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Continental rifting across the Southern Gulf of California


.This study of continental rifting in the Gulf of California has two aims: 1. To better understand the tectonic evolution of the Gulf of California, and 2. To gain insight into continental rifting processes. The combination of seismic reflection and refraction techniques provides a robust method to analyze crustal structure, allowing for the amount, style, and temporal evolution of continental extension over conjugate rifted margins to be assessed. Multi-channel seismic data spanning 600 km image basins created by upper crustal extension. These basins appear to have formed in two phases: an initial phase beginning at or near 14 Ma, forming large basins and a second phase beginning around 6 Ma forming smaller half-grabens. There is a reflective, ropey layer seen mantling basement along much of the transect; modeling of velocity-depth profiles in the upper crust show that this layer has a velocity of 2.5 km/s and is a few hundred meters thick. This layer is interpreted as the arc-related 20-11 Ma Comond'u formation, and its continuity across the Tamayo Bank and Trough suggests it was deposited either synchronous or after faulting of the Tamayo Trough. This provides compelling evidence for onset of NW-SE oriented extension in the Gulf before 11 Ma. Refraction data spanning an 881 km profile over conjugate rifted margins between the Baja peninsula and mainland Mexico were used to construct a seismic velocity model. This model shows that the crust has experienced an overall pure shear mode of extension and, on average, has thinned to half its original thickness. Estimated total opening across the southern Gulf of California is 495 km, indicating an alternative tectonic evolution characterized by oblique NW-SE extension and dextral slip in the Gulf since $\sim$14 Ma. Average lower crustal velocity of $\ sim$6.3 km/s on both margins indicates there was no large- scale magmatism during rifting, and significant Moho topography suggests brittle deformation of the whole crust and no lower crustal flow, indicating rifting of cold lithosphere. However, some magmatism is seen at the in the regions of greatest crustal thinning at the continent- ocean transition and allowed for the transition from rifting to seafloor spreading

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