Memoir 46—Field excursions to volcanic terranes in the western United States, Volume I: Southern Rocky Mountain region

Edited by C. E Chapin and J. Zidek, 1989, x + 486 pp., 48 tables, 403 figs.

This publication is produced for the International Association of Vocanology and Chemistry of the Earth's Interior (IAVCEI) General Assembly held in Santa Fe, New Mexico, on 25 June-1 July 1989 and contains a combination of scientific papers and field guides designed to familiarize the reader with the geology of a particular volcanic field and then lead him through it. Memoir 46 consists of nine excursions to volcanic fields in New Mexico, Arizona, Colorado, and Texas. The individual excursions were compiled from contributions of three to 11 authors each; a total of 105 authors contributed to the 16 excursions of Memoir 46 and Memoir 47. The technical papers form integral parts of the excursions, contain much new data, and are expected to serve as important references for many years. This set includes Bulletin 131, and Memoirs 46 and 47. Bulletin 134 is a companion to this set.

Excursions in this memoir include:

Miocene to Holocene volcanism and tectonism of the southern Colorado Plateau, Arizona, by G. E. Ulrich, et. al. A five-day trip along the southern perimeter of the Colorado Plateau provides an overview comparison of the eruptive styles, lava compositions, and tectonic settings of five volcanic fields in Arizona. An added stop in the Zuni-Bandera field of New Mexico will be made on the sixth day en-route to Santa Fe. The contrasts and similarities to be found illustrate the variety of volcanic processes that have operated over the past 17 m. y. Attention will be directed mainly to volcanic features that are less than 8 Ma, and the youngest volcano, Sunset Crater, which was active about 900 years ago.

Eocene–Miocene Mogollon–Datil volcanic field, NM, by J. C. Ratte, et. al. This trip begins in Socorro, New Mexico, where Day 1 will be spent examining Oligocene to modern magmatism and structure along the Socorro accommodation zone, a leaky transverse structure in the Rio Grande rift. During Day 2 we travel westward from Socorro along the northern edge of the Mogollon-Datil volcanic field to view late Eocene and early Oligocene volcaniclastic rocks of the alluvial apron constructed around the nucleus of the volcanic pile. Days3-7 will comprise a circumnavigation of the Mogollon Plateau, the central core of the Mogollon–Datil volcanic field.

From silicic calderas to mantle nodules: Cretaceous to Quaternary volcanism, southern Basin and Range province, Arizona and New Mexico, compiled by J. S. Pallister. Mesozoic to Quaternary volcanism in southern Arizona and New Mexico is closely related to the structural and tectonic evolution of the Basin and Range province. Although the volcanic centers described in this guidebook are compositionally and temporally diverse, they share a common basement framework. This overview attempts to establish a broad tectonic perspective from which to consider volcanism in the southern Basin and Range, and to point out similarities in eruptive processes at several volcanic centers.

Magmatism associated with lithospheric extension: Middle to late Cenozoic magmatism of the southeastern Colorado Plateau and central Rio Grande rift, New Mexico and Arizona, by W. S. Baldridge, et. al. Magmatism is an integral feature of continental extension and, in particular, of continental rifting. Some rifts, such as the East African rift, are associated with huge volumes of volcanic rocks. Others, such as the Rio Grande rift, have relatively small volumes of magmatic rocks exposed at the surface, although large quantities may exist at depth. Regardless of the volume, the compositions of magmatic rocks and their distribution and secular variation are of key importance in understanding the processes involved in extension of continental lithosphere.

Oligocene to Holocene magmatism and extensional tectonics, central Rio Grande rift and southeastern Colorado Plateau, New Mexico and Arizona, by W. S. Baldridge, et. al. The area of the central Rio Grande rift and southeastern Colorado Plateau is particularly important for the study of magmatic and tectonic processes because it spans several tectonic provinces, (e.g. Great Plains, Colorado, Plateau, Rio Grande rift, Basin and Range), each with its unique geologic history and lithospheric characteristics. Therefore, although this region is a relatively restricted area, it is possible to discern features that are unique to, or common to, these different tectonic provinces. This excursion is designed to show essential features of the middle Oligocene to Holocene magmatism of the central rift and the transition zone of the southeastern Colorado Plateau, and the relationship of magmatism to regional tectonic setting. Oligocene and early Miocene magmatic rocks are exposed mainly as dikes and small intrusions.

Mid-Tertiary silicic alkalic magmatism of Trans-Pecos Texas: Rheomorphic tuffs and extensive silicic lavas, by C. D. Henry, et. al. This trip provides an overview of the silicic, alkalic, and commonly peralkaline magmatism of Trans-Pecos Texas, particularly in the Davis Mountains and Big Bend National Park. Emphasis is on caldera development, a variety of pyroclastic deposits including strongly rheomorphic welded tuffs, and unusually widespread silicic lavas. Additionally, we will visit two calderas that illustrate contrasting styles of development and a wide variety of caldera-fill volcanic rocks and breccias. Much of the trip will examine a group of rocks that have outcrop features of lava flows but areal extents and aspect ratios commonly associated with ash-flow tuffs; these include both true lavas and strongly rheomorphic tuffs, as well as some units of debatable origin. Geochemistry and petrogenesis of the rocks will also be addressed, both in the field and in discussion sessions scheduled for several evenings of the trip.

Roots of ignimbrite calderas: Batholithic plutonism, volcanism, and mineralization in the Southern Rocky Mountains, Colorado and New Mexico, by C. M. Johnson, et. al. The Questa, Mount Aetna, and Grizzly Peak calderas and associated volcanic and plutonic rocks provide a view of crustal magmatism from the surface through 4-8 km (2-5 mi) of crust. The petrologic relations between spatially and temporally related volcanic and plutonic rocks will be studied in the 28-19 Ma Latir volcanic field and associated Questa caldera. Deeply eroded ring-zone structures and brittle and ductile deformation are the focus of the trip through the 34-35 Ma Mount Aetna caldera in southern Colorado. Inter-caldera structures related to caldera collapse and resurgence, in addition to petrologic relations between high-level intrusive stocks and ash-flow magmatism, will be the topic of discussions at the 34 Ma Brizzly Peak caldera north of Mount Aetna. The Questa caldera occupies the eastern flanks of the Rio Grande rift in northern New Mexico, and the Mount Aetna and Grizzly Peak calderas occupy the western flank of the extreme northern part of the rift in central Colorado.

Oligocene–Miocene San Juan volcanic field, Colorado, compiled by P. W. Lipman. The San Juan Mountains are the largest erosional remnant of a composite volcanic field that covered much of the southern Rocky Mountains in middle Tertiary time. The field consists mainly of intermediate-composition lavas and breccias, erupted about 35-50 Ma from scattered central volcanoes, overlain by about 15 widespread voluminous ash-flow sheets erupted 30-26 Ma from caldera sources. At about 26 Ma, volcanism shifted to a bimodal assemblage dominated by trachybasalt and silicic rhyolite, concurrent with the inception of regional extension during establishment of the Rio Grande rift zone.

Volcanic and hydrothermal evolution of Valles caldera and Jemez volcanic field, by F. Goff, et. al. Valles caldera has become world famous as an example of a resurgent caldera and as the location of a high-temperature geothermal system of volcanic origin. Although the caldera and its modern hydrothermal system result from events that have happened during the last 1.12 Ma, the Jemez volcanic field has experienced continuous volcanism for more than 13 Ma and is known to have had at least three major periods of distinct hydrothermal activity. The field-trip guide presented here will emphasize the results of volcanic, tectonic, and geochemical research pertaining not only to the Valles caldera but to the entire history of the Jemez volcanic field. The last one and one half days of the trip will also emphasize drill-hole data that have recently given researchers a wealth of information on extra- and intra-caldera structure, stratigraphy, hydrothermal alteration, and fluid chemistry in the most explored Quaternary caldera complex in the United States. The accompanying paper is not intended to review exhaustively all aspects of research on the Jemez volcanic field; rather, it stresses significant discoveries and interpretations that have occurred since about 1980.

Rio Grande rift volcanism: Northeastern Jemez zone, New Mexico, by M. A. Dungan, et. al. Magmatism, a predictable consequence of asthenospheric upwelling associated with lithospheric attenuation, is integral to continental rifting. The injection of mantle-derived magma into extending crust may have a profound effect on the rheology of the crust and therefore the style of deformation associated with extension. Rift-related magmatism encompasses much of the diversity of terrestrial-magma types. Compositions of mafic magmas range from tholeiite to some of the most silica-undersaturated magmas found on the continents, and their differentiates range over the entire spectrum of intermediate to salic compositions to include high-SiO2 rhyolite, trachyte, and phonolite. Large effusive eruptions from fissures are typical of some rifts, whereas others may be dominated by central vent cones or even silicic caldera complexes. Most of these aspects of rift volcanism and a wide range of mafic to salic magma compositions are represented in the Rio Grande rift; many will be seen on this trip.