Tectonic deformation can be partitioned between narrow zones of localized deformation and broad areas of distributed strain. In this second configuration, rivers can act as reliable markers of the horizontal deformation.

In order to better understand how rivers respond to horizontal motion, we develop analog experiments of deformation and erosion, inspired by the natural example of the Southern Alps of New Zealand.

We use a table covered by a thin carbon-kevlar film pulled by a step by step engine, with sprinklers positioned above to model the development of an eroding doubly-vergent wedge built in oblique collision. The experiments were performed in Geosciences Montpellier (France).

At the end of the experiments, the drainage network is clearly rotated: these experiments confirm that rivers can record horizontal distributed motion.

Then, we looked at the geometry of the drainage network and explore the χ patterns along the streams (Perron and Royden, 2013; Willett et al., 2014; Whipple et al., 2016). In a stable configuration, χ values should be similar accross a drainage divide. If not, the divide is unstable and a path toward equilibrium is to move toward the high χ value. (Willett et al., 2014).

The natural time and space scales do not allow for a direct test of this theoritical framework. Therefore, we took advantage of our unique setup and experimental data set. We analyzed the χ maps of our experimental landscapes to determine

1) if χ patterns can reflect the imposed horizontal deformation

2) if a difference in χ accross a divide does mark an area prone to reorganization.

From a statistical analysis of the maps, we observed that most basins have higher χ values on their left side than on their right side. This is in agreement with the imposed deformation: χ maps can be used to reveal horizontal, distributed tectonic distribution.

Then, we showed that most divides affected by a difference in χ will evolve as expected from this gradient (toward the high χ value). This geometric parameter appears as a strong predictive tool of drainage reorganization.