ARE THERE TOO MANY RIFT BASINS IN MEXICO?

Lower-Middle Jurassic redbed in Mexico have been related to continental rifting pre-opening of the Gulf of Mexico. These include, from north to south, La Boca and La Joya formations of the Sierra Madre Oriental, the Cahuasas Formation in east Mexico, and the Todos Santos Formation of Veracruz, Tehuantepec isthmus, and Chiapas. La Joya Formation in the Mexican Altiplano and redbeds in the Sabinas basin have also been interpreted as rift sequences. According to available models, rift basins record: (1) separation of the Maya block from North America; and, (2) separation of South America from the Maya block and openning of the proto-Caribbean. Detrital zircon (DZ) geochronology data suggest similar ages and lithological associations for rocks of La Boca and Todos Santos in Chiapas. These include from base to top an older volcanic arc sequence, a middle volcanoclastic sequence, an arcosic sequence, and an uppermost conglomeratic sequence. The latter contains a diverse zircon population and predates flooding of the gulf, as it changes transitionally into evaporite deposition. Lithological associations and DZ provenance for Todos Santos strata in Tehuantepec are, however, different in that they lack Jurassic grains and contains abundant Proterozoic populations; they lack volcanic and volcanoclastic assemblages. We interpret the Huizachal sequence to record initial separation of the Maya Block from northern Mexico; whereas Todos Santos in Chiapas represents both: deposition during extension pre-rotation of the Maya Block and contemporaneous with it. Todos Santos in Tehuantepec and Veracruz is likely recording rifting of South America from the Maya Block, whilst La Joya in the Mexican Altiplano and redbeds in central Coahuila may record back arc extension related to trenchward migration of magmatism of the Nazas arc and initial opening of the Arperos basin. This process is followed by creation of a passive margin in Durango, Coahuila, Zacatecas and San Luis Potosí.