TECTONIC IMPLICATIONS OF ABANDONED AND ACTIVE BASIN ARCHITECTURE IN THE NORTHERN GULF OF CALIFORNIA RIFT

The transtensional regime of the Pacific-North America (PA-NA) plate boundary was established in the Late Oligocene (~30 Ma), but only since ~ 6 Ma has the extensional deformation become localized in the Gulf of California. We use a grid of 48-fold, 6 sec two-way travel time (TWTT) multichannel seismic reflection data collected by Petróleos Mexicanos in the northern gulf. This portion of the current PA-NA rift includes a newly recognized fossil basin system: the Tiburón and Tepoca, located southeast of the presently active Wagner-Consag-Delfín Basin. The Tiburón Basin accrued subsidence between Isla Tiburón and the Puertecitos Volcanic Province; slightly later, the Tepoca Basin started developing to the north. The combined basins synchronously subsided forming an elongated NNE sigmoidal shape system extending ~ 70 x 220 km, ~ 45° clockwise oblique to the direction of extension, reaching ~ 4.8 s and 3.6 s TWTT of clastic sedimentation, respectively. The axis of subsidence subsequently re-localized northwest ~ 120 km, initiating the Wagner-Consag-Delfín Basin and leaving a NNE trending inter-basin high. The lower seismostratigraphic units of the new system draped Tiburón and Tepoca basins. The abandoned system comprises two pull-apart basins clearly bounded by large NW faults that account for most of the subsidence. Major faults are linked by NNE-striking faults that accommodated transtension; the basement is well defined. The active basin system has similar dimensions to the fossil one (~ 70 x 200 km), is formed by several linked basins and is dissected by high density NNE fault zones that accommodate transtension. In contrast, the main boundaries and inter-basin limits are diffuse; the basement is poorly defined; the seismic reflectors are distorted by high-impedance, non-coherent units interpreted as discrete intrusions and the sediments are cut by small andesitic to rhyolitic volcanic edifices derived from differentiated magmas. We conclude that the early extension in the Gulf of California was accommodated by lithospheric thinning producing deep, rapidly filled sedimentary basins. The extension was re-localized and the basins were abandoned. The area currently accommodating extension developed on thinned lithosphere actually forming transitional crust evolving into nascent oceanic rift basins.