Abstract The Sierra de Catorce in northern San Luis Potosí, Mexico, represents an uplifted block with exposures of the oldest rocks of the region which include Upper Triassic turbidites interpreted as deposits of a submarine fan system (“Potosí Fan”) and overlying Lower Jurassic volcanic and volcaniclastic strata interpreted as a record of the Early-Middle Jurassic volcanic arc (“Nazas Arc”) of western North America. These lower Mesozoic units, recognized in several exposures in the region, are interpreted as remnants of the ancient western margin of Pangea prior to accretion of Late Jurassic—Early Cretaceous magmatic arc complexes and associated marginal basins that constitute the Guerrero composite terrane in western Mexico and that resulted in construction of a new Pacific margin. A field trip in the Sierra de Catorce and surrounding exposures of the Upper Triassic—Lower Jurassic succession allows observation and discussion of key features that demonstrate the sedimentary and tectonic history of the western equatorial margin of Pangea.

Abstract The continental interior of Mexico is characterized by a Late Cretaceous prominent fold-thrust belt that shows characteristics of an eastward-tapering orogenic wedge. According to structural data and geothermometry of the deformation, this wedge is the result of horizontal stresses directed from the west (Pacific domain). The orogenic wedge is bounded to the west by the Guerrero Terrane, which is the second largest juvenile terrane accreted to the North American Cordillera. The possible linkage between the accretion of the Guerrero Terrane and the regional shortening in the Mexican interior is examined in detail in the region comprised between the Sierra de Guanajuato and the Peña de Bernal—Tamazunchale areas. In order to test the accretion hypothesis, we present key stratigraphic, structural, and geochronologic data from the Mexican Cordillera in central Mexico, and discuss the problems that exist in connecting the accretion of the Guerrero Terrane to the orogenic deformation of the Mexican continental interior.

Abstract Silver and gold mineralization was discovered in the late 1600s in the epithermal sulfide-rich veins of Mineral de Pozos Mining District, northeast Guanajuato state, Mexico. The main exploitation period of this mining district was between 1888 and 1922, with sporadic activities until 1942. Exploitation of the deposit is estimated at 1,200,000 tons of ore with average of 202 g/ton silver, and 11.83 g/ton Au. Mineral waste materials (more than 1 million tons) are scattered along the area on the main creeks and in the ancient processing plants ( Haciendas de Beneficio ). In this field guide, we present brief descriptions of the mineralization, the geology of the area, some of the ancient processing plants, and the potential dispersion of metals derived from the mine tailings into the environment (soil, sediments, and groundwater). Despite the relatively high concentrations of As and Pb in groundwater (0.011–0.090 mg/l As and 0.025–0.035 mg/l Pb), we consider that these values represent natural background values rather than contamination derived from anthropogenic input. However, we consider that Zn is the only metal potentially derived from mining activities being released to the environment. The historical processing plants offer a very interesting perspective of the more than a century old mining activities.

Abstract Los Azufres Volcanic Field hosts the second most important geothermal field of Mexico, with a production of 188 MW of electricity. Based on fieldwork and new geochronological data ( 14 C and 40 Ar/ 39 Ar) we define that activity at Los Azufres Volcanic Field started some 1.5 Ma with the emission of basaltic to rhyolitic lavas, and pyroclastic material. The late Pleistocene explosive activity in the southwest sector (Guangoche volcano area) of Los Azufres occurred in a narrow period of time between >31 and <26 ka. The pyroclastic stratigraphy of the S, SW, and W sectors is represented by diverse deposits of dacitic and rhyolitic composition, including a debris avalanche deposit related to a sector collapse of San Andrés volcano, several pyroclastic sequences associated with plateau forming lavas, and Guangoche volcano. Guangoche volcano was the focus of late Pleistocene eruptive activity with two Plin-ian and one subplinian events that deposited pumice-rich falls and pyroclastic flows and surges. These deposits are informally named the White Pumice (29 ka), which originated from a 23-km-high eruptive column and the ejection of 1.7 km 3 of tephra that covered an area of at least 223 km 2 with a mass discharge rate of 9 × 10 7 kg/s; the Ochre Pumice fall (<26 ka), deposited from a 16-km-high eruptive column involving 1.3 km 3 of tephra at a mass discharge rate of 1.9 × 10 7 kg/s; and the Multilayered fallout (<<26 ka) that resulted from an 11-km-high eruptive column with 1 km 3 of tephra at a mass discharge rate of 4.6 × 10 6 kg/s. The complete late Pleistocene stratigraphy suggests that explosive events at Los Azufres Volcanic Field have been intense. They are the subject of ongoing investigations to better understand this kind of large magnitude eruptions.

Abstract The eastern Trans-Mexican Volcanic Belt is characterized by a diversity of volcanoes that are related to different processes and eruptive styles. The spectacular exposures of late Pleistocene and Holocene volcanism provide a unique opportunity to explore a variety of volcanic features and deposits that may be relevant for volcanic hazard assessments within the area. This three-day field guide describes selected representative examples of the regional volcanism showing volcanic features including thick pyroclastic successions derived from the explosive activity of Los Humeros caldera volcano, caldera-rim effusions, alternating explosive and effusive activity of a vitrophyric rhyolite dome (Cerro Pizarro), and the eruptive activity of two composi-tionally contrasting maar volcanoes: Atexcac, a classic basaltic maar and Cerro Pinto, a rhyolitic tuff ring—dome complex.

Abstract The modern Mexico Megacity occupies almost a third of the surface of the Valley of Mexico, and it is exposed to natural and man-induced hazards affecting many aspects of urban development. Land subsidence is a geo-hazard imposing important constraints in the urban development by the gradual decrease in elevation of the land surface. This is caused either naturally, by the extraction of water, oil, minerals, or gas from the subsurface, or by the interaction between natural and anthropogenic forces. In this field trip guide we examine regional land subsidence and the vulnerability to fracturing of the lacustrine soils. Groundwater has been over-exploited for human consumption in Mexico City during the past 70 years, leading to a dramatic decline of piezometric levels and the associated land and subsoil deformation. Interdisciplinary research from geologists and engineers may play an important role in understanding the relationship between geological processes and the suitability of land for urban use.

Abstract Guanajuato has a long history (450 years) of mineral exploitation and remarkable silver and gold production from a complex system of fault-veins. Despite this, it is only in the past 40 years that the systematic study of its geology has been conducted. Mid-Tertiary epithermal veins occur in all the Mesozoic and Paleogene rock units exposed in the mining district, and mineralization seems to be the result of the combination of several geologic factors, such as the occurrence of greenschists in the basal complex, a thick sequence of Early Paleogene red beds overlain by a thick succession of Oligocene volcanic rocks with the existence of one or more paleolakes when the volcanoes were active. The systematic study of the greenschists and associated plutonic and sedimentary rocks in the basal complex of Sierra de Guanajuato has contributed significant information to the concept of accretion of the Guerrero terrane to the SW end of the North American craton in the Early Cretaceous. Research on the Eocene red bed sequence suggests that early extension occurred creating fault patterns that later were reactivated during Neogene Basin and Range pulses. Immediately east of the city of Guanajuato, a thick volcanic sequence is exposed, with two pyroclastic units formed by felsic ignimbrites that almost certainly are related to a nearby caldera, which was active immediately prior to Ag-Au mineralization. The first activity pulse of the caldera produced the Bufa ignimbrite, a massive unit that displays very large thickness variations (300 to <10 m) in short distances, which we interpret as a signal that it may be an intracaldera deposit. The second explosive pulse originated the Calderones formation, a unit formed by an undetermined but large number of ignimbrites, surge deposits, layers with accretionary lapilli, and epiclastic-volcanic deposits. The Calderones formation is characterized by pervasive chloritization, which points out toward the presence of external water in the system, probably related to one or more shallow lakes within the caldera previously formed by the Bufa eruption. Lithofacies variations and stratigraphic arguments suggest that the Guanajuato caldera was probably located near the Cerro Alto de Villalpando and La Peregrina lava dome complex. Morphological and structural evidence of the caldera are masked by several pulses of younger normal faulting which affected the southern portion of the Mexican Basin and Range Province (i.e., Mesa Central).

Abstract The Sierra Nevada Volcanic Range includes, from south to north, the active Popocatépetl (5452 m), Iztaccíhuatl (5272 m) with several volcanic edifices, Telapón (4000 m), and Tlaloc (4150 m) volcanoes. It has been generally assumed that volcanic activity has migrated from Tlaloc (north) to Popocatépetl (south) over time. New evidence obtained from previous studies, field reconnaissance, and radiometric dating indicate that magmatism at the Sierra Nevada Volcanic Range likely started at 1.8–1.4 Ma with the construction of Paleo-Tlaloc volcano, which is today buried by younger deposits. The activity continued between 1.07 and 0.89 Ma with the emplacement of dacitic domes (Puico, Yahualica, Yeloxochitl, Tearco, and Torrecillas), lavas and associated pyroclastic flows as the San Francisco (1 Ma), and Chicoloapan (0.9 Ma). Afterwards, the main edifice of modern Tlaloc was built up through the emission of dacitic lava flows (0.94–0.84 Ma). Iztaccíhuatl began its activity ca. 1.1 Ma with the formation of several volcanic edifices up to 0.45 Ma, time during which a Mount St. Helens—type event destroyed the southeastern flank of Los Pies Recientes cone, producing a debris avalanche and pyroclastic deposits. Telapón volcano formed approximately between 0.38 Ma and 0.34 Ma ago with the emplacement of lava flows and a dome that become quiescent afterwards. Apparently, ca. 0.32 Ma, Popocatépetl began its eruptive activity that continues today. Strikingly, Tlaloc reawakened with the emission of rhyolitic magma at 0.129 Ma followed by the emplacement of the El Papayo dacite (118 ka) to the south and Téyotl summit lavas (80 ka). Activity continued at Tlaloc with the generation of five explosive eruptions at 44, 38, 33, 31, and 25 ka and the growth of the summit dome. Coevally, Popocatépetl, at the southern end of the range, had collapsed twice to the south and had intense volcanic activity up until today. Holocene activity has taken place at Iztaccíhuatl with the 9 ka Buenavista dacitic lava flow and repetitive Plinian eruptions of Popocaté-petl including some historic events and the 1994—present eruption. Popocatépetl's reawakening reminded authorities, scientists, media, and the public of its potential risk. In fact, on 22 January 2001, the rapid collapse of an eruptive column generated scoria-rich pyroclastic flows that partly melted the glacier producing lahars. From the above considerations it is obvious that magmatism of the Sierra Nevada Volcanic Range did not keep a continuous north to south migrating path, but it rather shifted back and forth chaotically throughout its evolution. It is worth mentioning that major gaps presented in the eruptive sequence are most likely due to poor radiometric coverage of the area that may improve in the future.

Abstract Prepared in conjunction with the 2012 GSA Cordilleran Section Meeting, Querétaro, Mexico, this volume's eight field guides showcase three aspects of the geology of the southern end of the North America cordillera: Mid-Tertiary and Quaternary volcanology, environmental geology, and Mesozoic tectonics. Field Guide 25 explores the Cenozoic stratigraphy of Sierra de Guanajuato, one of the most important Mexican mining districts, and addresses a controversial topic, the accretion of the Guerrero terrane and its possible role in the Late Cretaceous—Early Tertiary orogeny. Three guides related to the Trans-Mexican Volcanic Belt, an active magmatic arc related to subduction of the Rivera and Cocos plates, include new data about the recent volcanic history, physical volcanology, and volcanic hazards in Mexico's most densely populated area. Bringing the geosciences into societal problems, one guide presents data on ground deformation related to water extraction in urbanized areas of the Mexico City basin, and another explores the ghost town of the Mineral de Pozos mining district and the effect of mine tailings on groundwater.

Abstract Despite the fact that most fold–thrust belts around the world share many features, successfully explained by the critical wedge model, the details of their geometric evolution and tectonic style development are poorly understood. In the classic section of the southern Canadian Rocky Mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices. In the Mexican fold–thrust Belt (Central Mexico), the age of deformation, the overall structural pattern and the total amount of shortening are similar, but the individual thrust sheets exhibit much more internal deformation as manifest by metre-scale buckle folds. One of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly-bedded basinal limestone in the Central Mexico section. Strain is concentrated toward the margins between platforms and basins. In Canada, thick platform carbonates form continuous resistant units across the Front Range. Possible reasons for the differences in tectonic style between the two sections include the dominant lithology, distribution of lithologies, taper angle of the tectonic wedges, amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks.

Abstract We present analogue models that illustrate the tectonic evolution of the continental margin of southwestern Mexico and the Early Cenozoic deformation of the Xolapa complex. Together with geological data they suggest that oblique convergence caused distributed deformation and mountain building near the present-day margin of southern Mexico in a general left-lateral transpressional regime. A similar deformation is also observed north of the Xolapa complex in Maastrichtian to Paleocene sedimentary and volcanic rock units. Since post-Oligocene exhumation of middle crust does not significantly affect Late Eocene to Oligocene volcanic rocks, we infer that the evolution of the transform margin led to the formation of discrete boundaries that eventually decoupled exhumed mid-lower crust from the onshore upper-crust sequences since the Late Eocene.

Along the coast of Oaxaca between Puerto Angel and Santiago Astata, a suite of rocks is exposed corresponding to the Xolapa metamorphic complex, which is intruded by granitoid igneous rocks of Paleogene to Miocene age. Both of these rock units are in fault-contact along coast-parallel shear zones with granulite facies metamorphic rocks of the Proterozoic age Oaxaca Complex and with cover rocks that unconformably overlie this crystalline basement. The largest of these shear zones is the Chacalapa shear zone exposed to the north of the town of Pochutla and formed by ultramylonites, mylonites, protomylonites, pseudotachylytes, phyllonites, and cataclasites. The kinematics of this shear zone are dominantly left-lateral. The mylonitic rocks of the Chacalapa shear zone indicate temperatures ∼500 °C (as evidenced by crystal-plastic deformation of plagioclase feldspar) that decrease systematically to a strictly brittle deformation regime. Quartz crystallographic textures associated with crystal-plastic deformation processes are locally asymmetric, indicating left-lateral shear. Brittle structures also exhibit left-lateral shear-sense indicators, as do active deformation structures. The age of the shear zone is constrained between 29 ± 0.2 Ma and 23.7 ± 1.2 Ma. The upper limit is the age of the Huatulco granodiorite, whose northern contact is sheared, and the lower bound is fixed by the K-Ar age of hornblende separated from granodioritic dikes that cut the mylonitic lineation. A lo largo de la costa de Oaxaca, entre Puerto Ángel y Santiago Astata, afloran rocas metamórficas del Complejo Xolapa intrusionadas por rocas ígneas de edad Paleógeno a Mioceno sin metamorfismo regional. Ambas unidades se encuentran en contacto tectónico a lo largo de zonas de cizalla con rocas Proterozoicas del Complejo Oaxaqueño al norte de Pochutla y con rocas sedimentarias Mesozoicas discordantes que sobreyacen los gneises. La principal de estas zonas de cizalla en el área de estudio es la falla Chacalapa, expuesta al norte de Pochutla, que está constituida por ultramilonitas, milonitas, protomilonitas, pseudotaquilitas, filonitas y cataclasitas en orden cronológico de desarrollo. La cinemática de esta zona de cizalla vertical es pre-dominantemente lateral-izquierda. Las rocas miloníticas de la falla Chacalapa registran temperaturas de recristal-ización dinámica de ∼500 °C (deformación cristal-plástica de feldespatos) que dis-minuyen sistemáticamente hasta el régimen netamente quebradizo. Asociadas a la recristalización dinámica se desarrollaron texturas cristalográficas de cuarzo en las rocas miloníticas que, cuando son asimétricas, exhiben un sentido de cizalla izquierdo. Las rocas del régimen quebradizo también registran una cinemática izquierda, al igual que las fallas activas. La edad de las rocas miloníticas de la falla Chacalapa se ubica entre los 29 ± 0.2 Ma y los 23.7 ± 1.2 Ma. El límite de edad superior lo constituye la edad del intrusivo Huatulco (U-Pb de circones, que se observa milonitizado en su margen sep-tentrional y el límite de edad inferior corresponde a la edad K-Ar de hornblenda procedente de diques granodioríticos porfídicos que truncan la milonita).