Cenozoic volcanism in Southwestern New Mexico.

Abstract

Tephrochronology is the study of volcanic ash (tephra) beds for the purpose of correlating and dating volcanic and other geologic events. Large magnitude silicic eruptions can disperse tephra up to thousands of kilometres from the vent, producing a near instantaneous marker horizon. In addition to their geochronological value, tephra beds are a major source of data on the eruption frequency and geochemistry of large rhyolitic volcanoes. The Quaternary Taupo volcanic zone (TVZ) in New Zealand is one of the most frequently active rhyolitic centres on Earth, and for much of the 20th century its tephra beds have been the focus of study. As a result, fingerprinting and dating techniques have been fine-tuned, and tephra beds now under-pin the late Cenozoic chronology for a wide variety of disciplines from volcanology to sequence stratigraphy and archaeology. The key to tephrochronology is the identification and correlation of tephra horizons. In the proximal setting (<50 km from vent), tephra beds can often be identified by their lithology, stratigraphic position and ferromagnesian mineralogy. Farther from source these features became less diagnostic as units become thinner and mineral depleted, and geochemical fingerprinting must then be employed. Grain-specific techniques are commonly required to assess the compositional homogeneity of the tephra, and to avoid xenolithic and detrital contaminants. Criteria that are valuable for identifying both source volcano and individual eruptive events from grain-specific electron microprobe techniques include: glass and Fe–Ti oxide chemistry, and eruption temperature and oxygen fugacity (estimated from oxide equilibrium pairs). Deposits of the contemporaneously active Taupo, Okataina, Maroa and Mayor Island caldera centres in the TVZ can be distinguished on the basis of such criteria. If a tephra lacks contaminants, trace- and rare earth element (REE) compositions of purified glass separates provide additional criteria. Similar approaches can be used on samples of the chilled, nonwelded bases of ignimbrites, while welded zones often display a characteristic thermal remnant magnetism (TRM) direction that can assist in correlation. Late Miocene and Pliocene rhyolitic tephra from the Coromandel region in New Zealand are compositionally indistinguishable from Quaternary TVZ deposits, but deposits from other SW Pacific provinces such as Antarctica and the Tonga–Kermadec arc are compositionally distinct. In the TVZ, rhyolitic tephra erupted during intervals of 10–20 ka from the same volcano are compositionally similar, while over longer periods or following large-volume eruptions the tephra are compositionally and mineralogically distinct. Most TVZ fall deposits are compositionally homogeneous, and there are no temporal compositional trends for individual volcanoes. Tephra beds are also valuable because they can be dated by a variety of methods, and therefore can provide direct age control for sequences that are not normally amenable to radiometric dating. For late Cenozoic tephra beds that are beyond the range of 14C (ca. 40 ka), and are fine-grained and/or lack K-rich phenocrysts, the isothermal plateau fission-track (ITPFT) method using glass can provide an accurate and precise age. 40Ar–39Ar can be employed for high-resolution dating, however this may require single crystal laser fusion to detect relict and partially degassed crystals that are common in pyroclastic deposits, even within single pumice blocks. The New Zealand tephrostratigraphic record is not complete in space or time. Only the post-64 ka record is well established. However, there are chronologically well-constrained tephra beds dating back to 10 Ma. Some tephra beds cover most of the North Island of New Zealand, are found in the South Island, and extend as much as 1400 km from vent into the Pacific Ocean. Widespread and/or stratigraphically important units include: Kaharoa (665 years BP); Kawakawa (22 ka); Rangitawa (0.33 Ma); Potaka (1 Ma) and Pakihikura (1.6 Ma). Ages on these and other tephra beds provide a framework for global climatic and sea-level change recorded in New Zealand, and allow direct correlation between the marine and terrestrial realms.