A magnetotelluric transect across the Sonora-Arizona border: Identification of the northeastern boundary of the Gulf of California rift

Abstract

The eastern limit of the Gulf of California rift is poorly defined, especially at its northern end in the Mexico-US transborder area. In order to define its location in this region, we have generated a 2D electrical resistivity model from magnetotelluric data acquired at 16 stations along a NE-SW-oriented transect, approximately 175 km long, from Puerto Peñasco on the northern coast of Sonora, Mexico, to the vicinity of Ajo in southwestern Arizona, USA. The model reveals important features of the crust in the study area. First, the significant contrast in electrical conductivity between the lower crust and the upper mantle allows visualizing the electric Moho, which is consistent with estimates from other geophysical methods in previous studies. Secondly, the combination of changes in the geometry of the electrical bodies and the marked lateral variation in resistivity along the transect allows dividing it into two sections, with the boundary located near the Quitobaquito Hills in Arizona, close to the border. The southwestern section is notably less resistive on average than the northeastern section and is traversed by a series of narrow, highly dipping transcrustal conductive bodies which are related to strike-slip faults cutting through a thinned crust. In contrast, the northeastern section lacks these types of electrical bodies, is also less heterogeneous, and tectonic structures in this section are not clearly defined. However, the geometry of two large resistive bodies that make up a significant portion of this section suggests the presence of listric faults rooted in the mid-lower crust. We interpret that the southwestern section corresponds to the Gulf of California tectonic province and the northeastern section to the Basin and Range province. The proposed boundary between these two Cenozoic provinces is located in a structurally complex area where older tectonic boundaries have been previously reported (Yavapai-Mazatzal Paleoproterozoic suture zone), as well as large structures of different ages (Upper Jurassic Mojave-Sonora Megashear, Paleocene Quitobaquito Hills thrust system). This underscores the importance of the geological inheritance on the localization of the northeastern boundary of the modern Gulf of California rift in the studied section.