Cenozoic volcanism and extension in northwestern Mesa Central, Durango, México

the santiago papasquiaro region is located in the northwest portion of the mesa central and is characterized by tertiary sierra madre occidental lithology. geologic mapping and dating of key units using u-pb laser ablation method on zircons has identified three different lithostratigraphic groups. these include: (1) early eocene felsic volcanic rocks, consisting of the antigua ignimbrite [51.75 +0.35/-0.45 ma]; (2) late eocene - early oligocene felsic to intermediate volcanic rocks, consisting of the altamira ignimbrite [38.8 ± 1.0 ma], el cazadero andesite [ages of 37.2 +0.30/-0.40 ma and 35.95 +0.45/-0.5 ma], a dioritic intrusion, the venadita rhyolite, puente negro ignimbrite [34.0 +0.50/-0.70 ma], los fresnos ignimbrite and balín ignimbrite [33.2 +0.50/-0.20 ma]; and (3) late oligocene to quaternary sedimentary and volcanic mafic rocks comprising the santiago sedimentary formation, basalts equivalent to metates formation and neogene to quaternary continental non-consolidated deposits. within the studied area, the main volcanic pulse of the sierra madre occidental volcanic province corresponds to the second group, which lasted ca. 5 ma. older rocks are represented by a single outcrop of the antigua ignimbrite, and the younger units are mainly continental clastic sediments intercalated with sporadic mafic alkaline volcanic rocks that record a major change in the tectonic regime. extension that began in late eocene - oligocene gradually tilted the late eocene - early oligocene volcanic sequence and formed the nnw-trending santiago papasquiaro half-graben, which is the northernmost and earliest structure of the río chico-otinapa graben. nw-striking faults of the san luis-tepehuanes fault system were synchronous with the development of the santiago papasquiaro half-graben. the deformation propagated gradually to the south, forming the nnw río chico-otinapa graben by the early-middle miocene, and to the east-southeast in the santiaguillo graben. a recent minor extensi