Mode of extension and rifting history of upper Tiburón and upper Delfín basins, northern Gulf of California

17 pages, 11 figures, 1 tableThe crustal structure of the northern Gulf of California transtensional margin has been investigated by a 280-km-long NW-SE profile, including deep multichannel seismic reflection and densely sampled refraction/wide-angle reflection seismic information combined with gravity modeling. The seismic and gravity modeling constrains two thinned crustal areas, corresponding to the upper Delfín and the upper Tiburón basins. On both sides of the profile, toward the Baja California Peninsula and the Mexico mainland, a progressive thickening of the continental crust is observed. Our results indicate that the crustal thickness is 19 km below the coastline, and it decreases to 14 and 17 km below the upper Delfín and upper Tiburón basins, respectively. In the area between both basins, the crust thickens to 19.5 km. There are significant lateral thickness variations for the different levels of the crust. The interpreted structure is consistent with the existence of an aborted rift below the upper Tiburón basin. Prominent dipping reflections in the multichannel data under upper Tiburón basin and the ridge between upper Tiburón and upper Delfín basins can be explained as a mylonite like zone related to a detachment fault. This interpretation suggests that the structural evolution of upper Tiburón basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust. The mode and locus of extension have evolved from a core complex in upper Tiburón to a narrow rift mode in upper Delfín basin.This work could not have been possible without the aid of the captains and crews of R/Vs Hespe´rides, Altair, and Humboldt. Special thanks to Hubert Fabriol (CICESE-BRGM), RemyLouat (ORSTOM-IRD) and Tony Monfret (ORSTOM-IRD). Also, we thank many technicians and students that participated in the different activities inland. Acquisition and processing of the gravity and seismic data were funded by CICYT Spanish project ANT-182/94-C01/02; the Secretaría de Marina of Me´xico; CICESE projects 644107 and 644114; CONACYT projects 0894PT, 28317T, and 28318T; and Convenio CONACYT/CSIC and INSU-France. The first author also thanks funding by Dirección General de Investigación Científica y Enseñanza Superior EX96-02876832, Ministerio de Educación y Cultura (Spain). Finally, we would like to thank the anonymous reviewers for their constructive comments.Peer reviewe