Morphotectonic Analysis of Uplifted Watersheds in the Santa Rosalía Basin, Baja California Sur, México.

The Santa Rosalia Basin (SRB), in the Baja California peninsula, Mexico, is related to the transtensional rifting of the Gulf of California and shows evidence for being tectonically active at least during the Pleistocene. The basin has characteristic sequences of non-symmetrical Pleistocene fluvial and marine terraces with remarkable stream incision and widening of the watersheds. Morphotectonic analysis of four major watersheds in the SRB is conducted to relate their spatial variation of marine-fluvial terraces with the regional tectonic deformation. Six to seven different terrace levels were identified within the Infierno, Boleo, Purgatorio, and Santa Agueda watersheds. Field mapping supported by measurements using RTK-GPS, which were linked to soil chronosequences profiles and luminescence dating provided morphochronology of the different terraces. The elevation of the terrace levels correlated between watersheds display a southward variation from ~6 to ~9 meters in the Boleo-Purgatorio profiles analysis and a difference in height from ~28 to ~50 meters in the Purgatorio-Santa Agueda profile. Geomorphic indices such as asymmetry factor (AF) and transverse topographic symmetry factor (TS), and hypsometric integral curves (HI) were to obtained using GIS. Watershed hypsometric curves show a southward increase of watershed maturity in the arroyos of the SRB. The AF displays variation of 33.5% for Arroyo Boleo, 36.23% for Arroyo Purgatorio, and 55.09% for Arroyo Santa Agueda, suggesting a southward decreasing in the watershed tilting across SRB. The average of TS for each watershed is consistent with the southward variation of the AF index, values of TS varies from 0.42, 0.33 to 0.23 (Arroyo Boleo, Arroyo Purgatorio, and Arroyo Santa Agueda, respectively). Correlation of geomorphic indices and fluvial geomorphology elements of the SRB indicates a possible influence of a differential tectonic uplift towards La Reforma Caldera, displaying a southward tilting of the watersheds. In addition, the delineation of main watersheds of the SRB and mapping of its marine-fluvial terraces can be helpful for future studies on fluvial geomorphology and tectonics influence in the region.