EVIDENCE OF SYNVOLCANIC CRUSTAL EXTENSION DURING THE MID-CENOZOIC IGNIMBRITE FLARE-UP IN THE NORTHERN SIERRA MADRE OCCIDENTAL, CHIHUAHUA, MEXICO

The timing and spatial extent of continental rifting in relation to mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province is relatively unknown, but it is an important consideration since crustal extension may have an influence on the generation of the large volumes of silicic magma. Our new geologic mapping and zircon U-Pb laser ablation ICP-MS ages from the Temoris-Guazapares region in the core of the northern Sierra Madre Occidental recognized three Oligocene synextensional volcanic units deposited during the mid-Cenozoic ignimbrite flare-up. The 29 – 27.5 Ma lower silicic ignimbrite unit is a >1 km-thick succession composed primarily of ignimbrite outflow sheets erupted from distal sources during the latest part of the main late Eocene – early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental. At least one of these ignimbrites is likely correlated to the widespread Divisadero tuff, described ~50 km to the northeast by Swanson et al. (2006). Erupted following the main ignimbrite pulse, the ~27.2 – 25 Ma middle intermediate volcanic unit consists of interbedded locally-derived lava flows and fluvial sedimentary rocks, with an upper section of thin distal silicic ignimbrites. The 25 – 24.5 Ma upper silicic volcanic unit is composed of silicic vent-proximal ignimbrites, lava flows, and plugs. In the Temoris-Guazapares region, there is evidence of synvolcanic half-graben deposition of the lower and middle stratigraphic units, as well as fault-controlled fissure volcanism of the upper unit erupted during continuing crustal extension. Based on vertical fault offset, we estimate a minimum of 15% horizontal extension in this region since the emplacement of the lower silicic volcanic unit. The synextensional nature of the mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental suggests a genetic relation between lithospheric extension and silicic volcanism.