Gerardo J. Aguirre-Díaz

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.

The Acambay graben: Active intraarc extension in the trans‐Mexican volcanic belt, Mexico

The trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench. The central part of the belt is being deformed by the Chapala-Tula fault zone, an approximately 450-km-long and 50-km-wide zone of active extension. The volcanic arc and the arc-parallel Chapala-Tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the Mexican Basin and Range. The Acambay graben, about 40 km long and 15 km wide, is located approximately 100 km northwest of Mexico City and is one of the major troughs within the Chapala-Tula fault zone. The border faults of the Acambay graben, Acambay-Tixmadeje in the north and Pastores in the south, are separated in the west by stepovers from range-bounding faults of similar orientation, Epitacio Huerta in the north and Venta de Bravo in the south. The stepovers occur at the intersection of these faults with an older system of Basin and Range faults. An early-stage right-lateral component of motion along the Venta de Bravo and Pastores faults is inferred on a map scale from a left-stepping en echelon array of normal fault segments. The divergence of the en echelon segments from the general fault trend decreases gradually from west to east, suggesting that the early extension was rotational. The present relative displacement along the southern margin of the system, on the other hand, results in a left-lateral strike-slip component. This is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left-oblique slip. The striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component. This is supported on a map scale by the structure of the right stepover between the Acambay-Tixmadeje and Epitacio Huerta faults, which shows no evidence of local extension or shortening. The divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation. This could explain why no active extension has been observed to the east of the Chapala-Tula fault zone, in the eastern part of the trans-Mexican volcanic belt. During the Ms = 6.9 Acambay earthquake of November 19, 1912, surface rupture occurred along both margins of the graben at the base of multiple-event scarps. Along the Acambay-Tixmadeje fault, the coseismic rupture is 41 km long. The vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum. Furthermore, the change in the vertical surface offset along the fault is approximately proportional to the change in height of the Acambay-Tixmadeje multiple-event fault scarp. The easternmost part of the ground rupture passes through a plain and not at the base of a multiple-event scarp as farther west. It may therefore correspond partly to an increase in length of the Acambay-Tixmadeje fault during the 1912 earthquake. The slip rate along the southern border of the Acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40Ar/39Ar age of 0.4±0.1 Ma. This basalt may be displaced up to 15 m by the Pastores fault, which indicates a middle-late Quaternary slip rate of ≤0.04 mm/yr. Furthermore, based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake, we estimate a recurrence interval of ≥5000 years for major earthquakes along the faults of the Acambay graben.

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.

The volcanic section at Nazas, Durango, Mexico, and the possibility of widespread Eocene volcanism within the Sierra Madre Occidental

Descriptions of volcanic rocks in the Sierra Madre Occidental of western Mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the Oligocene ignimbrite flare-up. However, much less attention has been given to the pre-Oligocene volcanic sequence beneath this ash flow blanket. At Nazas, Durango, on the eastern flank of the Sierra Madre Occidental, a well-exposed section includes voluminous felsic volcanic rocks of both Eocene and Oligocene age. The oldest igneous rocks at Nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie Cretaceous limestones and apparently were deformed with them during Laramide tectonism. More commonly, the limestones are overlain by Tertiary volcanic rocks and continental clastic deposits. The Tertiary volcanic section is about 800 m thick, not including intercalated fanglomerates. The section records three distinct magmatic episodes: from 51 to 40 Ma, comprising felsic ash flow tuffs and intermediate lava flows and domes; a pulse at about 30 Ma, comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m; and from 24 to 20 Ma, comprising alkalic basalts. The Ahuichila Formation, a molasse-type conglomerate up to 200 m thick that underlies the Tertiary volcanic section, was deposited during or just after the Laramide deformation. The Santa Ines Formation is a widespread fanglomerate, up to 110 m thick, that underlies the basalts and apparently accumulated during normal faulting. Eocene volcanism in the Nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes. A similar sequence of Eocene volcanic rocks has been mapped in central Chihuahua state, about 350 km to the northwest of Nazas. Felsic tuffs interbedded with intermediate volcanic rocks older than 40 Ma are also exposed at Tayoltita, Durango, about 200 km southwest of Nazas. These occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the Eocene than during the Oligocene in the Sierra Madre Occidental. Eocene volcanism was more typical of orogenic magmatic belts developed at continental margins, whereas the Oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension. In addition to these three areas, there are several other localities in western Mexico having volcanic rocks with reported ages between 40 and 53 Ma. It is possible that the volcanic field during the Eocene in Mexico was comparable in extent to that of the Oligocene. The Eocene volcanism in western Mexico was in part contemporaneous with the Challis volcanic episode of northwestern United States and its extension into western Canada. However, Eocene magmatism in the Pacific Northwest apparently developed in an extensional tectonic setting, whereas in Mexico no evidence for Eocene extensional deformation is known.

Nature and timing of faulting and synextensional magmatism in the southern Basin and Range, central-eastern Durango, Mexico

The Nazas area, in the central-eastern portion of the state of Durango, is within the poorly studied southern half of the Basin and Range province in Mexico. At Nazas, high-angle normal faults that cut part of the mid-Tertiary volcanic sequence strike between N 20° and 70° W, with 40% of them between N 40° and 50° W. Tilting of faulted blocks varies from 5° to 35° to the northeast, most commonly around 25° NE. A few blocks are tilted to the southwest as much as 25°. Fault offsets range from 40 m to nearly 300 m. The earliest faulting occurred between 31 and 29 Ma, before the emplacement of mid-Tertiary ash-flow tuffs had ended. The Santa Ines Formation, a widespread fanglomerate, was deposited after the earliest faulting episode and is overlain by 24 to 20 Ma alkalic basalt flows. Although not cut by faults, the flows are adjacent to or cover normal faults. Some of the mapped faults could have been coeval with basalt eruption, as is the case in Trans-Pecos Texas, where alkalic basalt having similar age and composition to that in the Nazas area erupted contemporaneously with normal faulting. The Nazas alkalic basalt also has similar age and stratigraphic position as, but is compositionally distinct from, the Southern Cordilleran Basaltic Andesite (SCORBA), a widespread mafic suite in southwestern North America that has been linked to regional extension.

Volcanic evolution of the Amealco caldera, central Mexico

The Amealco caldera is a well-preserved Pliocene volcanic center, 11 km in diameter, located in the central part of the Mexican Volcanic Belt. It is one of seven calderas known in the belt. Compared to those of the other calderas, the Amealco products are less evolved, and include only a minor volume of rhyolite. According to the stratigraphic record and K-Ar data, the inferred volcanic history of the Amealco caldera is as follows. Caldera-related activity started ca. 4.7 Ma with eruptions of pumice fallout and pyroclastic flows apparently of Plinian type. These events were followed by eruption of far-reaching surges and pyroclastic flows that deposited three widespread ignimbrites named Amealco I, Amealco II, and Amealco III. By about 4.7 Ma at least 77 km 3 (Dense rock equivalent, DRE) of trachyandesitic-trachydacitic magma were evacuated from the magma chamber and caused caldera collapse. After this climatic stage, pyroclastic activity continued, probably as tephra fountains from ring-fracture vents, that erupted pumice flows and fallouts that were accompanied by mud flows forming deposits of local extent. Both tephra and mud-flow deposits make up a DRE volume of 2.35 km 3. This was followed by 4.3 Ma trachyandesitic lava domes that were emplaced through several ring-fracture vents, making up a DRE volume of 3.8 km 3; the domes form the caldera’s present topographic rim. At about 4.0 Ma, a modest-sized volcano had formed on the western flank of the caldera that erupted several trachyandesitic lava flows and fallout tephra (both lava and tephra deposits = 0.8 km 3). Between 3.9 and 3.7 Ma, 10 intracaldera lava domes were emplaced, accompanied by tephra eruptions that produced relatively small deposits (volume not quantified) that were later reworked and redeposited as lake deposits within the caldera; five of these lava domes are trachyandesitic (4.3 km3) and five are rhyolitic (2.4 km 3). The central lava domes are interpreted here as the viscous, gas-poor magma that usually erupts at the end of a caldera cycle, and thus may mark the end of the volcanic evolution of the Amealco caldera. Volcanic activity continued adjacent to the caldera for at least 1.6 m.y. after the emplacement of the central lava domes. These events include bimodal volcanism at 3.7 Ma of basaltic-andesite lava from a volcano just north of the caldera rim (Hormigas volcano) and emplacement of several rhyolitic lava domes to the southwest of the caldera (Coronita rhyolite). At 2.9 Ma a rhyolite (obsidian) lava dome complex was

The Sanfandila earthquake sequence of 1998, Queretaro, Mexico: activation of an undocumented fault in the northern edge of central Trans-Mexican Volcanic Belt

Abstract A sequence of small earthquakes occurred in Central Mexico, at the northern edge of the Trans-Mexican Volcanic Belt (TMVB) in the State of Queretaro, during the first 3 months of 1998. Medium to large events in the continental regime of central Mexico are not common, but the seismic history of the region demonstrates that faults there are capable of generating destructive events. The sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics. Employing a waveform inversion scheme adapted from a method used for regional studies, we found that the source mechanisms conform to the style of faulting (i.e. extension in the E–W direction) representative of the Taxco–San Miguel Allende Fault system. This system has been proposed as the southernmost extension of the Basin and Range (BR) Province. The spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334° with almost pure dip slip. Since events which occurred just south from this region show features which are consistent with TMVB tectonics (i.e. extension in an N–S direction), the sequence may mark the boundary between the TMVB and BR stress domains.

Nature and P-T Conditions of the Crust Beneath the Central Mexican Volcanic Belt Based on a Precambrian Crustal Xenolith

A granulite xenolith was encountered in an ignimbrite of Amealco caldera, México. This sample is one of three granulite occurrences in the central Mexican Volcanic Belt. It is a medium-grained, equigranular rock, with plagioclase (60 vol%), quartz (10%), orthopyroxene (8%), clinopyroxene (2%), accessory ilmenite (<1%), apatite and zircon (both <1%), and glass (20%). The glass represents partial fusion, probably due to decompression. Absence of alumina-rich phases or graphite indicates an igneous protolith. Isotope values of 143Nd/144Nd = 0.512681 ± 16 (Nd = +0.84) and 87Sr/86Sr = 0.705874 ± 39 confirm a volcanic arc tectonic setting. The granulite yielded a model age of 683 Ma. Whole-rock chemistry indicates a dacite composition. REE patterns show a positive Eu anomaly with a general negative slope from La to Lu, and HFSE such as Nb are relatively depleted, as expected for subduction-generated magmas. Fluid inclusion (FI) studies performed mainly on feldspars revealed four FI types. Type 1 is represented by very rare FI composed of CO2 ± water. Other types (2, 3, and 4) are essentially composed of CO2. Type 2 comprises large (up to 60 μm) FI, whereas type 3 FI are distributed along planes crosscutting the crystal (quartz and feldspar). Type 4 are complex low-maturity FI surrounding type 3 FI. Densities in type 3 FI are between 0.07 and 0.75 g/cm3. This scattering is interpreted as being due to decrepitation of FI during the decompression stage of the sample. Using the highest recorded densities, maximum trapping pressure was estimated at 2.9 to 3.2 kbar, assuming a temperature of 800-900°C, corresponding to a lower to upper crust pressure (5−10 km depth) following the decompression stage. A granulitic basement of arc affinity and Precambrian age is inferred at depths of >5 km beneath Amealco caldera. A fragment of this basement was incorporated into an ascending pulse of mafic magma that was injected into the relatively shallow magma chamber of Amealco caldera, and was erupted together with voluminous pyroclastic flows.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.

The 1970 eruption on Deception Island (Antarctica): eruptive dynamics and implications for volcanic hazards

In the southern winter of 1970, a phreatomagmatic eruption occurred in the northern part of Deception Island (South Shetland Archipelago, Antarctic Peninsula). The eruption, with no eye-witnesses to the event, occurred in the same general area as the 1967 eruption, but with new, more widely distributed vents. Two contrasting groups of craters were formed in the 1970 eruption, showing that different active fissures and eruptive dynamics were operating. One group consists of ‘maar-like’ craters, whereas the other comprises conical edifices. The 1970 eruption can be classified as volcanic explosivity index (VEI) 3, with mainly phreatomagmatic phases that generated a bulk volume of about 0.1 km3 of pyroclastic material and an eruptive column at least 10 km high, from which fallout deposits are recognized more than 100 km to the NE. The 1970 eruption was similar to that of 1967 and together these two eruptive events show how eruption dynamics can be controlled by the uppermost part of the volcano substrate and the width and orientation of the eruptive fissure. These influence magma–water interaction and hence may imply different eruptive phases and associated volcanic hazards. Supplementary material: Granulometric and component histograms of the samples that are not shown in Figure 4 are available at http://www.geolsoc.org.uk/SUP18761.