INITIATION OF GULF OF CALIFORNIA AS A MID-MIOCENE BACK-ARC EXTENSIONAL BELT

The initial rifting in the Gulf of California has been considered a late Miocene process in which the interaction between Pacific and North America plates was accommodated by a partition between right lateral deformation along the Tosco-Abreojos fault system west of Baja California (BC) and an almost orthogonal extension east of it, in the so called Gulf Extensional Province (GEP) (e.g. Stock and Hodges, 1989). In this model, mostly based on observations in BC, the initial extension in the GEP occurred almost entirely after the end of subduction (~12.5 Ma in southern BC). Our study of the geology of the western Nayarit and Sinaloa backed by Ar-Ar geochronology and scaled physical models point to an earlier beginning of the rifting process in the southern part of the Gulf. The eastern boundary of the GEP in northern Nayarit and in Sinaloa is made by a NNW-SSE to N-S trending and ~50 km wide belt of high angle extensional faults that bounds to the west the Sierra Madre Occidental ignimbrite plateau. The fault system is segmented in 150 to 190-km-long domains with alternate dipping separated by perpendicular transfer zones. Block tilting is to the ENE in northern Sinaloa and Northern Nayarit and WNW in southern Sinaloa between El Rosario and Piaxtla. The age of the NNW extensional belt is constrained by a regional pulse of geochemically uniform mafic volcanism found almost continuously all over the eastern side of the GEP. This consists of NNW striking dikes feeding flat lying lava flows that rest in unconformity over the SMO tilted block. Seventeen K-Ar and ten Ar-Ar ages constrain this mafic volcanic pulse between ~12 and 10 Ma. The basaltic lavas are exposed just above the present sea level, indicating a minimum of 2.2 km of vertical offset of the SMO ignimbrites, whose youngest Ar-Ar age is 18.7 Ma. These results are consistent with the inception of the Los Cabos extensional fault in southernmost BC, where footwall rocks indicate rapid cooling before ~10 Ma (Fletcher et al., 2000) and the older sediments in the associated basin are assigned to the middle Miocene on paleontological basis. A middle Miocene age for the initial extension of the Gulf makes it concurrent with the development of the Comondú andesitic arc to the west. In this case the initial rifting would have been formed in a back arc setting during the final stage of subduction.