Guillermo Labarthe-Hernández

Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico

The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24‐21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic arc, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest‐trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to ~20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.

Fissure ignimbrites: Fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting

The Sierra Madre Occidental is mostly composed of middle Tertiary large-volume ignimbrites. From the United States‐Mexico border (;318N), the Sierra Madre Occidental extends southward to its intersection with the Mexican volcanic belt (;218N). Ignimbrites of equivalent age extend into southern Mexico as discontinuous outcrops. Considering the average thickness of 1000 m for these ignimbrites based on representative measured sections, a conservative estimate of their total volume is ;393,000 km 3 . Fewer than 15 calderas have been identified in this province, and the source of most of these ignimbrites has been an unsolved problem. We present geologic evidence indicating that fissures, most of them with the regional trend of Basin and Range faults, served as conduits for the ignimbrites. These fissures can be several kilometers long and are represented by pyroclastic (ignimbrite) dikes, rhyolitic lava dikes, linearly aligned lava domes, and elongated coignimbrite lithic-lag breccias adjacent to Basin and Range faults. Considering that the Basin and Range extension overlapped in time and space with the ignimbrite flare-up, we propose a model in which batholith-sized magma chambers reached shallow crustal levels and erupted their contents when they reached Basin and Range normal faults. The faults acted as vents and caused fast decompression when the system was opened, and large volumes of silicic magmas were explosively erupted. Finally, devolatilized rhyolitic magmas were emplaced as domes or dikes. We propose the term ‘‘fissure ignimbrites’’ for ignimbrites formed in this way.

Tectono-volcanic control of fissure type vents for the 28 Ma Panalillo Ignimbrite in the Villa de Reyes graben, San Luis Potosí, México.

The volcano-tectonic events at the Villa de Reyes Graben (VRG), in the southern Sierra Madre Occidental, Mexico, include 1) a regional NNE fault system developed before 32 Ma, 2) this pre-32 Ma faulting controlled the emplacement of 31.5 Ma dacitic domes, 3) NE faulting at 28 Ma that displaced the 31.5 Ma dacitic domes and formed the VRG, as well as the oblique grabens of Bledos and Enramadas oriented NW, 4) emplacement of Panalillo ignimbrite at 28 Ma filling the VRG and erupting from fissures related to the oblique grabens, and eruption of Placa basalt apparently also from fault-controlled vents.

Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome

Pena de Bernal is a natural monument located near the town of Bernal, in Queretaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Pena de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by United Nations Educational, Scientific, and Cultural Organization (UNESCO). In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics, and chemical composition. It is a leucocratic-igneous rock intruding marine Mesozoic sedimentary rocks and has been misinterpreted as a pluton of Eocene or older age. However, this study shows that Pena de Bernal is a dacitic dome with SiO 2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Pena de Bernal body includes three plugs that crop out in an ∼3.5 × 1.5 km area elongated N40°E. Texture of the rock is porphyritic, nearly holocrystalline (80 vol% crystals and 20 vol% glass), with a mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Pena de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug.