Synchronous Changes in Rift-Margin Basins and Initiation of the Alarcón Spreading Ridge and Related Transform Fault, Southwestern Gulf of California

How oceanic transform faults form is not well understood, and the rift-to-drift hypothesis is not well tested. The southern Gulf of California provides insights into these processes. Many studies have shown that the western margin of the Gulf is still active despite sea-floor spreading on the Alarcón ridge since 2 - 3 Ma. Magnetic anomalies on the southeastern side of the Alarcón ridge show that it changed to seafloor spreading at 3.7 Ma. On the northwestern side, magnetic anomalies indicate formation of new oceanic crust began at 2.5 Ma. We assume that at that time the adjacent well-defined transform fault formed north of the ridge and the Tamayo fracture zone to the SW. The transform and fracture zone extend to the NW into a transtensional array of N, NW, and uncommon NE striking faults. The fault array ends against the Loreto and San Jose Island transtensional basins. The San Jose Island basin had its most rapid subsidence from 3.6 ± 0.5 Ma to 2.5-2.4 Ma. Basin margin faulting died and moved east at or shortly soon after 2.5-2.4 Ma the same time as the Alarcón ridge along the same domain. Loreto basin faulting increased at 2.5-2.4 Ma and decreased at 2.0 Ma, implying that the Pescadaro basin started spreading and Farallon transform formed after the Alarcón ridge and transform. Late Quaternary marine terraces suggest that faulting rates slowed greatly at both locations since the fault reorganizations at 2.5 and 2.0 Ma. The sequential model suggested from these data is (i) transtensional fault array, (ii) new spreading ridge and transform fault form while transtensional array decreases to low rates, (iii) rifting continues at margins possibly due to gravitational potential from high rift escarpment/margin to marine deep at spreading ridge.