Tectonic gradients and geologic structures control planform geometrical changes in drainage basins: Simulations, experiments, and field observations

Spatial gradients in tectonic uplift rate and exhumation of inherently heterogeneous geologic structures are key in inducing changes in river orientation and planform basin geometry. However, the processes that link basin geometrical evolution to the tectonic and structural forcings are complicated by the interactions between neighboring basins and sub-basins. Here we study the planform evolution of drainage basins and flowlines in two end-member cases: The first is forced by tectonic tilting and the second by geologic structures of orthogonal fracture sets. We combine experiments and simulations to explore the planform evolution of drainage basins under tectonic tilting at the mountain-range scale. The experiments are performed in DULAB (Differential Uplift LAndscape evolution Box), a physical apparatus for landscape evolution capable of inducing time-dependent spatial gradients in the uplift rate. The simulations are based on the DAC (Divide And Capture) numerical landscape evolution model. We measured basin spacing ratio (the ratio between basin length and outlet spacing) and found that it remains uniform and constant during basin size and shape change in response to tectonic tilting that induces main water divide migration. An important process that facilitates the preservation of basin spacing ratio during divide migration is incorporation of new basins to the main divide along the shrinking mountainside and the abandonment of basins at the extending side. Contrary to previous expectations, we observed that the incorporated and abandoned basins play only a passive role: they remain constant in size while the divide migrates toward and away from them. To explore the planform landscape evolution in the presence of geologic structures, we studied the evolution of a canyon system in the Dead Sea basin, Israel. The area is transected by orthogonal sets of hundreds of clastic dikes -- mode-I fractures, filled with injected sediments. We found that the orthogonal sets of clastic dikes force the drainage network to evolve into a rectangular pattern with right-angle junctions and bends. Field observations and experiments revealed that the clastic dikes dictate the tributary orientation through systems of sinkholes and caves formed by internal erosion along the clastic dikes.