CONTINENTAL RUPTURE DELAYED BY SEDIMENTATION AND LOW-ANGLE NORMAL FAULTS IN THE NORTHERN GULF OF CALIFORNIA: ANALYSIS OF SEISMIC REFLECTION PROFILES IN THE TIBURON AND DELFIN BASINS

The transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern Gulf of California across the obliquely conjugate Tiburon–Delfin basin segment. Structural mapping on a 5-20 km grid of seismic reflection lines (48 channels, 6s TWTT) of Petroleos Mexicanos demonstrates that ~200% extension is accommodated on a series of NNE-striking listric-normal faults that merge at depth into a detachment fault here named Angel de la Guarda Detachment (AGD). The main AGD extends ~50 km along strike and has listric geometry that flattens below 3 seconds (TWTT). It juxtaposes late-Neogene marine sequence over thinned continental crust and contains an intrabasinal divide defining its western limit. The AGD is bounded on both ends by two NW-striking, dextral-oblique faults, the Tiburon and De Mar faults that shear the continental crust on both margins nearly parallel to the tectonic transport. Active extension in Delfin basins occurs on northeast-striking, northwest dipping normal faults that cut the detachment fault on the western flank of an intrabasinal structural high that defines the northwest limit of the Tiburon basin. A poorly defined transtensional fault zone bound the eastern limit of Upper Delfin basin. Dextral shear occurs along the Volcanes fault zone that cuts the peninsular margin north of the Ballenas transform faults and bounds the Lower and Upper Delfin basins to the west. Above the intrabasinal structural high a regional unconformity in the upper 0.5 seconds (TWTT) and crest erosion of roll over anticlines above the detachment indicates that dislocation and thermal buoyancy forces warped the footwall of the AGD during the lithospheric rupture in the active Delfin basins to the west. The maximum length of new oceanic crust in the Delfin basin is less than 60 km based on the lack of an acoustic basement beneath a sedimentary lid that reaches >7 km in thickness. Our results indicate that extreme crustal thinning and lower crustal flow in oblique rifts occurs along detachment faults bounded by lateral shear zones and increases in volume toward the rift axis. We propose that in the north insulation and thermal softening produced by the thick sedimentary lid allowed lower crustal flow, which compensated large magnitude of crustal thinning produced by low angle normal faults.