Late Miocene transition from extension to dextral transtension in the southern Coastal Sonora fault zone, Gulf of California rift, Mexico

The Coastal Sonora fault zone (CSFZ) in northwestern Mexico records the Pacific-North America plate boundary reorganization during the Miocene from subduction to a transform boundary. This study presents the structural architecture of the Sierra El Aguaje-San Carlos -Guaymas region along the southern part of the CSFZ. The study area shows a complex fault pattern divided into four tectonic domains: ENE-WSW left -lateral, normal, and oblique faults in the Sierra El Aguaje and Cerro El Vigia domains and N-S domino -type normal faults in the El Parral and San Carlos domains, which are separated by the NW -striking dextral San Carlos fault. Based on a new fault -slip dataset and K-Ar ages, we propose a structural evolution model in which deformation progressively evolved from extension to dextral transtension. The deformation history involved -E-W pure extension associated with N-S normal faults from -15.3 Ma to -11-10 Ma, followed by transtension and the development of strike -slip faults after -11-10 Ma. That change induced the reactivation of preexisting faults and local strain partitioning between normal, oblique, and strike -slip faults. Transtension became strike -slip dominated during the latest Miocene and the Pliocene, accumulating -8 km of post -6.3 Ma dextral slip along the San Carlos fault. Paleostress analysis demonstrates that during the extensional phase, 02 and 03 were horizontal and oriented -N-S and -E-W, respectively. The transition to transtension required that 02 reached the lithostatic stress value, its magnitude approached 01, and its orientation remained -N-S. These conditions implied a horizontal tectonic force directed towards the north began after -11-10 Ma in northwestern Mexico. We propose that this new stress regime was related to the end of the Farallon-North America convergence and the inception of the Gulf of California oblique rifting with 01 oriented N-S.