. Cooling ages for apatite fission-track and (U-Th)/He plotted
against paleodepth. Each system reveals a PAZ or PRZ below which is a vertical zone of
∼
12-Ma ages. Their maximum
vertical extent (
∼
2 km for the (U-Th)/He ages) indicates the approximate magnitude of erosion at 12 Ma. A more
recent interval of Pliocene faulting, uplift, and cooling is recorded by the lowest helium samples.
