Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, similar to 70 x 200 km, marked by active distributed deformation and six similar to10-km-wide segmented basins lacking well-defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense ( up to 18 faults in 5 km) complex network of mainly oblique-normal faults, with small offsets, dips of 60 - 80degrees and strikes of N-N30 degreesE. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfin Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfin Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid-ocean ridges. On the basis of the near-surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfin Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain-partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.
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