Geomorphometric dating of uplift: The case of the NW European foreland of the Alpine arc
O 3.3 in Interplay of endogenous and exogenous processes, climate change, and their feedbacks
Though topography probably contains as much information as the sediment record about the uplift history of elevated regions, separating the time factor from other relief-forming factors and quantifying it remains difficult and uncertain tasks. While inverse modelling of river long profiles (Pritchard et al., 2009) has been increasingly used recently to produce uplift histories of large basins, a concurrent approach relies on catchment-scale morphometry for a range of catchment sizes in order to date the most recent uplift event that imposed rejuvenation of fluvial landscapes. A composite metric R describing the degree of landscape incision based on three nested hypsometric integrals (of whole catchment, drainage network, and trunk stream) has been shown to be predominantly controlled by catchment size A and uplift age (Demoulin, 2011), leading to take the slope of the linear relation between R and log A as a derived index SR directly related to the uplift age (Demoulin, 2012).
This approach appeared especially adapted to explore the causes of Plio-Quaternary uplift in NW Europe, through mapping the recently uplifted areas and evaluating relative uplift ages. Indeed, it is still debated whether uplifted regions across NW Europe attest to lithospheric buckling in front of the Alpine arc or were randomly produced by a swarm of baby plumes. We calculated R for more than 7000 basins >15 km2 and mapped SR spatial variations by moving a 60-km-wide window along five N- to NW-trending zones of alternating Paleozoic massifs (Massif central-Brittany; Rhenish shield; Bohemian massif) and Meso-Cenozoic basins (Paris basin; Franconian basin) covering the whole NW European platform in front of the Alpine arc. The resulting 350 to 750 km-long SR profiles seem to provide the most meaningful time information, better than that obtained with noisier higher-resolution SR maps. Their preliminary analysis especially evidences a systematic increase in SR from south to north across the contiguous Paris basin and Rhenish shield zones, suggesting northward propagation of an uplift wave that started from ~200 km north of the alpine collision front in Pliocene times and travelled across this part of the European platform at a rate in the order of 80 km/My. The Bohemian Massif and Massif central-Brittany zones show more complex SR patterns possibly linked to marginal alteration of uplift propagation by interferences with, e.g., compression in front of the Pyrenees and the Western Carpathians. Surprisingly, in contrast with the Paris basin, the Franconian basin displays fairly uniform low to moderate SR values suggesting that no tectonic perturbation occurred there since the late Early Pleistocene. Overall, the SR map points to an uplift signal coherent at the continental scale but not directly compatible with lithospheric folding. In conclusion, the R/SR geomorphometric approach of uplift chronology provides a wealth of data, whose in-depth analysis will help get fresh insight into the timing and causes of Plio-Quaternary uplift in NW Europe.
Demoulin A., 2011. Basin and river profile morphometry: A new index with a high potential for relative dating of tectonic uplift. Geomorphology 126, 97-107.
Demoulin A., 2012. Morphometric dating of the fluvial landscape response to a tectonic perturbation. Geophysical Research Letters 39, L15402, doi:10.1029/2012GL052201.
Pritchard D., Roberts, G., White N., Richardson C., 2009. Uplift histories from river profiles. Geophysical Research Letters 36, L24301, doi:10.1029/2009GL040928.
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