Deciphering the active deformation of the Guayllabamba Depression of Ecuador using fluvial morphometric studies

The Guayllabamba Depression is one of the most active environments of the Interandean Valley of Ecuador. Located just north of Quito, the Depression is collocated with the Pliocene-Pleistocene Guayllabamba basin, which is filled with lacustrine fluvial and volcanic deposits. The Depression has generally been interpreted as a pull-apart basin controlled by a strike-slip system, however, the surface evidences of this fault system is not clear. The present research proposes that the formation of the Depression was due to the collapse of incised valleys and enhanced by the high erosion rates of the Pisque and Guayllabamba rivers, resulting in its characteristic geometry that is symmetric about the Pisque river. To test this hypothesis, we map the main geomorphic features in the area, such as landslides scarps and fluvial terraces, calculate minimum erosion rates by combining the volume of eroded material with the ages of the youngest eroded volcaniclastic deposits, and carry out a quantitative study of the fluvial morphology of the Guayllabamba and Pisque river basins using high resolution digital elevation models, as well as ArcGIS and LSDTopoTools software. The fluvial morphometric analysis includes extracting longitudinal profiles of the trunk rivers and their tributaries, to calculate chi, steepness and concavity plots and knickpoint identification. We use these results to constrain the active tectonic deformation and fluvial processes in the Guayllabamba Depression, and we compare them to adjacent watersheds to highlight differences in active tectonic and surface processes there.