The Gulf of California in northwestern Mexico is an oblique-extensional plate boundary that formed in a region with a complex geological history. The pre-rift lithosphere of the Gulf was influenced by Mesozoic arc magmatism followed by the shutting down of subduction and foundering of slab fragments that have been imaged beneath parts of Baja California. This was followed by a ~12-6 Ma period of broadly distributed deformation involving normal, strike-slip and transtensional faulting that spanned the width of the Gulf Extensional Province and extended into the continental borderland west of Baja California. The Gulf of California opened roughly ~6 Ma as the plate boundary shifted ~250 km inland rupturing weakened continental crust and separating Baja California from mainland Mexico.

Despite the rapid localization of strain along the full length of the Gulf in a region that now has mostly new crust, the plate boundary shows a wide variation in deformation styles suggesting that magmatism, sediment supply and the local stress field may be key players. The northern segment has the characteristics of a wide rift, with a plate boundary jump at ~2 Ma, evidence for lower crustal flow, and broadly distributed extensional strain and small gradients in topography and crustal thinning.  New oceanic crust here is limited to the lower crust in the rift basins. In contrast, the central segment has > 7 km thick, ~280 km wide oceanic crust, and is a magma-rich, narrow rift¹ and the only Gulf basin with on axis regions of partial melt  at 26-50 km depth. The Alarcon Basin in the south has ~6 km thick oceanic crust, but is a magma-poor, wide rift¹. Its margins have substantial Moho topography suggesting no lower crustal flow during rifting. Interestingly, tomographic images show asthenospheric upwelling beneath the Alarcon and the northern rift basins, but in the central Gulf upwelling is instead focused beneath a transform fault. 

¹ Lizarralde et al., Nature 448, 466-469.