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During the Cenozoic, the northern extra-Andean Patagonia was characterized by an extensive magmatism, mainly Oligocene (25–28Ma) and basaltic, i.e. the Somún Curá plateau. Younger, post-plateau magmatism produces several polygenetic volcanic complexes (Alta Sierra de Somún Curá, Sierra de Apas, Sierra de Telsen, Sierra de Talagapa, Sierra Pire Mahuida, and Sierra de Chacays) and minor monogenetic centres (Trayén Niyeu Volcano, Dos Hermanos Hills), developed from Late Oligocene through Miocene times. Up to the present, the Agua de la Piedra Volcanic Complex has been considered a post-shield eruptive assemblage. This volcanic building is composed of basal pyroclastic flows of trachytic composition and lava flows and domes which span from trachyte and lesser amounts of rhyolite to late basaltic flows. The series is characterized by its alkaline composition and displays a typical compositional gap in the mesosilicic realm. The evolution from the basic rocks to the trachyte-rhyolite differentiates was controlled by early olivine-clinopyroxene fractionation and later by plagioclase + K feldspar crystallization. There are scarse geocronological data of this volcanic series. In order to precisely date the bimodal sequence, a trachyte lava flow (Puesto Navarrete; 32.32 +0.48 -0.98 Ma), which constitutes the core of the Complex, and another rhyolite flow (Puesto Suárez; 32.01 +0.51 -0.34 Ma) were selected to perform U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) analyses. The basaltic facies was dated by K-Ar methodology at 24.4 ± 0.9 Ma. All in all, the ages pose new time constraints in the Agua de la Piedra Volcanic Complex within the evolutive context of the Somún Curá Magmatic Province. The new geochronological information obtained indicate that the trachytic and rhyolitic rocks would have formed synchronously or shortly before the main event that formed the plateau, while the ages of the basaltic rocks are bound to be coeval with other post plateau volcanic complexes.

Fragments of scleritomes of Chancelloria eros from the middle Cambrian of Sonora, Mexico are reported for the first time. Chancelloriid scleritomes partially articulated and isolated sclerites belonging to Allonnia tintinopsis and Archiasterella sp. are widely distributed in reddish calcilutite of the El Gavilán Formation, in the Sahuaral section, central Sonora. The faunal association is represented by chancelloriids, spicules of reticulosan sponges, linguliform brachiopods, hyoliths and trilobites. The El Gavilán Formation is an open shelf sequence of 65 m thick, composed of calcilutite interbedded with thin-bedded marly limestone. Trilobite association allowed assigning the open shelf deposit to the middle Cambrian, Altiocculus Subzone, Ehmaniella Zone (Series 3, Stage 5). Chancelloriid genera of the El Sahuaral section may be compared with the biota of San Jose de Gracia, in central Sonora, México, and also with other mid-Cambrian shelf deposits around Laurentia as Burgess Shale in Canada and formations from Utah, USA. Cambrian rocks of Sonora were part of the southern-western border of Laurentia craton, which was located near the paleo-equator during Cambrian times.

The Mercurio structural dome is a poorly exposed and complex structure located in the transitional region between the Coahuila Calcareous Platform and the San Pedro El Gallo sector of the Sierra Madre Oriental, Mexico. It is located in the State of Chihuahua, close to the limits with Coahuila and Durango, Mexico. The dome is a circular structure, ~16 km in diameter, that can be seen in air-photos, satellite images, and shaded relief maps, but that has a subtle topographic expression on the ground. As seen in the field, the most conspicuous topographic features in the area are several hills with the morphology of volcanic necks that rise up to 250 m above the surrounding terrain. The deformation fringe of the dome is a series of cuesta-like low hills, less than 30 m high, where a poorly lithified volcano-sedimentary succession (litharenites, polymictic conglomerates, and ignimbrites) is almost completely masked by desert pavement, which is mainly constituted by well-rounded calcareous clasts derived from the Mesozoic sedimentary marine rocks and by less abundant Paleogene volcanic rocks exposed in the region. Inside the dome the following units are exposed: 1) the pre-volcanic basement in a NW-trending, upright, open anticline developed in limestone of the Aurora Formation, 2) a series of hills where is exposed a succession of epiclastic and volcanic rocks, which are similar, in age and lithology, to some facies of the Ahuichila Formation, and 3) a NW-trending dike, exposed at Cerro Dinamita, which is interpreted as an offshoot of the buried subvolcanic body that created the dome. The deformation fringe around the buried intrusive has a quaquaversal array in the bedding and forms a simple monocline-like structure in the NE part of the dome. A set of SE- and NW-trending plunging folds forms the SE and SW portions of the dome, respectively. The NW part of the fringe is nearly completely masked by volcanic rocks, but there is a ~W plunging syncline in the area. Geophysical data show a broad gravimetric high in the region, and there is a distinct aeromagnetic anomaly inside the dome. The morphological expression of the dome lies just east of a NW-trending lineament of gravity and magnetic anomalies, which may be the buried portion of a normal fault shown in geologic maps of the region northwest of the studied area. Another possible cause is an alignment of buried intrusive bodies suggested by the regional aeromagnetic data, a small diorite outcrop south of Sierra El Diablo, and presence of volcanic necks in the northern portion of Sierra Los Alamos. Available geological and geophysical information was used to model a near-surface, irregular intrusive body with variable magnetic susceptibilites. This variation in susceptibilities is consistent with observed differences in rock composition in the exposed volcanic rocks and with evidence that the structure was formed by a bimodal (andesite-rhyolite) magmatic system where mixing/mingling occurred. As a whole, the set of structures is interpreted as a dome formed by forceful magma injection into a previously folded Paleogene volcano-sedimentary succession. U-Pb zircon ages were used to bracket the age of the deformation pulses registered in the rocks. Litharenites from the deformed volcano-sedimentary succession yielded an Ypresian zircon age of ~51 Ma. A tilted, lithic-rich ignimbrite collected near the top of the exposed volcano-sedimentary succession has mean age of 46.4 +0.8/-1.6 Ma, and the Cerro Dinamita dike has a mean age of 29.37 ± 0.24 Ma. Thus, the youngest pulse of Laramide deformation in the area is younger than ~46 Ma and the re-folding, associated with emplacement of the dome occurred at ~29 Ma. Detrital zircon U-Pb ages from Mercurio sandstones suggest dominant sediment sources from plutonic and/or volcanic rocks exposed along western Mexico. Likely subordinate sources are Mesozoic sedimentary rocks in northern and central Mexico. Distribution of detrital zircon U-Pb ages in the studied samples is similar to that documented in sandstones of the Difunta Group at the Parras and La Popa basins, except that older grains (>1.0 Ga), documented in the clastic rocks of these basins, are scarce in the sandstones of the Mercurio area.

A sedimentological and ichnological analysis carried out in the members Tierras Blancas and Rancho Viejo of the Concepción del Oro Formation (Turonian-Early Campanian), in the region of Concepción del Oro, Zacatecas, allows documenting the sedimentological characteristics, the sedimentary evolution, and its relationship with the orogenic belt. The Tierras Blancas Member (Turonian- Early Coniacian) is made up of successions of deltaic coastlines (i.e., delta fronts influenced/dominated by rivers or by waves/storms), with ichnofaunistic assemblages corresponding to the Skolithos and Cruziana ichnofacies, and a variable Bioturbation Index (BI: 0 – 4), represented by traces of organisms that feed on material in suspension and substrate, with small to moderate traces, controlled by the conditions of salinity, oxygenation, nutrient supply and energy from the sedimentary environment. The Rancho Viejo Member (Late Coniacian-Early Campanian) consists of mixed-type underwater channels, depositional-type submarine channel, channel overflow or abandoned canal, lobe-channel transition, depositional lobe, and mass transport complex (MTC) deposits of type II, characterized by ichnogeneous of the Cruziana icnofacies in its distal variation, Zoophycus and Nereites, with a BI variable between absent and abundant (IB 0 – 5), characterized by traces of organisms of moderate to large sizes that feed on suspended material, which combine the activities of feeding and dwelling. The average paleocurrent vectors in structures related to wave processes suggest a deltaic coastline with a strike almost SE 20°-NW 20°, while the unidirectional structures associated with sediment transport indicate that the deltaic system progradation was preferably toward the NE20°, perpendicular to the orogenic belt. The orientation documented in flute and tool marks structures in the deep systems (Rancho Viejo Member) determine a preferential strike toward the SE45° and SW30°, indicating that the transport of the sediments was perpendicular and axial to the orogenic belt.

In this paper, we carried out a kinematic analysis of the Cerralvo Extensional System (SEC), southern Gulf of California, and propose a crustal extension model from the processing and interpretation of 57.4 km of two-dimensional multichannel seismic-reflection data. The seismic image shows that in the SEC the main faults dip eastward with angles between 10–25° forming an asymmetric rift. The amount of horizontal extension measured in Cerralvo is 28.0 km, which corresponds to a crustal thinning factor of 2.0. The average subsidence rate in the SEC is 0.3 mm/yr, which is three times the marine sedimentation documented in this basin (0.1 mm/yr). Along the SEC, the acoustic basement is delimited by a layer with semi-continuous reflectors, which may represent cataclastic and/or mylonitic fabrics associated with low-angle normal faults. The penetrative deformation, the development of basins bounded by low-angle normal faults, as well as the magnitude of horizontal extension across the SEC, suggest simple shear deformation resulted in crustal extension since the late Miocene to the present. It is also inferred that the SEC is the eastern continuation of the Los Cabos Block structural system (BLC). Thus, our regional model proposes that in the currently advanced phase of the continental rupture process and the opening of the GC, the BLC and SEC fault systems converge at depth forming a regional detachment surface along the upper crust, causing the hanging wall to rotate to lower dip angles forming asymmetric basins with an eastward tectonic transport. Our results were contrasted with the sea bottom topography to the SEC. Here we inferred that at the western boundary of the Pescadero Norte pull-apart basin, the regional detachment surface must acquire an antilistric geometry characterized by a high angle shear zone through the lower crust. This style of deformation might result in the formation of metamorphic core complexes associated with regional rolling hinge structures.

In this work an integral analysis of a pedo-sedimentary sequence is presented, found in the lower fluvial terrace of the arroyo Tinajas, Sonora. The objective of this research is to establish the mechanism of the sequence formation in order to reconstruct the paleoenvironmental conditions. The sequence consists on a modern soil and eleven paleosols interbedded with fluvial sediments. The physical (color, grain size distribution, rock magnetic properties), chemical (pH, electric conductivity, total calcium content), and micromorphological properties are evaluated. The clay mineralogy determined by X-ray diffraction is also presented. Practically all the paleosols are truncated. Due to the lack of A horizons, the chronological frame of the sequence was obtained by using a small amount of organic matter found in the paleosol 8. To concentrate the organic matter, an extraction by demineralization with hydrofluric acid was used. This concentrated organic matter is dated by 14C accelerator mass spectrometry (AMS), giving an age of 8415 ± 90 years cal BP. The results show that all paleosols are weakly developed. However, it is possible to establish the main pedogenetic tendencies: the lower part of the sequence is finer, with strong reductomorphic features and vertic properties. In contrast, the upper paleosols are coarser in texture, more porous and have clay coatings. From paleosol 7, a change in the relief energy is detected due to the sedimentological variations (inverse gradation, increase in grain size), which is more evident in the paleosols 2, 3, 4, due to the high amount of gravels, bad sorting and angular shapes, related to fluvio-colluvial processes. We suppose that this change in the pedogenetic trend and the sedimentological variations is related to regional climatic oscillations in the middle Holocene, because no recent tectonic activity has been documented in the area.

An extensive lake existed during part of the Oligocene and almost all the Miocene in the region located east of the southern end of the Sierra Madre Occidental (SMOc). The lake was formed in a tectonic depression associated with the Aguascalientes graben and the Teocaltiche half-graben. Sediments accumulated inside this continental basin are epiclastic, mainly derived from volcanic rocks of the SMOc, and from the volcanic cover of the Mesa Central, chemical and biochemical precipitates (fresh water limestone), and mixed sediments, as well as far less voluminous, but ubiquitous, volcanic ash-fall layers, which were used to obtain U-Pb ages from zircons. The age range of the basin-fill sediments is broad, varying from Rupelian (~28.6 Ma) to Tortonian (~7.6 Ma). Previously, on the basis of fossil fauna, the succession was considered only as late Miocene in age. The southern end of the SMOc includes several NNE-trending basins of tectonic origin that contained lakes. Based on the characteristics of the individual sedimentary successions, and on their fossil faunas, it is known that three of these basins (Tlaltenango, Juchipila and Teocaltiche) had lakes at the same time. The fourth basin, the Bolaños graben, located west and parallel to the Tlaltenango basin, probably also had a lake during the Neogene, but almost all its sediments have been eroded, so that little is known about them. In order to explain the simultaneous partial closure of all the drainage systems a common, regional, cause must be invoked. All the tectonic basins in the southern part of the SMOc end close to the tectonic depressions related to either the Tepic-Zacoalco or Tula-Chapala rifts. Isostatic uplift of the footwall blocks of these ~E-W-trending rifts may have provoked partial closure of hydrologic systems within the NNE-trending grabens. It is not known when the basins were opened and ceased to act as aggrading systems, but the apparent absence of Blancan age fossils within them may indicate that they could have been opened prior to ~4.75 Ma, which is the numerical value of the beginning of this faunal stage. A probable common cause for the simultaneous opening could be a marine invasion in the region now occupied by the Gulf of California. The new base level of erosion probably increased the stream gradients in the fluvial systems, providing enough energy to the axial rivers within the grabens to overcome the uplift associated with ~E-W rift activity.

The La Alberca-Teremendo fault is a 26 km-long, complex fault composed of an en échelon array of short crustal fault segments, belonging to the Morelia-Acambay fault system. This fault system shows parallel scarps with morphological evidence of recent activity such as drainage alteration, maximum throws of 50 m and minimum throws of 1.4 m that displace the recent soils. The fault acted as a conduit for the formation of the La Alberca de Guadalupe maar (23000 to 21000 years ago) and displaced afterwards its phreatomagmatic sequences. The paleoseismic analysis indicates that the La Alberca-Teremendo fault moved three times in the past 23000 years (age of the maar); this activity caused an average vertical displacement of 87 cm, and might have generated earthquakes with magnitudes Mw between 6.6 and 7, as well as volcano-tectonic earthquakes with magnitudes Mw between 4 and 5.5. The displacements were identified on the fault through the superposition of soils differentiated by a disconformity and an anomalous increase in the percentage of clay and organic matter. The La Alberca-Teremendo fault has dominant dip slip with a minor left-lateral component, a slip rate of 0.114 mm/year, and an average recurrence interval of 7726 ± 68 years. According to scaling relations that use the surface rupture length, if we assume that the La Alberca-Teremendo fault moves tectonically, it could generate earthquakes with maximum magnitudes of Mw between 6.7 and 7.3, however because of the active volcanic processes in the area, we could expect moderate volcano-tectonic earthquakes (Mw 4–5.5) rather than catastrophic ones.

The Sigmodontinae subfamily represents one of the most diverse groups of mammals in the world; this rodent group evolved in the open and arid ecosystems of the Miocene of North America and was the most successful legion of mammals in the Great American Biotic Interchange. Part of its diversification occurred in the Mexican Pliocene, in the Hemphillian-Blancan boundary, where Prosigmodon and Sigmodon species are very common. Recent molecular phylogenetic systematics research propose that Sigmodon is related to South American sigmodontines, while studies of classical morphometry in isolated molar teeth consider Prosigmodon as synonymy of Sigmodon, which modifies the biogeographic and chronostratigraphic interpretations of this group in America. In this paper, we describe a new species of Prosigmodon from the late Hemphillian (~4.89 Ma) of central Mexico based on jaws, maxillary, and complete isolated teeth. This is the most complete and austral record of the genus in North America. This species was compared with North American species of the Sigmodontinae and Neotominae subfamilies and we defined it as a new Prosigmodon species characterized by having a consistently present minute mesoloph in M1 and M2, in addition, there is an isolated metaconid from the protoconid in the m1 of young individuals. We performed a phylogenetic model focused on understanding the relationship between Prosigmodon (four species) and Sigmodon (eight species), where we included Baiomys (two species), Neotoma (two species), Peromyscus (two species), and Reithrodontomys (two species). Our results indicate that Prosigmodon is a monophyletic group if Sigmodon minor is included within the genus and P. chihuahuensis is excluded. The Mexican Prosigmodon species have more apomorphic characters with respect to S. minor and P. holocuspis. The species of Prosigmodon and Sigmodon are not closely related, Sigmodon is more related to the Neotoma species than the species of Baiomys, Prosigmodon, Reitrhodontomys, and Peromyscus. Based on the topology of our cladogram and the stratigraphic ranges of the species Sigmodontinae and Neotominae, we discuss that Baiomys, Prosigmodon, Reitrhodontomys, and Peromyscus probably diversified in the early Hemphillian, while Sigmodon and Neotoma did so during the late Pliocene.

Folds are geological structures present in many tectonic environments, their geometry allows us to obtain a quantitative approximation of the deformation that they accommodated. Chevron folds are an important tool to estimate shortening in orogenic systems, due to their particular geometry. In spite of this, a method to calculate the amount of shortening (dynamic and kinematic) consistent with the theory of chevron folds has not been proposed. In this work, we propose a new method for the calculation of total shortening in chevron folds that have a post-buckling deformation based on the theoretical considerations of Ramsay (1974). Our method is conceptually simple and general, so that it can be applied to estimate the amount of shortening in the orogenic systems around the world.