Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico

The Gulf of California is a prime example of a young oblique-divergent plate boundary. This type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings. A complexity in the Gulf of California is that the modern oblique rifting was preceded by a stage of orthogonal rifting. We have used extensive mapping as well as stratigraphic, geomorphic, and structural analysis to determine the history of deformation near Loreto on the southern Baja California peninsula during formation of the Gulf of California. Our data support the suggestion that middle to late Miocene (protogulf stage, 12 to ca. 6 Ma) orthogonal rifting was overprinted by transtensional structures during Pliocene to Quaternary time, though we cannot closely define the time of change. We further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults. The Loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned, down-tothe-east extensional monoclines and normal faults. These structures lie along, or as far as 4 km in front (east) of, a steep, 1000– 1600-m-high Main Gulf Escarpment that defines the western topographic margin of the rift zone. The Loreto segment is further defined by a rise in elevation of the Main Gulf Escarpment of up to 1 km and an ;500–800 m increase in structural relief of *E-mail: [email protected]. prerift strata in the escarpment from the segment boundaries to the center of the segment. Structural analysis of secondary faults of the Loreto segment shows that fault populations that are known to be Pliocene to Quaternary in age are mixed normal and dextral-normal faults with a bulk extension direction of west-northwest–eastsoutheast (2808–1008). In contrast, fault populations that cut only prerift, Oligocene–middle Miocene volcanic and sedimentary rocks are mainly normal faults with northeast-southwest to east-northeast– west-southwest bulk extension directions that average 2458–658. These faults are interpreted to be late Miocene in age and formed during protogulf orthogonal rifting. Thus, the Pliocene faults record a major change of extension direction of ;358 from latest Miocene to Pliocene time, compatible with the onset of oblique rifting in the Gulf of California. Oblique-divergent overprinting is mainly expressed as the Loreto fault and Loreto basin in the northern part of the segment. The Loreto fault and basin evolved with major fault reorganizations and partial basin inversion. Many observations suggest that the Pliocene Loreto fault was linked to two nascent transform faults in the gulf and that Carmen Island rotated ;358–408 clockwise within the complex fault system.