Geology Reference
In-Depth Information
only places loose constraints on the total tilt
and differential uplift of the range.
the time of deformation,
t
, the tectonic uplift
rate at a point is
U
t
i
/
t
=
[(
Z
i
−
Z
0
i
)
+
E
i
−
(
U
e
i
+
SL
i
)]/
t
(7.10)
Tectonic and surface uplift rates
The average of the tectonic uplift rates computed
for a series of points across a range then defines
a mean rate for the whole range. Note that the
data have to be collected across a broad enough
area to encompass the region that will respond
isostatically to the removal of material by erosion.
Thus, the rigidity of the crust and its effective
elastic thickness, which together set its flexural
wavelength, serve to define the approximate min-
imum dimensions of a study area. The isostatic
response to localized erosion will be compen-
sated and smoothed across this entire area.
Unfortunately, although the approach
described is conceptually straightforward, this
methodology can be applied to relatively few
ranges in the world. The availability of digital
topography makes it easy to calculate the present
mean surface elevation, but defining a reliable
paleoelevation for an ancient surface is commonly
impossible. The optimal strategy for making calcu-
lations of tectonic uplift rates (Fig. 7.26) appears to
require that we study ranges that have experi-
enced large changes in mean elevation, where
good dates are available for former surfaces, where
enough of an original surface is preserved to per-
mit calculation of the amount removed by erosion,
where the amount of sea-level change (SL) is small
compared to the change in mean elevation, and
where some data can serve to define paleoeleva-
tion (
Z
0
i
) of the pre-deformational surface.
A successful study in the Finisterre Range
of Papua New Guinea (Abbott
et al.
, 1997)
exploited a slightly denuded surface of carbon-
ates that had been recently uplifted from
considerable depths in the ocean. Foraminiferal
assemblages were used to estimate the deposi-
tional depth and age of these carbonates. These
estimates constrain both the paleoaltitude and the
duration of deformation (
t
). Although the former
depositional surface has been uplifted up to alti-
tudes of more than 2 km from water depths of
as much as 3 km, much of the original surface is
still preserved. By smoothly connecting these pre-
served surface remnants, an envelope was defined
that represents the pre-eroded, but uplifted,
Recall that the mean surface is the same as the
mean elevation, but that individual points on
the surface may increase in elevation at the
same time as the mean elevation or surface
height is decreasing (Fig. 7.2). Increases in the
mean elevation of a region can only occur in
response to tectonic processes of crustal thick-
ening, flexural support due to bending of rigid
lithosphere, or changes in the density distribution
of the crust and underlying mantle. When
assessing changes in mean surface elevation,
generally we would like to know how much the
mean elevation has changed and what role
isostatic uplift played in raising parts of the
surface.
Tectonic uplift
can be defined as that
portion of the total uplift of the mean surface
that is not attributable to an isostatic response
to unloading. In order to resolve these differ-
ent components of uplift, we need to know
the geometry of the surface at the start of
deformation, the present geometry, the volume
of material eroded off the original surface, and
any changes in the reference frame, such as in
sea level, that are used to assess displacement. If
we want to define rates, then ages for the begin-
ning and end of deformation are also needed.
Initially, we can note that the total bedrock
uplift for a point (Abbott
et al.
, 1997) (Fig. 7.26A)
is defined as
U
i
=
(
Z
i
−
Z
0
i
)
+
E
i
+
SL
i
(7.8)
where, at any point
i
,
U
i
=
total bedrock uplift,
Z
i
=
present topographic elevation,
Z
0
i
=
original
topographic elevation,
E
i
=
thickness of eroded
material, and SL
i
=
change in sea level between
the beginning of uplift and the present
(a sea-level rise is assumed to be positive). The
total bedrock uplift can be expressed as the sum
of a tectonic and isostatic component:
U
i
=
U
t
i
+
U
e
i
(7.9)
where, at point
i
,
U
t
i
=
tectonic component of
uplift and
U
e
i
=
isostatic component of uplift.
Combining the above equations and adding in
