Multi-stream order analyses in basin asymmetry: A tool to discriminate the influence of neotectonics in fluvial landscape development (Madrid Basin, Central Spain)

Abstract

Morphometric analysis of the Tajo drainage network using a DEM was carried out in order to determine neotectonic influence on the development of the Madrid Basin since the Pliocene. Statistical analysis of digitally-derived Transverse Topographical Drainage Basin Asymmetry (T Factor) was used to infer directions of ground tilting. A crustal block pattern was interpreted from a mean basin-asymmetry vector field that was computed using a new approach based on a gridded running average of the raw vector field. Measurement of T Factor for different order channels allows investigation of evolution rates of different order rivers and at the same time reveals possible factors controlling basin evolution. The vector data show clear differences between high and low orders. High-order asymmetry is consistent with fluvial terrace distribution (as it reflects channel migration and incision within the main valleys) and shows a general trend toward a central basin axis running SW–NE. A SW–NE trend is also defined by other morphographical approaches such as drainage density and relief analysis. Both approaches point toward the presence of two uplifted areas running SW–NE and a subsided zone between them, which is in agreement with previous crustal flexure models. In contrast, low-order asymmetry shows a clear trend indicating eastward tilting, which could be driven by recent changes in maximum principal stress direction and its influence on fault reactivations. In general, large rivers generating fluvial terraces are controlled by basement structure, with positive and negative crustal flexures driving their migration direction and thus controlling the spatial distribution of terraces, because the position of main streams is controlled by large tectonic lineaments that favor groundwater flow convergence. On the other hand, small tributaries have low capabilities to attain equilibrium, and T Factor values of low-order basins may reflect more recent block tectonics due to variations in stress fields or may reflect local controls like lithology.