Geology Reference
In-Depth Information
A
eroded material
undissected uplifted
surface
E
i
present topography
Z
i
paleosea level
sea-level change
present sea level
Rock, Surface, &
Tectonic Uplift
original surface prior
to deformation
Z
oi
B
isostatic
component
of uplift
mean surface
mean surface
tectonic
component
of uplift
original surface
Fig. 7.26
Topographic variables needed to calculate the tectonic component of uplift.
A. Inputs to a calculation of tectonic uplift. Observables are the present topography (
Z
i
) and present sea level.
Variables that are difficult to define can include the depth of eroded material (
E
i
) and the position of the original
surface (
Z
0
i
) with respect to a known reference plane (like former sea level), both of which are required to calculate
total rock uplift. Modified after Abbott
et al.
(1997). B. Effects of erosion on isostatic uplift. Given an identical amount
of bedrock uplift, the relative amount of erosion determines the proportion of the total rock uplift that is due to an
isostatic response. In each case, the isostatic contribution is about five-sixths of the mean erosion. The remaining rock
uplift is attributable to tectonic effects (crustal thickening or buoyancy effects).
surface. Subtracting the modern topography from
this envelope provides an estimate of the amount
of eroded material (
E
), and an isostatic response
to this unloading can be calculated. In the
Finisterre Range, Abbott
et al.
(1997) showed that
the uncertainties resulting from erosional isostatic
compensation and paleo-sea-level estimates are
small (
<
10%) compared to a tectonic uplift rate
that averages 1-2 mm/yr over the past 2-3 Myr. In
the Finisterre Range, the small contribution of ero-
sional isostasy to uplift is not surprising, because
the raised surface is still only slightly dissected. In
contrast, in settings where erosion has removed
all but a few remnants of a former surface, isos-
tasy may drive much of the observed rock uplift at
isolated points in the landscape (Fig. 7.26B).
Sometimes it can be possible to use large-scale
patterns of deposition to discriminate between
rock uplift due to tectonic loading versus
erosionally driven isostatic uplift. Both of these
mechanisms can drive local surface uplift of
peaks, but in the former case, uplift results from
crustal thickening, whereas in the latter case, it
results from erosional thinning of the crust.
Collisional mountain belts typically abut basins
that receive sediments derived from the adjacent
mountains. Two aspects of the basin fill and river
systems can distinguish between tectonic loading
and erosional unloading (Burbank, 1992). During
crustal thickening that is associated with tectonic
loading, maximum basin subsidence occurs near
the mountain front. This depression is commonly
occupied by a longitudinal river flowing parallel
to the mountain front in a medial to proximal part
of the basin (Fig. 7.27). Transverse rivers debouch-
ing from the mountains typically join this trunk
