Mid-Miocene to Early Pliocene Upper Crustal Extension in the Southern Gulf of California

600 km of multi-channel seismic data across conjugate rifted margins in the southern Gulf of California show many rifted basins. Analysis of these basins shows two distinct styles: the first consists of large basins with no obvious basin-bounding fault or large syn-rift sedimentary sequences and only small-scale basement faulting. The second style consists of smaller, simple half-grabens, mostly with lower syn-rift sequences and upper post-rift sequences. All recent sediments imaged appear to be post-rift, indicating no significant active faulting in this area at present. Rough estimates of basin age based on sedimentation rates indicate that these two styles correspond to two phases: the larger basins may have been formed beginning 14 Ma, whereas the simpler half-grabens are more likely formed beginning around 8 Ma. A highly reflective ropey layer is imaged at the top of basement along much of the transect; modeling of mommon-midpoint (CMP) supergathers indicates that this layer has a velocity of about 2.5 km/s and a thickness of 250-500 m. We interpret this layer as being the 20-11 Ma Comondu formation. In the Tamayo Bank and Trough region, adjacent to the southeast continent-ocean transition, this Comondu layer appears to continuously mantle the bank and trough, masking the large-scale faulting that created the trough, because despite the large size of this basin (1500 m sedimentary thickness) there are no syn-rift sedimentary sequences recording its creation. We interpret this as evidence for overlap between the end of the Comondu Arc and onset of NW-SE extension in the Gulf of California. Along with the estimated ages of the basins, all evidence points toward an alternative tectonic history for the Gulf of California, where significant NW-SE oriented extensions began possibly as early as 14 Ma.