444
ers, they indieate the amount of denudation (equal to
the thickness of the removed sequence) for the time
period between the volcanic aetivity and the age of
the present land. The amount of erosion on graben
shoulders (Fig. 9.15d; rift flank uplift) has been assessed in several regions. This is possible when strata
of the same age as the eroded ones are preserved
within the graben fill or at some lateral distanee from
the graben strueture (also see e.g. Roll 1979). Dating
of the rifting or relies of a former, higher landscape
along an esearpment may indieate the time involved
in esearpment retreat.
Digital topographic data in a geographical infonnation
system (GIS) can considerably help in refining landfonn
reconstruction and in assessing mass balances of eroded
rocks (e.g. Roessner and Strecker 1997). For example in
the Central Kenya Rift, East Africa, ashflow tuffs of known
age (~3 Ma) mark the remnants of a fonner plateau surface
which was subsequently affected by faulting and valley
incision. Using this method, the long-tenn mean denudation rate of an area of 710 km 2 was found to have been
about 12 mlMa.
A young example of escarpment retreat has been
described from the Gulf of Suez and its junction with the
Red Sea (Steckler 1994). The pre-rift strata are composed
of the Nubian sandstone (Paleozoic up to early Cretaceous
in age) and cliff-fonning Cretaceous-Eocene carbonates.
The relatively weak sands tone is more easily eroded than
the carbonates in the arid climate; the underlying basement
is intersected by wadis. The rate of escarpment retreat varies inversely with the sediment thickness (400 to 2000 m).
The average rate was 6 kmlMa during the past 20 Ma.
In southem Gennany the escarpment fonned by upper
Jurassic limestones and older Jurassie strata (500 to 600 m
in total thickness) has retreated at a rate of about 2 kmlMa
(based on data in Geyer and Gwinner 1986).
The amount and average rate of paleo-denudation
can also be derived for older time periods. For exampIe, the end of an aneient erosion period on top of
tilted and folded strata ean be dated by the overlying
rocks; the onset of erosion and the volume of eroded
material may be found from the age and structure of
the sediments below the uneonformity (Fig. 9.l5e).
Recently, thermobarometry and ehronothermometry,
indieating deep-reaehing exhumation, have been applied to a number of regions to determine long-term
rock uplift and denudation (Fig. 7.l5g; cf. Seet. 7.6
and Table 9.l). "Tectonic" erosion is not considered
here.
These are only some examples of a variety of
geomorphological and geologieal methods whieh can
be used to make estimates for loeal and more regional paleo-denudation rates.
Chapter 9 Denudation and Mass Transport
9.5.2 Long-Term Denudation Rates (Overview)
Data Base and Discussion
Some data for long-term average denudation rates
derived from landscape reeonstruction, sediment volumes (denudation-aceumulation systems, Seet. 11.2),
and exhumation studies are listed in Table 9.1. The
data are grouped aceording to regions of different
relief and time spans studied (Holoeene, Quatemary,
longer periods). Some brief eomments on these values are necessary:
- Denudation values for the Pleistoeene and to some
extent also for the Holoeene confirm the impact of
relief but the data vary signifieantly from region to
region.
Some ofthis scattering may be caused by drastic variations
in climate and the possibility that weathering and sediment
transport are still in a kind of transitional state. For exampie, glacial sediments within and at the foot of many mountain ranges may still being reworked and incorporated into
Holocene deposits.
The accuracy of longer-tenn paleo-denudation rates depends to a large degree on our knowledge about the geo10gical history and the chronostratigraphic dating of rocks
preserved in the study region (e.g. Gilchrist et al. 1994).
Some very high "denudation rates" found by the fission
track method in Alpine regions include "tectonic erosion"
(cf. Sect. 9.6.2, Rates ofUplift).
- Long-term denudation rates of large regions of
mixed relief are mostly in the same order of magnitude (20 to 70 mmlka), regardless of the time period
considered. This is true of the examples from North
Ameriea, Europe, Afriea and India (drainage areas of
the Mississippi river and the Blaek Sea, East Midlands in England, Southwest Germany, rifted margins
along the Atlantie and Indian oeeans).
- The lateral retreat of escarpments associated with
rift flank uplift or erosion along the front of foldthrust belts may operate faster than vertical exhumation (e.g. Masek et al. 1994; Einseie et al. 1996).
There are not sufficient data to direetly compare the
results from different time periods. The values from
the Alps, the Pyrenees, and Southwest Afriea indieate that there is no fundamental difference between
Holocene and long-period denudation rates, although
the data represent neither exaetly the same region nor
have been assessed by using the same method.
Erosion at the base of large iee sheets (40 up to
400 mmlka) does not reaeh the extremely high values
reported from mountain glaciers.
Further data for long-tenn denudation are compiled, e.g.,
by Biennan (1994), Walling and Webb (1996), and in
some further references listed above.
ers, they indieate the amount of denudation (equal to
the thickness of the removed sequence) for the time
period between the volcanic aetivity and the age of
the present land. The amount of erosion on graben
shoulders (Fig. 9.15d; rift flank uplift) has been assessed in several regions. This is possible when strata
of the same age as the eroded ones are preserved
within the graben fill or at some lateral distanee from
the graben strueture (also see e.g. Roll 1979). Dating
of the rifting or relies of a former, higher landscape
along an esearpment may indieate the time involved
in esearpment retreat.
Digital topographic data in a geographical infonnation
system (GIS) can considerably help in refining landfonn
reconstruction and in assessing mass balances of eroded
rocks (e.g. Roessner and Strecker 1997). For example in
the Central Kenya Rift, East Africa, ashflow tuffs of known
age (~3 Ma) mark the remnants of a fonner plateau surface
which was subsequently affected by faulting and valley
incision. Using this method, the long-tenn mean denudation rate of an area of 710 km 2 was found to have been
about 12 mlMa.
A young example of escarpment retreat has been
described from the Gulf of Suez and its junction with the
Red Sea (Steckler 1994). The pre-rift strata are composed
of the Nubian sandstone (Paleozoic up to early Cretaceous
in age) and cliff-fonning Cretaceous-Eocene carbonates.
The relatively weak sands tone is more easily eroded than
the carbonates in the arid climate; the underlying basement
is intersected by wadis. The rate of escarpment retreat varies inversely with the sediment thickness (400 to 2000 m).
The average rate was 6 kmlMa during the past 20 Ma.
In southem Gennany the escarpment fonned by upper
Jurassic limestones and older Jurassie strata (500 to 600 m
in total thickness) has retreated at a rate of about 2 kmlMa
(based on data in Geyer and Gwinner 1986).
The amount and average rate of paleo-denudation
can also be derived for older time periods. For exampIe, the end of an aneient erosion period on top of
tilted and folded strata ean be dated by the overlying
rocks; the onset of erosion and the volume of eroded
material may be found from the age and structure of
the sediments below the uneonformity (Fig. 9.l5e).
Recently, thermobarometry and ehronothermometry,
indieating deep-reaehing exhumation, have been applied to a number of regions to determine long-term
rock uplift and denudation (Fig. 7.l5g; cf. Seet. 7.6
and Table 9.l). "Tectonic" erosion is not considered
here.
These are only some examples of a variety of
geomorphological and geologieal methods whieh can
be used to make estimates for loeal and more regional paleo-denudation rates.
Chapter 9 Denudation and Mass Transport
9.5.2 Long-Term Denudation Rates (Overview)
Data Base and Discussion
Some data for long-term average denudation rates
derived from landscape reeonstruction, sediment volumes (denudation-aceumulation systems, Seet. 11.2),
and exhumation studies are listed in Table 9.1. The
data are grouped aceording to regions of different
relief and time spans studied (Holoeene, Quatemary,
longer periods). Some brief eomments on these values are necessary:
- Denudation values for the Pleistoeene and to some
extent also for the Holoeene confirm the impact of
relief but the data vary signifieantly from region to
region.
Some ofthis scattering may be caused by drastic variations
in climate and the possibility that weathering and sediment
transport are still in a kind of transitional state. For exampie, glacial sediments within and at the foot of many mountain ranges may still being reworked and incorporated into
Holocene deposits.
The accuracy of longer-tenn paleo-denudation rates depends to a large degree on our knowledge about the geo10gical history and the chronostratigraphic dating of rocks
preserved in the study region (e.g. Gilchrist et al. 1994).
Some very high "denudation rates" found by the fission
track method in Alpine regions include "tectonic erosion"
(cf. Sect. 9.6.2, Rates ofUplift).
- Long-term denudation rates of large regions of
mixed relief are mostly in the same order of magnitude (20 to 70 mmlka), regardless of the time period
considered. This is true of the examples from North
Ameriea, Europe, Afriea and India (drainage areas of
the Mississippi river and the Blaek Sea, East Midlands in England, Southwest Germany, rifted margins
along the Atlantie and Indian oeeans).
- The lateral retreat of escarpments associated with
rift flank uplift or erosion along the front of foldthrust belts may operate faster than vertical exhumation (e.g. Masek et al. 1994; Einseie et al. 1996).
There are not sufficient data to direetly compare the
results from different time periods. The values from
the Alps, the Pyrenees, and Southwest Afriea indieate that there is no fundamental difference between
Holocene and long-period denudation rates, although
the data represent neither exaetly the same region nor
have been assessed by using the same method.
Erosion at the base of large iee sheets (40 up to
400 mmlka) does not reaeh the extremely high values
reported from mountain glaciers.
Further data for long-tenn denudation are compiled, e.g.,
by Biennan (1994), Walling and Webb (1996), and in
some further references listed above.