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–Citlaltépetl (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 >100 km2. 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 ∼42 000 yr BP) at the eastern end of the CPVC. This cone fed an olivine-basaltic flow field of ∼20 km2 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 field>27 km2 of late-Pleistocene or Holocene age. The youngest group includes the westernmost flow, from Cerro Colorado, and a lava flow ∼2980 BP from the Rincón de Chapultepec scoria cone of the NVF. test 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 México 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 Rı́o Naolinco lava flow from the northwestern crater. This large-volume (∼1.3 km3) 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 variations in vent locations could produce future flows down one or more of more than a half dozen drainages with widely varying population densities.