TIMING AND GEOLOGIC SETTING OF ESPINAZO-EL SALTO IGNIMBRITE SUCCESSION, CENTRAL SIERRA MADRE OCCIDENTAL
Until 20 years ago the geologic knowledge about the central and southern Sierra Madre Occidental was mostly based on the pioneer studies of McDowell and Keizer (1977) along the Durango – Mazatlán highway. These authors defined as the Espinazo-El Salto Volcanic Sequence (EESVS) a succession of flat-lying ignimbrite exposed in the highest part of the transect. They recognized five units with K-Ar ages indistinguishable within analytical error clustering at 23.5 Ma. Samples from uppermost units were later dated by McDowell and McIntosh (2012) at 23.88 to 24.00 Ma by the Ar-Ar method. The study of the McDowell group was limited to the area surrounding the highway and did not reached the base of the succession.
Our mapping along the new Durango – Mazatlán toll highway and several dirty roads descending to the north and to the south into the valleys of the Presidio and Baluarte rivers, respectively, shows that the EESVS is up to 900 m thick to the west of the 150-km-long, NNW-SSE trending Tayoltita-Pueblo Nuevo extensional fault system (TPNFS). Here the ignimbrites filled pre-existing valleys and lie in angular unconformity on tilted blocks of the ~31-29 Ma Durango ignimbrite succession or cover a poorly sorted conglomerate with a dominant detrital zircon population in the ~32–25 Ma range. By contrast, to the east of this fault system, the EESVS does not exceed 250 m in thickness and the Durango ignimbrite sequence is almost horizontal. These geologic observations imply that the eruption of the EESVS took place after a period of ENE-WSW extension that activated the TPNFS. Ignimbrites with ages comparable with the EESVS are found in a NNW-SSE elongated, 145 x 70 km wide area covering a minimum of ~7,500 km2 in the westernmost part of Durango, central Sinaloa and northern Nayarit. No obvious caldera is found in this region, but the elongated distribution of the EESVS around the TPNFS suggests that it was fed by a series of fissure eruptions.
To obtain a more precise timing of the EESVS emplacement we have dated its base and top by Ar-Ar dating of single feldspar crystals. For each sample, ten to twenty-five total fusion analyses were obtained, and only those data above the limit of quantification were used for the age calculations. The upper part of the EESVS yielded tightly clustered individual ages with peaks between 23.78 and 23.88 Ma, consistent with the Ar-Ar ages of McDowell and McIntosh (2012). Samples from the basal unit, however, display a wide dispersion of ages ranging from ~27 to ~23 Ma, with peaks at 25.8 and 26.8 Ma. Given that the whole succession is stratigraphically continuous (no angular unconformity or paleosoils are observed between ignimbrites) we consider these ages biased by pre-existing crystals of slightly older shallow intrusions that were mixed with juvenile ones during the first massive eruption that opened the path to the surface along the TPNFS.