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[1] 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, $70 Â 200 km, marked by active distributed deformation and six $10-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 60-80°and strikes of N-N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfín 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 Delfín 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 Delfín 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.

S U M M A R YReceiver functions (RFs) from teleseismic events recorded by the NARS-Baja array were used to map crustal thickness in the continental margins of the Gulf of California, a newly forming ocean basin. Although the upper crust is known to have split apart simultaneously along the entire length of the Gulf, little is known about the behaviour of the lower crust in this region. The RFs show clear P-to-S wave conversions from the Moho beneath the stations. The delay times between the direct P and P-to-S waves indicate thinner crust closer to the Gulf along the entire Baja California peninsula. The thinner crust is associated with the eastern Peninsular Ranges batholith (PRB). Crustal thickness is uncorrelated with topography in the PRB and the Moho is not flat, suggesting mantle compensation by a weaker than normal mantle based on seismological evidence. The approximately W-E shallowing in Moho depths is significant with extremes in crustal thickness of ∼21 and 37 km. Similar results have been obtained at the northern end of the Gulf by Lewis et al., who proposed a mechanism of lower crustal flow associated with rifting in the Gulf Extensional Province for thinning of the crust. Based on the amount of pre-Pliocene extension possible in the continental margins, if the lower crust did thin in concert with the upper crust, it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin. In this case, we suggest that when breakup occurred, the lower crust in the margins of the Gulf was still behaving ductilely. Alternatively, the lower crust may have thinned after the Gulf opened. The implications of these mechanisms are discussed.

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 We developed a novel Bayesian array-based receiver function method that can leverage dense arrays for urban basin imaging;  Probabilistic representation of the receiver functions helps objective assessment of feature identification and geological interpretation;  Our method produces reliable and coherent basin images that can improve our understanding of subsurface structures.

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We integrated new geologic data with published geophysical data to document that the southernmost San Andreas fault zone, onshore of the Salton Sea, southern California, is a transpressional, 1-4-km-wide ladder-like structure. This newly identified Durmid ladder structure is a voluminous right-reverse fault zone that broadens across Durmid Hill around rotating domains of regularly spaced, left-and rightlateral cross faults. The active East Shoreline fault zone of the San Andreas fault forms the southwest margin of this fault zone, and it is generally parallel to the main strand of the San Andreas fault zone for >30 km, deforms Pliocene to modern sediment, and has an ~1-kmwide damage zone of strongly folded and faulted sedimentary rocks. Hundreds of left-and right-lateral cross faults and folds connect the two right-lateral strands of the San Andreas fault zone within the ladder structure for at least 25 km northward from Bombay Beach. Leftlateral cross faults strike east and likely rotated clockwise ~45°-60° from their original northeast strike. Transpression, clockwise rotation, and right-lateral shear between the East Shoreline fault and main strand of the San Andreas fault zone in the Durmid ladder structure exhumed a large amount of Pliocene-Pleistocene basin fill since the ladder formed in the Pleistocene. Strike-slip faults in the Durmid ladder structure cut latest Cenozoic to modern sediment and produced many of the ubiquitous folds in the area by fault-bend folding as they slipped past ramps and flats. Growth strata in the upper Brawley Formation are concentrated along the master right-lateral fault zones. Long, narrow zones of fractures displace modern sediment along both edges of the Durmid structure, perhaps due to the same kind of slow shallow creep that has been documented along the main strand of the San Andreas fault zone. Steep right and right-oblique faults and folds are the main structures in Pleistocene sediments deformed by the ~1-km-wide East Shoreline fault. Geophysical data sets and drill holes in Coachella Valley show that the East Shoreline fault probably persists into the subsurface as a northeast-dipping fault zone that defines the basinward edge of a complex three-dimensional flower structure along ~100 km of the San Andreas fault zone. The East Shoreline fault appears to continue northward for over 100 km past the Mecca and Indio Hills along the northeast margin of Coachella Valley, where southwest-dipping basin-fill deposits are being exhumed on its northeast side. Lines 4 and 5 of the Salton Seismic Imaging Project imaged faults that are along strike of the East Shoreline fault and occupy the same structural position as the East Shoreline fault relative to the San Andreas fault. These data are also consistent with the East Shoreline fault being related to the Garnet Hills fault of the San Andreas fault zone. Southward, the transpressional southernmost San Andreas fault zone changes gradually along strike into the transtensional Brawley seismic zone across an ~5-km-long transitional ...

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