CENTER FOR VOLCANIC AND TECrONIC STUDIES1 DEPARTMENT OF GEOSCIENCE University of Nevada, Las Vegas Las Vegas Nevidi.89154 Report No. 56 ANNUAL REPORT for the Period 10/1/90 to 9/30/91 Submitted to the Nudlear Waste Project Office State of Nevada December 14, 1991 'Eugene L Smith-Principal hvestigator. Daniel Feuerbach-Research Associate Terry aaumann-Research Associate 0 9210020232 911231 PDR WASTE WM-11 PDR INTRODUCION The annual report of the Center for Volcanic and Tectonic Studies (CVTS) contains a series of papers, maps, reprints and theses that review the progress made by the CVF`S between October 1, 1990 and September 30, 1991. During this period CVIS staff focused on several topics that had direct relevance to volcanic hazards related to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. These topics included: (1) The role of the mantle in controlling the location and composition of Pliocene- Holocene volcanism. (2) The nature of boundaries between compositionally different mantle domains. These domain boundaries may control the location of surface faults and volcanism. (3) The detailed geology of the Pliocene volcanic cones in Crater Flat (4) The detailed geology of bedrock to the east of Crater Flat on Yucca Mountain. (5) The structural controls and emplacement mechansims of Pliocene/Quaternary basaltic volcanic centers and dikes. (6) The study of young felsic volcanism in the southern Basin and Range. (7) Estimating the probability of disruption of the proposed repository by volcanic eruption (this topic is being studied by Dr. C Ho-UNLV). Activities CV1S presented papers at several professional meetings including (a) The Geological Society of America Cordilleran Section Meeting in San Francisco- March 25-27, 1991. (b) The Napoli '91 International Conference on Active Volcanoes and Risk Mitigation, in Naples, Italy - August, 1991. (c) The Geological Society of America National Meeting in San Diego, October, 1991. In addition CVWS staff participated in several technical exchanges and field trips with NRC, ANCW and NWTRB panels, and presented numerous invited talks, seminars and field trips to the public about volcanism and the proposed nuclear waste repository. 1 The CVIS staff feel that part of their responsibility is to provide the public with unbiased and alternative views of the issues related to the proposed Yucca Mountain high-level nuclear waste repository. Invited talk and lectures to public meetings and civic groups as well as large enrollment classes at UNLV provide the community with a source of information regarding the geology and geologic hazards related to the project. CVTS Staff During the period October 1, 1990 to September 30, 1991, CVIPS staff included Eugene L Smith (PI), Dan Feuerbach and Terry Naumann (Research Associates), and Tracey Tuna (Student Assistant). Jim Faulds (University of Iowa) received partial support to complete his field studies in the Malpais Mesa area of Arizona and in Crater Flat. In addition, two graduate students (Tracey Cascadden and Hayden Bridwell) were partially funded to complete Master's Thesis projects. Organization of the Annual Report This report includes the following contributions: (1) Evolution of a mantle domain boundary during regional extension: constraints from isotopic geochemistry of volcanic rocks in the Lake Mead area, Nevada and Arizonae by D.L Feuerbach, El. Smith (CVTS-UNLV) and JD. Walker (University of Kansas). (2) A summary of a paper entitled 'Isotope geocbemistry of the nafic lavas of the Reveille Range, Nevada- A window into the mantle of the central Great Basin" by T.R Naumann, El. Smith (CVTS-UNLV) and JD. Walker (University of Kansas). (3) The geologic map of Crater Flat, Nevada. The authors of the map are J. Faulds (University of Iowa), D. Feuerbach (CVTS-UNLV), A. Ramelli (UNR) and John Bell (UNR). This map and explanation will be published by the Nevada Bureau of Mines and Geology. (4) A summary of a paper regarding the structural control and emplacement chanism of mafic dikes by J. Faulds (University of Iowa).. (5) Copies of abstracts submitted by CVTS staff during the year of funding. These include: (a) Structural control of Pliocene volcanism in the vicinity of the Nevada Test Site, Nevada: an example from Buckboard Mesa" by Naumann, Feuerbach and Smith. Presented at the Geological Society of America Cordilleran Section 2 meeting in San Francisco. (b) The transiton from subalkalic to alkalic volcanism in the Lake Mead area of Nevada and Arizona: geochemical and isotopic constraints" by Feuerbach, Smith, Walker and Tangernan (Kansas). Presented at the Geological Society of America Cordilleran Section meeting in San Francisco. (c) The eastern boundary of the extensional allochthon in the eastern Basin and Range: volcanic and structural geology of the northern White Hills, Arizonae by Cascadden and Smith. Presented at the Geological Society of America Cordilleran Section meeting in San Francisco. (d) "New insights on structural controls and emplacement mechanisms of Pliocene/Quaternary basaltic dikes, southern Nevada and northwestern Arizona' by Faulds, Feuerbach and Smith. Presented at the Geological Society of America National Meeting in San Diego. (e) 'Intermediate and mafic volcanic rocks of the northern White Hills, Arizona: implications for the production of intermediate composition volcanic rocks during regional extension! by Cascadden and Smith. Presented at the Geological Society of America National Meeting in San Diego. (f) 'Horublende geobarometry from mid-Miocene plutons: implications regarding uplift and block rotation during Basin and Range extension! by Metcalf (UNLV) and Smith. Presented at the Geological Society of America National Meeting in San Diego. (g) Volcanic risk assessment studies for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, USA! by Smith, Feuerbach, Naumann and Ho (UNLV). Presented at the Napoli '91 International Conference on Active Volcanoes and Risk Mitigation in Naples, Italy. (6) Two papers describing new K-Ar dates from the Reveille Range and Fortification Hill field published in Isochron/West. Naumann, TR., Smith. E-1 Shafiqullah, M, and Damon, PE., New K-Ar ages for mafic to intermediate volcanic rocks in the Reveille Range, Nevada: Isochron West. Feuerbach, DL, Smith. E... Shafiquallah, M, and Damon, PE., New K-Ar dates for mafic late-Miocene to Pliocene volcanic rocks in the Lake Mead area, Arizona and Nevada: Isochron West. (7) A thesis entitled 'he Sloan Sag: A Mid-Miocene, volcanotectonic depression, north- 3 central McCullough Mountains, southern Nevada" by Hayden BridwelL This thesis deals with a section of young (<12 Ma) felsic volcanic rocks in the Mccullough Range 16 kilometers south of Las Vegas. (8) A thesis entitled 'Style of volcanism and extensional tectonics in the eastern Basin and Range Province: northern Mohave County, Arizona" by Tracey Cascadden. The thesis deals with the formation of mafic volcanic rocks in an extensional environment. If these theses are not included in your copy of the annual report and you wish more information, contact the CVTS (702) 739-3971. 4 EVOLUTION OF A MANTLE DOMAIN BOUNDARY DURING REGIONAL EXTENSION: CONSTRAINTS FROM ISOTOPIC GEOCHEMISTRY OF VOLCANIC ROCKS IN THE LAKE MEAD AREA, NEVADA AND ARIZONA by D.L Feuerbach, E.I. Smith Center for Volcanic and Tectonic Studies Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 89154 JD. Walker Department of Geology University of Kansas Lawrence, Kansas 66045 5 INTRODUCTION In the southwestern U.SA, three mantle isotopic provinces were defined by the use of Sr and Nd isotopic systems (Fitton et al, 1986; Menzies et al, 1983; Farmer et al, 1989) (Figure 1). These are: (1) Basin and Range characterized by Nd between +5 and +8 and intial 7Sr/6Sr =0.703 (Perry et al., 1987; Menzies et al, 1983; Farmer et al, 1989); (2) Transition zone between the Colorado Plateau and Basin and Range with '7Sr/'uSr between 0.7038 and 0.707 and Nd between -4 and +5 (Cooper and Hart, 1990; Fitton et al, 1986); and (3) Sierra Nevada or Western Great Basin province distinguished by "Sr/6Sr >0.706 and Nd between 0 and -11 (Menzies et al, 1983; Fitton et al, 1986; Fitton et al, 1991. Basalts with similar 97Sr/f6Sr and Nd in southern Nevada (Farmer et al, 1989) also lie within this province. The boundary between the Western Great Basin and Basin and Range provinces (Menzies et al, 1983; Fitton et al, 1991) and the contact between the OIB and EM2 mantle domains of Menzies (1989) pass through the Lake Mead area of southern Nevada (Figure 1). The Lake Mead area also contains an important crustal boundary. The Lake Mead fault zone (LMFZ) (Anderson, 1973; Bohannon, 1984; Duebendorfer and Wallin, 1991), a northeast trending set of left-lateral strike-slip faults separates the "amagmatic zone' from the Northern Colorado River extensional corridor (NCREC). The amagmatic zone is a region of minor igneous activity and little to moderate Tertiary extension that separates the Great Basin from the Sonoran Desert-sections of the Basin-and-Range province in the western U.S.A This region between 36° and 370 north, corresponds to a regional topographic slope and a gravity gradient with an amplitude of about 100 mgals (Eaton, 1982; Eaton et al, 1978). The zone also represents a boundary between contrasting migration directions of magmatism and extension (Bartley et al, 1988; Bartley, 1989; Glazner and Supplee, 1982; Reynolds et al, 1987). In contrast, the NCREC (Howard and John, 1987) experienced over 100%o extension and abundant magmatism during Tertiary time. The LIMFZ also separates thick sections of Paleozoic and Mesozoic sedimentary sections (to the north) from the NCREC, an area nearly devoid of these sections (Longwell et at, 1965; Anderson, 1971). This paper focuses on the boundary between the amagmatic zone and the NCREC. We use Miocene and Pliocene mafic volcanic rocks (SiO 2<55%) that span this mantle-crustal boundary as a probe into the mantle to determine (1) the nature of isotopic differences across the boundary, (2) the timing of formation of the boundary, and (3) any link between crustal and mantle processes.