The opening of the Gulf of California near Loreto, Baja California, Mexico: from basin and range extension to transtensional tectonics

Detailed structural field analyses of faults and fractures have led to the reconstruction of the tectonic evolution of the Loreto region (Baja California, Mexico) during the opening of the Gulf of California. Three main tectonic events have been recognized: (i) formation of large-scale radial structures along the future Main Gulf Escarpment, here interpreted as associated with emplacement of magmatic bodies at shallow levels during the beginning of extension processes in Early-Middle Miocene; (ii) NE-SW extension (Basin and Range event) active since 15 Ma, producing NW-SE normal dip-slip faults in the Oligo-Miocene volcaniclastic basement, and indicating high extension rates; (iii) transtensional tectonics recorded especially in the Plio-Quaternary sediments and associated with the activation of NW-SE dextral strike-slip faults and N-S normal faults. The tectonic structures related to the Basin and Range extension are sealed by the Plio-Quaternary sediments of the Loreto basin and by the volcanics of the Mencenares Volcanic Complex, a huge volcanic field interfingering with the sedimentary succession of the basin. © 1994.