Active deformation and shallow structure of the Wagner, Consag, and Delfín Basins, northern Gulf of California, Mexico

Abstract

[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.

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

“…Crustal deformation in the Gulf varies from classic ridgetransform structures in the south to diffuse deformation in the north, where a large number of shallow small-offset normal faults exist (Persaud et al 2003;Lizarralde et al 2007). Variations in the crustal structure and composition along the Gulf and beneath Baja are found in several studies.…”

Dissolved methane in the sills region of the Gulf of California

“…If no hydrothermal activity is present, then anoxic diagenesis of sedimentary organic matter can also form methane gas in the sediments, which may be released as bubbles to the water column. Release of methane bubbles from the sediments could be triggered (Canet et al 2010) by the extensive faulting known to be present in the BC sill area (Persaud et al 2003). Measurements of gas composition (i.e., ethane and propane) and methane stable isotope ratios could help to clarify the relative importance of the potential biogenic, thermogenic, and abiogenic sources and could also help constrain the importance of methane oxidation, but we were unable to obtain these measurements with our samples.…”

“…Known tectonically-active zones in our study area are the Ballenas Transform Fault (BTF) system within the Ballenas Channel (BC), and Delfín Basin. The BTF is part of the plate boundary and is thought to be the major locus of present transform movement in the Midriff Islands region (Bischoff and Henyey 1974), and the Delfín Basin contains an active spreading center (Lonsdale 1989, Persaud et al 2003. Esta rápida deposición de sedimentos inhibe la erupción del fondo marino, como se observa en las dorsales oceánicas, y la intrusión de umbrales y diques basálticos calientes en los sedimentos ricos en materia orgánica es la principal actividad ígnea (Lonsdale y Becker 1985).…”

“…El sistema de FTB se sitúa en el límite de placas y se considera el sitio principal del movimiento de placas tectónicas en la región de las grandes islas (Bischoff y Henyey 1974). La cuenca Delfín contiene un centro de dispersión activo (Lonsdale 1989, Persaud et al 2003.…”

“…Este trabajo se llevó a cabo en un crucero de oportunidad y sólo fue posible obtener un conjunto de muestras en un área limitada. (Persaud et al 2003). Faulting associated with the BTF system and intrusion of hot basalt into the upper sediment layers (e.g., volcanic knolls) would provide conduits for gas transport from the subsurface to the surface.…”