Center for Volcanic & Tectonic Studies Department of Geoscience University of Nevada, Las Vegas 4505 Maryland Parkway Las Vegas NV 89154-4010, USA -s Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 99154-4010 (702) 895-3971 (702) 895-4064 FAX [email protected] (E-Mail) March 2, 1994 Dear Carl, Enclosed is the annual report for the Center for Volcanic and Tectonic Studies for 1993. I left it unbound so that you can copy it. If you need additional copies, let me know. Eugene I. Smith Professor of Geology MAR 419- 9 MAR 4 994 Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 89154-4010 (702) 739-3262 ORGANIZATION OF THE ANNUAL REPORT OF THE CENTER FOR VOLCANIC & TECTONIC STUDIES (CVTS) (sections separated by colored paper) INTRODUCTION CVTS Activities CVTS Staff CVTS Bibliography-1993-1994 Future Directions CONTRIBUTION OF E.I. SMITH Paper The role of the mantle during crustal extension: constraints from the geochemistry of volcanic rocks in the Lake Mead area: Nevada and Arizona (Reprint from December '93 Geological Society of America Bulletin) Abstracts of presentations CONTRIBUTION OF T.K. BRADSHAW Paper Polygenetic volcanism at Crater Flat, Nevada (Manuscript in review for publication in the Journal of Volcanology and Geothermal Research) Further Geochemical modeling: Crater Flat. Paper Basaltic volcanism in the southern Basin and Range: no role for a mantle plume. (Reprint from Earth and Planetary Sciences Letters) Abstracts of presentations Data tables for chemical analyses Final thoughts CONTRIBUTION OF G.M. YOGODZINSKI: Summary of CVTS efforts in the Reveille Range, south-central Nevada Paper Magmatism and tectonic development of the western Aleutians: an oceanic arc in a strike- slip setting. (Reprint of paper published in the July '93 Journal of Geophysical Research) Abstracts of presentations CONTRIBUTION OF S.A. MORIKAWA Masters of Science Thesis The Geology of the Tuff of Bridge Spring: Southern Nevada and Northwestern Arizona." INTRODUCTION The annual report of the Center for Volcanic Studies (CVTS) contains a series of papers, reprints and a Master of Science thesis that review the progress made by the CVTS between October 1, 1992 and February 1, 1994. During this period CVTS staff focused on several topics that have direct relevance to volcanic hazards related to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. These topics include: (1) Polygenetic/polycyclic volcanism in Crater Flat, Nevada (2) The role of the mantle during crustal extension. (3) The detailed geology of Crater Flat, Nevada. (4) Pliocene volcanoes in the Reveille Range, south-central Nevada. (5) Estimating the probability of disruption of the proposed repository by volcanic eruptions. This topic is being studied by Dr. C.H. Ho at UNLV. CVTS Activities CVTS staff presented papers at several professional meetings including: (a) International Association of Volcanology and Chemistry of the Earth's Interior General Assembly in Canberra Australia, September, 1993. (b) Geological Society of America Cordilleran/Rocky Mountain Section Meeting in Reno, Nevada: May 19-21. 1993. (c) Arizona/Nevada Academy of Sciences Meeting, Las Vegas, Nevada: April 17, 1993. (d) The Forth International Conference on Volcanism and Volcan de Colima, Colima, Mexico, January 2428, 1994. (e) American Geophysical Union, Fall Meeting, San Francisco, Califomia, December 6-10, 1993. In addition, CVTS staff participated in several technical exchanges with NRC, State of Nevada and NAS panels. These include presentations to: (a) the DOE-NRC Technical Exchange on Volcanism, Las Vegas, June 9, 1993. (b) the National Academy of Sciences Board on Radioactive Waste Management, Las Vegas, November 9, 1993. CVTS staff also presented several invited talks, seminars and field trips to the public about volcanism, local geology and the proposed nuclear waste repository. CVTS staff feel that part of their responsibility is to provide the public with unbiased and alternative views of the issues related to the proposed high-level nuclear waste repository at Yucca Mountain. CVTS Staff During the period October 1, 1993 to February 1, 1994, CVTS staff included Eugene I. Smith (PI), and Tim Bradshaw (October 1, 1992 to September 30, 1993), Jim Mills (January 1, 1993 to August 10, 1993), Shirley Morikawa (October 1, 1993 to present) and Gene Yogodzinski (October 1, 1993 to present) as research associates. During this period two graduate students, Shirley Morikawa and Alex Sanchez, were funded to do thesis research. JD. Walker (University of Kansas) continued to do isotopic analyses in cooperation with CVTS staff. In addition, Mark Reagan and Dan Feuerbach (University of Iowa) did U-series dating of the Lathrop Wells cone, Jim Faulds (University of Iowa) completed a geologic map of the Crater Flat quadrangle, and Mark Martin (Geological Survey of Chile) completed a geological map of the Reveille Quadrangle. Future Directions Volcanic risk studies and probability calculations require an understanding of the origin and eruption dynamics of small volume mafic volcanoes. The following list is a sampling of those studies that will have a significant impact on future probabililty calculations. 1. Volatile content studies for basaltic systems: Cinder cone eruptions are commonly characterized as strombolian (quiet, effusive), but our historical experience tells us that cinder cone-forming events can be violent, with plinian and sub-plinian eruptive phases. Based on field observations, there is evidence for explosive activity during cinder cone formation in Crater Flat and at Lathrop Wells The principle control of the explosivity of an eruption will generally be the volatile content of the melt. The presence of amphibole in lavas at Crater Flat indicates a relatively high pre-eruption volatile content for those magmas. We are therefore looking to the volatile content of basalts in southern Nevada for an improved understanding of eruption dynamics and cinder cone formation. Our approach will be through ion probe and FIIR (fourier transform infrared spectroscopy) studies of volatile species (mostly H 20 & C0 2) in volcanic glass and especially glass inclusions in minerals. 2. Volcanic events and probability models: Evaluating the probability of a future volcanic eruption is made difficult in part by the definition of the term 'volcanic event' and its use in statistical modeling. We can imagine cases where a lava flow, a cinder cone, or a chain of cinder cones might reasonably be considered a single volcanic event. In general, refinement of the 'volcanic event' definition will be limited by the information that can (practically) be obtained through geologic mapping and radiometric dating. We are investigating the practical limits of this approach in our study of the small volume, mafic volcanic field in the Reveille and Pancake ranges of south-central Nevada (see contribution by G.M. Yogodzinski in this report). 3. Structural controls on volcanism and the distribution of volcanoes in a volcanic field: Volcanic risk zones in the vicinity of Yucca Mountain are based in part on assumptions about the relative importance of structural control over the spatial distribution of cinder cone eruptions. In the vicinity of Yucca Mountain, the relative importance of NNE versus NW-trending structural control has been debated at length, but the issue remains unresolved. We have argued that analogous volcanic fields within the Great Basin (e.g., the Reveille Range) show unambiguously the importance of control on NS-to-NNE-trending structures. In the future we will expand our analysis of cinder cone spatial distribution to other volcanic fields within the region. 4. Polygenetic/polycyclic activity at small volume maic systems. Cinder cone are traditionally considered to have formed by eruptions lasting from 1 to 10 years. Recently, it has become apparent that Cinder Cones are complex volcanoes that erupt from more than one vent over long periods of time (up to 10,000 years). Vents can be focused in one locality or they may form volcanic chains. Our studies in Crater Flat suggest that Black and Red cones are both polycyclic and polygenetic (see the contribution by Tim Bradshaw in this report). We propose to study cinder cones in volcanic fields that are now dormant or inactive, so that we can observe the complete eruptive and chemical history of individual cones. We will determine whether cinder cones in other areas are polycyclic and/or polygenetic. Our studies will take place in the Reveille Range, Nevada; the Fortification Hill field, Arizona and possibly the San Francisco volcanic field, Arizona and in the Trans-Mexican volcanic belt, central Mexico. Our studies will continue in Crater Flat. Studies of this type will help us understand the origin of small volume mafic volcanoes and allow us to better define a volcanic event 5. Nature of the mantle In the Great Basin: The trace element and isotopic features of basaltic rocks erupted in the vicinity of Yucca Mountain show the influence of continental lithosphere. It is widely believed that this influence is derived entirely from the lithospheric mantle, but some geochemical features suggest that there may be a significant contribution from the continental crust. The geochemical source of the magmas may hold important clues with regard to the depth of melt storage in small volume mafic systems, and the transport time between melt generation and eruption. We will continue to seek an improved understanding mantle versus crustal source contributions with the aim of improving our overall understanding of the plumbing systems that feed small volume mafic volcanic fields. CVTS Bibliography-1993-1994 Papers Bradshaw, T.K., Hawkesworth, C.J., and Gallagher, K., 1993, Basaltic volcanism in the southern Basin and Range: no role for a mantle plume: Earth and Planetary Science Letters, v. 116, p. 45-62. Feuerbach, D.L., Smith, E.I., Walker, J.D. and Tangeman, J.A., 1993, The role of the mantle during crustal extension: constraints from geochemistry of volcanic rocks in the Lake Mead area, Nevada and Arizona: Geological Society of America Bulletin, v 105, p.