| Geomorphometry may be a powerful tool to describe the characteristics of the landscape’s
response to tectonic signals, but the meaning of morphometric indices is often obscured by
the interplay between the many variables controlling the geomorphological evolution.
Moreover, although the so-called hypsometric integral refers to the basin scale, most indices
are generally derived from the river long profiles and thus focus mainly on the short-term
response of a drainage network to base level change, providing limited information in regions
of older and/or moderate uplift. Here, using the Rhenish shield (western Europe), an area of
moderate Quaternary uplift, as a test case, I attempt first to build an index yielding a
comprehensive view of the stage attained by the landscape’s response and, indirectly, an
evaluation of the timing of the triggering base level change. This index, called R1, is a ratio
of differences between the three integrals linked respectively to the classical basin’s
hypsometric curve, to the main river’s long profile, and at the intermediate level,
to a ’drainage network’s hypsometric curve’. While its ratio form minimizes the
lithological effect on R1, this index is strongly correlated with basin size (regional
correlation coefficients are in the range 0.88-0.93), reflecting the way an erosion wave
propagates from the outlet of a basin toward its headwaters. Therefore, it is not
directly usable as a proxy for relative uplift age. However, one can show that the
relation between R1 and basin size is theoretically expected to change with time.
Following uplift, the slope Sr of the linear relation R1 = f(lnA) first increases
rapidly but briefly, then it gradually diminishes over several million years. This is
fully confirmed by the analysis of R1 and Sr in the Rhenish shield study area.
Once its initial increase is completed (assumedly in a few ten thousand years),
Sr appears to be a reliable indicator of relative uplift (or any other cause of base
level lowering) age. The next natural step of the research is to estimate the R1 and
Sr values in areas worldwide where the age of a recent base level change is more
or less well constrained, in order to assess whether a quantitative relation might
be derived, that would allow one to infer absolute ages of uplift from the index
values. At present, the Mattole and Big River catchments near the Mendocino triple
junction, and the drainage networks of eastern central Scotland, of the Montagne Noire
(southern Massif Central), and of several subareas of the Bohemian massif have been
investigated. The new data tend to indicate that, provided one corrects the indices for
second-order effects such as catchment elongation (influencing the rate at which erosion
diffuses in the entire catchment), uplift age has an exponential dependence on Sr. |