Geomorfometría del Escarpe Principal del Golfo de California. Análisis comparativo entre dos segmentos del rift: Sierra San Pedro Mártir y Sierra Juárez, Baja California, México

Abstract

The Main Gulf Escarpment is a fundamental tectonic and geomorphologic feature defining the western edge of the Gulf Extensional Province. This escarpment reaches 2.5 and 1.7 km in the eastern mountain front of the Sierra San Pedro Mártir and the Sierra Juárez rift segments, respectively. Although their overall appearance is similar, these two segments are controlled by active faults with contrasting geometry and kinematics. The escarpment of San Pedro Mártir is in the foot-wall of a down-to-the-east, high-angle normal fault system, whereas the Sierra Juárez escarpment lies in the hanging-wall of a down-to-the-west low-angle normal fault system. The characterization of these two escarpments using geomorphic analysis of the mountain fronts reflect fundamental differences in the tectonic control by two master faults with opposed polarity. The Smf and Vf indices show that both mountain fronts are tectonically active. In the Sierra San Pedro Mártir segment, the changes in the patterns of topography, slope, slope-area relationship (SA), normalized steepness index (Ksn) and concavity index (θ) are strongly correlated with the down-to-the east slip of the master fault. These geomorphic indices indicate a significant increase in the displacement gradient and tectonic uplift towards the central portion of the San Pedro Mártir fault. In contrast, the same geomorphic indices suggest asymmetry, and lower uplift gradient in the Sierra Juárez, exhibit an eastward dipping plateau which we named ‘Mesa Juárez Oriental’. The plateau dip to the east, opposed to that of the pre-rift erosional plain, once dissected by rivers with a paleo-flow from Sonora to the Pacific coast during Eocene and Oligocene times. Hypsometric analysis of the ‘Mesa Juárez Oriental’ at the uppermost part of the scarp is consistent with the geometry of a rollover anticline formed in the hanging-wall of the Cañada David detachment and Laguna Salada high-angle normal-dextral fault. The irregularities in the shape and size of large basins are probably controlled by lithological heterogeneities that promote the lateral expansion and drainage capture of adjacent sub-basins. This study documents the distinctive geomorphological differences between two adjacent rift escarpments structurally controlled by master fault with opposed sense of slip.