Geology Reference
In-Depth Information
4-6 km
3 km
Hanging-Wall Erosion
fluvial strata
syn- & post-uplift strata
eroded to expose
Permian source beds
Potwar Plateau:
piggyback basin
conglomerate, including
Permian clasts
incipient
Salt Range
foreland strata
of known thickness
and taper
fluvial molasse sediment
E ocene to Paleozoic strata
Eocambrian salt
P recambrian
basement
footwall ramp (reactivated normal fault)
footwall ramp
N
2.5-3 km of strata eroded over ~0.5 Myr
(mean rate: 5-6 mm/yr)
S
Fig. 7.8 Erosion rates from uplifted hanging-wall strata.
Calculations of erosion rates based on combined structural and stratigraphic data in the Salt Range, northern Pakistan.
Three ingredients to calculate rates are available here: thickness and shape of the uplifted hanging wall; timing of
initiation of fault slip; and timing of the first appearance of clasts eroded from the Permian rocks beneath the
foreland strata. These data define erosion rates that essentially balance rock uplift rates, a common occurrence
when poorly lithified rocks are raised above base level. Modified after Burbank and Beck (1991).
This unroofing requires about 3 km of erosion
during 0.5 Myr, a rate of approximately 6 mm/yr
or 6 km/Myr!
Such rapid erosion rates are not atypical on a
global basis for sites where Cenozoic terrestrial
foreland strata are uplifted above local base level
(Burbank et  al. , 1999; Dadson et  al. , 2003). In
general, erosion rates of these weakly cemented
strata appear to be closely equivalent to rock
uplift rates, such that little topographic relief
develops within the uplifted foreland strata
(Plate 1D). Consider, for example, the Himalayan
foreland in Nepal where shortening has exceeded
10 km and rock uplift rates exceed 10 mm/yr
(Lavé and Avouac, 2000). Despite these very
rapid rates, relief in the hanging wall of active
foreland thrust faults (the Main Frontal Thrust) is
typically just a few hundred meters, less than 5%
of the total rock uplift. Hence, rates of erosion
and rock uplift are nearly in balance. In such
sites, if the fault geometry and the slip rate along
it can be determined and if hanging-wall relief is
small, rates of erosion can be readily deduced.
Several challenges confront this type of
geometric analysis. In order to reconstruct the
eroded mass, its pre-deformational geometry
must be assumed or reconstructed. In regions
where predictable variations in stratal thick-
nesses occur, such as in many foreland basins
or passive margins, reconstructions can be done
with considerable confidence. Given dated strata
that either are cut by or overlie the fault that
accommodated the displacement, the duration of
deformation can be defined. Commonly, however,
one does not know how closely the dates limit
the deformation; at numerous sites, either the
beginning or end of deformation, but more
rarely  both, can be defined. In many terrestrial
sequences, moreover, it is very difficult to
determine accurately the relevant stratigraphic
ages. In these sorts of studies, magnetic polarity
stratigraphy has provided a powerful tool for
creating precise ages for Cenozoic terrestrial
strata and in calculating rates of deformational
and erosional processes (Burbank and Raynolds,
1984; Fang et al. , 2005; Jordan et al. , 1988).
Topographically constrained erosion rates
If the geometry and age of a former land surface
can be reconstructed, a straightforward differ-
encing with the modern land surface yields a
mean erosion rate. For example, individual grow-
ing folds experience dissection as they are ele-
vated above local base level. Typically, dissection
is focused along river valleys and concavities,
such that segments of the pre-deformational
land surface may be preserved in a folded, but
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