Approximately 21,700 yr B.P., after a period of quiescence of ∼4800 yr, Nevado de Toluca volcano erupted, producing the Lower Toluca Pumice deposit. The activity generated a 24-km-high Plinian column that lasted ∼11–13 h and dispersed 2.3 km 3 (0.8 km 3 dense rock equivalent) of tephra toward the NE, blanketing the Lerma basin, an area occupied today by the city of Toluca, with up to 5 cm of ash. Subsequent eruptive pulses were sub-Plinian in style, accompanied by phreatomagmatic explosions that emplaced surge deposits. Finally, the column collapsed toward the NE with the emplacement of a pumice flow deposit. The high vesicularity of the pumice from the basal Plinian layer, up to 83% by volume, indicates that exsolution was dominantly magmatic, and that pressurization of the magma chamber was probably due to a magma mixing process. Evidence for this includes the compositional range of juvenile products (61–65 wt% SiO 2 ), as well as the presence of two types of plagioclase, one in equilibrium and the other one with disequilibrium textures and reverse zoning. This suggests input of an andesitic liquid into the dacitic magma chamber. Based on the eruptive record, the most likely future eruptive activity at Nevado de Toluca volcano will be Plinian. Although quiet for more than 3250 yr, Plinian activity could occur after a long period of quiescence, and it could represent a hazard for the entire Toluca basin, where more than one million people live today.

Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km 2 , and total erupted volumes range between 0.5 and 2 km 3 /cone. An average eruption rate for the entire Chichinautzin was estimated at ∼0.5 km 3 /1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract In 1977, Guillermo P. Salas, general director of the Mineral Resources Council, commissioned special studies manager José Luis Lee Moreno to evaluate the geologic-mining potential of the Neovolcanic Axis Metallogenetic Province of Salas (1975). Detailed studies of ERTS-1 satellite images over approximately 10,000 km 2 of the province had revealed important tectonic features suggesting as yet undiscovered ore deposits. The evaluation project began in 1977 under Raúl Cruz Ríos. Regional geology (Nieto and others, 1977) indicated extensive volcanic-sedimentary outcrops at the boundary with the Sierra Madre del Sur province, and exploration for volcanogenic massive sulfides was recommended. By mid-1978 the first Tizapa samples had tested successfully, and the Metamorphic Rocks Project was set up in January 1979. Since that time, exploration and preliminary evaluation of the deposit have continued without interruption. Tizapa lies 67 km directly 60° southwest of Toluca and 4 km southeast of Zacazonapan in southwestern Mexico State (Fig. 1). Highway 130 from Toluca leads to Temascaltepec, from where unpaved roadways reach Zacazonapan. Fieldwork included a semidetailed regional geologic survey using part of the “Valle de Bravo” Sheet E-14-A-46 (Direccion General de Estudios del Territorio Nacional, scale 1:50,000) as topographic base; subsequent 1:1,000 topographic mapping over approximately 1 km2 as a basis for detailed geological and aerial geophysical electric surveys; direct diamond-drilling with core recovery; and a 30-m prospecting tunnel.

The Inner Piedmont is a large, composite, sillimanite-grade terrane that extends from near the Virginia–North Carolina border to central Alabama and consists of the eastern Tugaloo and Cat Square terranes. It is bound to the west by the Brevard fault zone and to the east by the central Piedmont suture. It is the core of the Neoacadian (360–350 Ma) orogen in the southern Appalachians and records Late Devonian–Mississippian closure and high-grade metamorphism (sillimanite I and II) of Siluro-Devonian sediments deposited in the remnant Rheic ocean basin. The Cat Square terrane is bounded by the younger-over-older Brindle Creek fault to the west and the central Piedmont suture to the east. It consists of a unique sequence of Siluro-Devonian metapsammite and pelitic schist that was intruded by Devonian anatectic granitoids (Toluca Granite, ∼378 Ma, and Walker Top Granite, ∼366 or ∼407 Ma). Rare mafic and ultramafic rocks occur in the eastern Cat Square terrane. Minimum sediment thickness is estimated at 4 km (13,000 ft). Detrital zircons indicate that Cat Square terrane rocks have a maximum age of ∼430 Ma, with both Laurentian (2.8, 1.8, 1.4, 1.1 Ga) and peri-Gondwanan (600, 500 Ma) affinities. Deposition on oceanic crust explains the existence of several mafic and ultramafic bodies and the absence of continental basement in the Cat Square terrane. The Cat Square terrane petrotectonic assemblage represents a Siluro-Devonian remnant ocean basin between Laurentia and the approaching Carolina superterrane. Metapsammite and pelitic schist may represent turbidites shed from approaching tectonic highlands on both flanks of the closing ocean. Palinspastic restoration of the Inner Piedmont constrains the location of the Cat Square basin to the Pennsylvania embayment, and links the mid-Devonian to Mississippian deformation in the Neoacadian core to the SW-migrating pulses of the diachronous Acadian-Neoacadian clastic wedge. Location and SW migration of the clastic wedge in concert with structural patterns in the Inner Piedmont support a transpressive NW-directed collision of the Carolina superterrane with the New York promontory and zippering the basin shut from NE to SW.