Basin Plan-view Geometry is Dynamic but Reflects No Age

Observations from around the globe show that drainage basins maintain geometric regularity regardless of external tectonic and climatic conditions. This regularity is commonly quantified using geometric-geomorphic scaling relations such as Hack's Law and basin spacing ratio (the ratio between basin length and the distance between basin outlets). The origin of this regularity is not well understood, and as a consequence, it is not clear whether and how it is preserved during modifications of basin shape and size following changes in the tectonic conditions. Hence, our ability to interpret planform geometry of basins with relation to fluvial transiency in response to external perturbations is critically hampered. To address these gaps, we combine experiments using a landscape evolution experimental apparatus, DULAB, and numerical landscape evolution simulations to isolate autogenic plan-view dynamics when tectonic forcing is perturbed. Our experiments and simulations consist of two distinct stages. First, uniform uplift is applied, and basins emerge and grow by incising toward an uplifting and shrinking plateau. Second, tectonic tilting is applied, and basin size and shape adjust following migration of the main drainage divide. Results show that during the first stage, scaling relations converge toward steady-state values and remain within a range of values reported for natural basins from the very onset of the experiments and simulations. Thus, basin lengthening and widening occur at approximately similar rates. Several processes are identified that assist in maintaining the scaling relations, including differential basin growth and processes of reorganization. During the second stage, we observe only minor basin width adjustments. On the extending mountain flank, the spacing ratio increases due to basin lengthening, and on the shrinking mountain flank, the spacing ratio remains steady via introduction of new basins to the main divide.