The upper Miocene Boleo Formation (Baja California Sur, México) provides a record of tectonic controls on mixed evaporite-siliciclastic sedimentation in a transtensional pull-apart basin during continental break-up and onset of seafloor spreading in the central Gulf of California. The thin basal limestone, recently dated at 6.35 ± 0.21 Ma, records abrupt onset of subsidence and rapid marine transgression over a formerly emergent landscape. The siliciclastic member contains coarsening-up mud-sand-gravel parasequences that record progradation of fan deltas into a saline shelf. The laterally equivalent gypsum member consists of shallowing-up parasequences formed by evaporative drawdown in the distal evaporite depocenter. Thin, laterally extensive mudstone and breccia units, which host Cu-Co-Zn stratiform ore deposits, record short-lived pulses of marine transgression, fault activity, hydrothermal activity, and mineralization. Growth strata, internal unconformities, and paleocurrent data record NE tilting on a network of NW-striking syn-depositional normal faults during sediment transport to the northeast. Our results support a model for syn-basinal growth of a large monocline above the tip of a propagating oblique-dextral normal fault at the southwest margin of the Santa Rosalía basin. Large-scale displacement across the monocline was the primary mechanism of subsidence, which likely was enhanced by loading related to mafic intrusions and seafloor spreading in the adjacent Guaymas basin. We propose that marine flooding was geologically instantaneous within existing age uncertainties (± 0.2 m.y.) for 1000 km along the Pacific−North America plate-boundary fault system, consistent with models for rapid acceleration of transtensional strain when the plate boundary became localized in the modern Gulf of California at ca. 7−6 Ma.
Dorsey et al. (Thu,) studied this question.