RIFT-TO-DRIFT TRANSITION IN THE NORTHERN GULF OF CALIFORNIA

Asymmetry is a conspicuous feature throughout the length of the Gulf of California rift; here the active deformation occurs along the western margin. Based on the interpretation of ~3600 km of multichannel seismic reflection data collected by PEMEX, we show that the eastern side of the northern Gulf hosts a now-inactive, but well-developed system of rhombochasmic basins, which includes the Altar, Adair-Tepoca and Tiburón basins, with depths ranging from ~4.5 to ~6.0 km. Subsidence in these basins was controlled by the Altar, Southern Cerro Prieto, Amado, De Mar, and Tiburón faults. Nowadays these faults show little activity, but may have accommodated large amounts of both dip-slip and strike-slip motion during the early transtensional evolution of the Gulf of California in late Miocene? to Pliocene time. The inactive basin system in the eastern Gulf is separated from the active Consag-Wagner, Upper and Lower Delfín basins in the western Gulf, by a broad antiform structure cored by an inferred basement high. The stratigraphic relationships across this antiform indicate that the uppermost deposits in the inactive basins correlate with the lowermost deposits in the active basins, evidencing that sedimentation of the now abandoned basins immediately preceded deposition in the active basins.

We suggest that the locus of strain changed in late Pliocene time towards zones of lower yield stress, produced by the higher thermal conditions in the western Gulf, where discrete volcanism has occurred during late Miocene-Pliocene and Quaternary time. We interpret that the antiform structure dividing the active and inactive basins developed as an accommodation zone during the westward re-localization of deformation. The shifting of coherent strain left behind a strip of abandoned basins along the length of the eastern Gulf of California rift, which now forms an incipient drifting margin offshore Sonora and Sinaloa, while rifting is ongoing offshore Baja California.