Gerardo Carrasco-Núñez

Space-time patterns of Cenozoic arc volcanism in central Mexico: From the Sierra Madre Occidental to the Mexican Volcanic Belt

ABSTRACTA histogram of 778 isotopic ages of magmatic rocks younger than Eocene in centralMexico shows a multimodal distribution with peaks at about 30 Ma, 23 Ma, 10 Ma, and 5 Ma.The sample suite displays systematic spatial variations with age that likely reflect the pro-tracted transition from the north-northwest–trending arc of the Sierra Madre Occidental tothe east-west–trending Mexican Volcanic Belt. The reorientation of the arc is accompanied bya change in the dominant composition of the products from silicic ignimbrites and rhyolites toandesitic and basaltic lavas. The observed transition is related to the Miocene reorganization ofthe subduction system following the cessation of subduction off Baja California and the east-ward motion of the Caribbean–Farallon–North America triple junction along the southeast-ern margin of Mexico. Our data support an early–middle Miocene age for the initiation of sub-horizontal subduction in southern Mexico and confirm that the locus of arc volcanism wasprimarily controlled by the geometry of plate boundaries and the thermal structure of the sub-ducting slab.

A voluminous avalanche-induced lahar from Citlaltépetl volcano, Mexico: Implications for hazard assessment

Abstract During the late Pleistocene the ancestral edifice of Citlaltepetl volcano (also known as Pico de Orizaba) collapsed to form a clay-rich deposit that extends 85 km from its source, has a volume of 1.8 km 3 , and covers an area of 143 km 2 east of the volcano. The deposit has clay content ranging from 10 to 16% and contains secondary alteration minerals such as smectite and kaolinite. The deposits features suggest that it had an origin as a sector collapse of hydrothermally altered rock that transformed from a debris avalanche to a cohesive lahar very close to its source. The presence of glacier ice and a hydrothermal system during late Pleistocene times apparently provided a source of pore water which enhanced the hydrothermal alteration of the summit of Citlaltepetl and was the origin of most of the water for the lahar. This deposit and several others suggest that glaciated volcanoes are sites where hydrothermal alteration and resulting cohesive lahars are most likely. Although cohesive lahars and debris avalanches both have origins as sector collapses, cohesive lahars are more mobile than similar-sized debris avalanches. Thus potential hazard of edifice collapse at glaciated volcanoes, especially those with large volumes of hydrothermally altered rock, includes the possibility of large-volume cohesive lahars.

Late-Pleistocene to precolumbian behind-the-arc mafic volcanism in the eastern Mexican Volcanic Belt; implications for future hazards

An area of widespread alkaline-to-subalkaline volcanism lies at the northern end of the Cofre de Perote^ Citlaltepetl (Pico de Orizaba) volcanic chain in the eastern Mexican Volcanic Belt (MVB). Two principal areas were active. About a dozen latest-Pleistocene to precolumbian vents form the 11-km-wide, E^W-trending Cofre de Perote vent cluster (CPVC) at 2300^2800 m elevation on the flank of the largely Pleistocene Cofre de Perote shield volcano and produced an extensive lava field that covers s 100 km 2 . More widely dispersed vents form the Naolinco volcanic field (NVF) in the Sierra de Chiconquiaco north of the city of Jalapa (Xalapa). Three generations of flows are delineated by cone and lava-flow morphology, degree of vegetation and cultivation, and radiocarbon dating. The flows lie in the behind-the-arc portion of the northeastern part of the MVB and show major- and trace-element chemical patterns transitional between intraplate and subduction zone environments. Flows of the oldest group originated from La Joya cinder cone (radiocarbon ages V42 000 yr BP) at the eastern end of the CPVC. This cone fed an olivine-basaltic flow field of V20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of Jalapa, the capital city of the state of Veracruz. The Central Cone Group (CCG), of intermediate age, consists of four morphologically youthful cinder cones and associated vents that were the source of a lava fields 27 km 2 of late-Pleistocene or Holocene age. The youngest group includes the westernmost flow, from Cerro Colorado, and a lava flow V2980 BP from the Rincon de Chapultepec scoria cone of the NVF. The latest eruption, from the compound El Volcancillo scoria cone, occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in Mexico and resemble paired aa^pahoehoe flows from Mauna Loa volcano. The El Volcancillo eruption initially produced the high effusion rate, short-duration Toxtlacuaya alkaline aa lava flow from the southeastern crater. This 12-km-long hawaiite (average 50.5% SiO2) flow was followed by extrusion of the calc-alkaline R| ¤o Naolinco lava flow from the northwestern crater. This large-volume (V1.3 km 3 ) tube-fed basaltic pahoehoe flow (average SiO2 49%) traveled 50 km. Inferred effusion rates suggest emplacement over a decade-long period. Flows of all three age groups are transected by Highway 140 and the railway that form major transportation arteries between Jalapa and Puebla. This area has not previously been considered to be at volcanic risk, but volcanism here has continued into precolumbian time. Future eruptions of similar magnitude and location to those documented here could pose significant hazards to transportation corridors and to densely populated areas in and to the north of Jalapa. Slight

Probabilistic hazard analysis of Citlaltepetl (Pico de Orizaba) Volcano, eastern Mexican Volcanic Belt

Citlaltepetl or Pico de Orizaba is the highest active volcano in the North American continent. Although Citlaltepetl is at present in repose, its eruptive history reveals repetitive explosive eruptions in the past. Its relatively low eruption rate has favored significant population growth in areas that may be affected by a potential eruptive activity. The need of some criteria for hazards assessment and land-use planning has motivated the use of statistical methods to estimate the time and space distribution of volcanic hazards around this volcano. The analysis of past activity, from late Pleistocene to historic times, and the extent of some well-identified deposits are used to calculate the recurrence probabilities of eruptions of various size during time periods useful for land-use planning.

Eruption of a major Holocene pyroclastic flow at Citlaltépetl volcano (Pico de Orizaba), México, 8.5–9.0 ka

Abstract Multiple volcanic eruptions occurred between 8500 and 9000 yr. B.P. from the central crater of Citlaltepetl Volcano generating a series of pyroclastic flows that formed a deposit with a total volume of about 0.26 km3 (D.R.E.). The flows descended in all directions around the crater, but they were mostly controlled by topography and deposited in valleys or local topographic depressions up to about 30 km from vent. Although the flows were apparently emplaced without much violence, some features of the deposits reveal local turbulent conditions and an expanded fluidization that can be related to moderate flow velocities. The deposit has two members: the lower one consists of multiple flow units, and the upper one includes a single flow unit and a thin basal pumice-fall. Both members are lithologically similar and dominated by dense, andesitic scoriae with minor amounts of different pumice types (andesitic, dacitic, and banded), and lithics in a silty matrix. The eruption probably had a low-pressure ‘boiling-over’ mechanism and was possibly triggered by mixing of dacitic and andesitic magmas. Juvenile material in the pyroclastic-flow deposit is compositionally similar to that of Holocene lava flows at Citlaltepetl, which have apparently resulted from magma homogenization of mafic and silicic end members. Because the system is continuously injected with new basaltic-andesitic magma, a recurrence of explosive activity is possible in future eruptions.

Geology and geochemistry of Amealco Caldera, Qro., Mexico

Amealco Caldera, located in the central part of the Mexican Volcanic Belt (MVB), about 130 km NW of Mexico City, is characterized by a quasi-circular, ∼11-km-diameter structure whose southern portion is partly truncated by a regional fault. The caldera was formed (< 5 m.y. ago) due to a voluminous eruption of 65 km3 (eq. magma volume) of ignimbrites which covered an area of ∼1800 km2. This was followed by eruption of ∼2 km3 of andesitic and dacitic lava flows from the border of the caldera and emplacement of a total of 8 silicic (rhyolitic) domes (∼8 km3) inside the caldera. Finally, postcaldera volcanism (∼4 km3) consisted of extrusion of andesitic and basaltic andesite lavas and scoriae. We have analyzed 17 representative rock-samples for their major- and trace-element contents by X-ray fluorescence spectrometry and for rare earth elements by high-performance liquid chromatography. All samples were also analyzed for their Sr and Nd isotopic compositions which range as follows: 87Sr/86Sr 0.70379–0.70451, 143Nd/144Nd 0.51267–0.51278. The chemical and isotopic data are used to constrain the origin and evolution of magmas in the caldera. No combination of altered MORB and overlying sediments from the subducted slab (Cocos plate) can produce the Sr and Nd isotopic compositions observed in rocks from Amealco caldera. Further modelling reveals that the mantle-derived primary magmas in Amealco caldera may have assimilated up to ∼10% of a crustal component during their evolution from basaltic-andesites to dacites, but no further assimilation appears to have taken place during the evolution of the rhyolitic magmas. The structure and genesis of the Amealco caldera are comparable to some other calderas of the MVB.

Reappraisal of the Geology and Geochemistry of Volcá n Zamorano, Central Mexico: Implications for Discriminating the Sierra Madre Occidental and Mexican Volcanic Belt Provinces

We report new geological, 40Ar/39Ar geochronology, and geochemical data (major and trace elements, and Sr, Nd, and Pb isotopic ratios) on andesitic to dacitic rocks collected from Volcán Zamorano and on rhyolitic ignimbrites from the surrounding area. These data better constrain the relationship of two major volcanic provinces of Mexico: the mid-Tertiary Sierra Madre Occidental (SMO) and the Miocene to Recent Mexican Volcanic Belt (MVB). Structurally, the SMO and MVB show different extensional styles. The SMO was affected by east-west extension mainly during the Oligocene and middle Miocene, whereas the MVB was affected by extensional deformation during middle-to-late Miocene and Plio-Quaternary times, although reactivation of the older structural systems, such as the NNW-SSE system, has occurred in more recent times. The new geological data from the study area are consistent with the 40Ar/39Ar dates. The andesitic and dacitic rocks from Volcán Zamorano are about 10 Ma. These rocks, along with nearby andesitic and basaltic volcanoes, mark a relatively early phase of the MVB. The basaltic to dacitic rocks show the following ranges for Sr, Nd, and Pb isotopic ratios: (87Sr/86Sr)i 0.70362-0.70474; (143Nd/144Nd)i 0.51258-0.51286; 206Pb/204Pb 18.38-18.81; 207Pb/204Pb 15.57-15.63; and 208Pb/204Pb 38.27-38.70. For SMO ignimbrites (∼30-28 Ma) cropping out in the Zamorano area, (87Sr/86Sr)i are slightly higher (0.70407-0.70557), and (143Nd/144Nd)i are somewhat lower (0.51250-0.51253). Pb-isotope ratios for these ignimbrites are also higher (about 18.84-18.87, 15.65, and 38.78-38.82, respectively) than for basaltic to dacitic rocks from Volcán Zamorano and the surrounding area. Most studied rocks from both provinces seem to contain a crustal component. The ignimbrites are chemically and isotopically similar to rhyolitic and ignimbritic rocks sampled from other areas of the SMO. Our study of Volcán Zamorano shows that the transition from SMO to MVB was rather sudden and not a gradual process.

Holocene block-and-ash flows from summit dome activity of Citlaltépetl volcano, Eastern Mexico

Abstract A major eruption produced several block-and-ash flows about 4,100 years B.P. at Citlaltepetl volcano (Pico de Orizaba), an ice-capped, 5670-m-high, andesitic, active stratovolcano located at the eastern end of the Mexican Volcanic Belt. Repetitive gravitational collapse of a dacitic dome at the summit crater produced a series of block-and-ash flows, lahars, and floods, which were channeled through two main river-valleys on the west and south flanks of the volcano. The total erupted volume is estimated to be at least 0.27 km 3 . The deposits in both areas are similar in composition, and size, but they differ in the area covered, distribution, and structure. The western deposits form a large fan, cover a larger area, and include numerous laharic and fluviatile deposits. In contrast, the southern deposits form prominent terraces where confined in narrow channels, and have associated laharic units in distal areas, where the flows reach a maximum distance of 30 km from the vent. Directed disruptions of a central summit dome occurred, possibly first to the west and then to the southeast, perhaps due to minor modifications of the summit dome morphology, producing the voluminous block-and-ash flow deposits documented here. The flows were strongly controlled by topography, influencing the deposition of the moving particles. Grain-size variations along the flow paths are hardly detectable suggesting no evident lateral downstream transformations. Because sudden changes in dome morphology may cause significant variations in the direction of future dome collapse, specific areas of potential affectation cannot be predicted. Therefore, about 350,000 inhabitants living within a radius of 35-km from the vent could be potentially impacted if catastrophic block-and-ash flows were to recur in the future from similar summit dome activity. Recognition of these deposits is therefore important for hazard assessment because some seemingly safe areas may be at high risk.

Lava flow growth inferred from morphometric parameters; a case study of Citlaltepetl Volcano, Mexico

Recent (Quaternary) lava fields, such as those of Citlaltepetl (Pico de Orizaba) volcano in Mexico, are excellent places to obtain precise measurements of flow-field dimensions that can be used to estimate volume, eruption duration and effusion rates. The relationship between these parameters and the influence of some other interrelated features such as lava composition, superficial structures and lava type are important tools that can help to infer conditions when the lavas were active and thus improve understanding of how flow fields grow. The Holocene lavas of Citlaltepetl volcano are homogeneous in composition (dacites) and are generally blocky with well-developed levees. The eruption duration obtained for the Citlaltepetl lavas by a method proposed by C. Kilburn and R. Lopes presents a good correlation with the different lava types morphologically classified here. Results from that method compare favourably with the inferred effusion rates estimated by an empirical cooling method (Graetz). The lavas show different behaviour, mainly controlled by fluctuations in the effusion rate that promote changes from single- to multiple-flow style. The maximum distance achieved by a flow is directly proportional to the effusion rate in Citlaltepetl lavas, but it is always lower for multiple flows, independent of the volume of erupted lava. Observations of Citlaltepetl lavas can be used to understand how lava flow growth occurs on other volcanoes.

High incidence of lung cancer and malignant mesothelioma linked to erionite fibre exposure in a rural community in Central Mexico

Objective To report the high incidence of lung cancer (LC) and malignant mesothelioma (MM) linked to environmental exposure to erionite fibres in a rural village of central Mexico. Methods This is a retrospective survey of clinical and mortality records from the years 2000–2012, accompanied by an environmental survey for nine Group-1 lung and pleura carcinogenic agents listed by the International Agency for Research on Cancer (IARC). Results Out of a total of 45 deaths between 2000 and 2012, 14 deaths correspond to different neoplasms of the lung, and at least four deaths to MM. The ages at diagnosis of MM were between 30 and 54 years. Annual age-standardised mortality rates per thousand due to LC and MM in the village (age >20 years) are 7.09 and 2.48 for males, and 4.75 and 1.05 for females, respectively. Erionite fibres were found in exposed rocks and soils, which can easily become airborne and be carried into streets and recreational areas near schools and homes. Other carcinogenic elements and minerals are found only in trace amounts, except for quartz dust and asbestos (chrysotile) cement sheeting, which are also present in the neighbouring villages. Conclusions These results indicate that environmental exposure to erionite is the main cause of the high rates of MM mortality in the Village of Tierra Blanca, supporting previous similar reports for people exposed to erionite fibres in villages in Turkey.