PRElMlWk-RV EVALUATION OF THE GEOLOGIC RELATIONS AND SEISMOTECTONIC STABILITY OF THE YUCCA MOUNTAIN AREA NEVADA NUCLEAR WASTE SITE INVESTIGATION (NNWSI) PROGRESS REPORT 30 SEPTEMBER 1991 CENTER FOR NEOTECTONIC STUDIES MACKAY SCHOOL OF MINES UNIVERSITY OF NEVADA, RENO INDEX TO YUCCA MOUNTAIN PROGRESS REPORT 1 OCTOBER 1990 - 30 SEPTEMBER 1991 I. General Task Introduction and Summary of activities conducted during the contract period. II. Task 1. Ouaternarv Tectonics Summary of activities conducted during the contract period. Results of research for the contract period. Appendix A Final report on Varnish Samples collected from the Monte Cristo Area. Appendix B The maximum background earthquake for the Basin and Range province, western North America. Appendix C Historical surface faulting in the Basin and Range province, western North America: implications for fault segmentation. Appendix D An explanation to accompany the geologic map of Crater Flat, Nevada (First Draft). Appendix E Late Quaternary faulting at Crater Flat, Yucca Mountain, southern Nevada. III. Task 3. Mineral DeRosits. Volcanic Geologv Introduction and summary of activities conducted during the contract period. Appendix A Magmatic and Hydrothermal Activity, Caldera Geology, and Regional Extension in the Western Part of the South- western Nevada Volcanic Field. Appendix B Abstract. Geologic and tectonic setting and Miocene volcanic stratigraphy of the Gold Mountain - Slate Ridge area, Southwestern Nevada. Appendix C Abstract. Structural geology and Neogene extensional tectonics of the Gold Mountain-Slate Ridge area, southwestern Nevada. Appendix D Abstract. Multiple Episodes of Au-Ag mineralization in the Bullfrog Hills, SW Nevada, and their relation to coeval extension and volcanism. Appendix E Contrasting styles of epithermal precious-metal mineral- ization in the southwestern Nevada volcanic field. 1 Appendix F Compositional controls on the gold contents of silicic volcanic rocks: 15th International Geochemical Exploration Symposium Program with Abstracts. Appendix G Ash-Flow Volcanism of Ammonia Tanks Age in the Oasis Valley Area, SW Nevada: Bearing on the Evolution of the Timber Mountain Calderas and the Timing of Formation of the Timber Mountain II Resurgent Dome. IV. Task 4 Seismology Summary of activities conducted during the contract period. Appendix A Real-Time Analogue and Digital Data Acquisition through CUSP. V. Task 5. Tectonics. Neotectonics Highlights of major research accomplishments and brief summaries of research results. Appendix A Structure of the western Pahranagat Shear System. Appendix B Structural analysis of Bare Mountain, southern Nevada. Appendix C Mesozoic and Cenozoic structural geology of the CP Hills, Nevada Test Site, Nye County, Nevada; and regional implications. Appendix D Neotectonics of the southern Amargosa Desert, Nye County, Nevada and Inyo County, California. Appendix E Quaternary tectonics and basin history of Pahrump and Stewart Valley, Nevada and California. Appendix F Mesozoic deformation in the Nevada Test Site and vicinity: A new perspective from the CP Hills, Nye County, Nevada. Appendix G The origin of large local uplift in extensional regions. VI. Task 8. Basinal Studies Executive Summary of activities conducted during the contract period. Appendix A The Mississippian Antler Foreland and Continental Margin in southern Nevada: the Eleana Formation Reinterpreted. 2 Appendix B Mississippian Stratigraphy and Tectonics of East- Central Nevada: Post-Antler Orogenesis. Appendix C Mississippian through Permian Orogenesis in eastern Nevada: Post-Antler, Pre-Sonoma tectonics of the western Cordillera. 3 Annual Progress Report-General Task Prepared by C. H. Jones 1 October 1990 to 30 September 1991 Introduction This report provides a summary of progress for the project "Evaluation of the Geologic Relations and Seismotectonic Stability of the Yucca Mountain Area, Nevada Nuclear Waste Site Investigation (NNWSI)." This progress report was preceded by the progress report for the year from 1 October 1989 to 30 September 1990. Initially the report will cover progress of the General Task, followed by sections describing progress of the other ongoing Tasks which are listed below. Task 1 - Quaternary Tectonics Task 3 - Mineral Deposits, Volcanic Geology Task 4 - Seismology Task 5 - Tectonics, Neotectonics Task 8 - Basinal Studies General Task Staff Steven G. Wesnousky, Project Director, Peizhen Zhang (until 1 Nov. 1990), Postdoctoral Fellow, Craig H. Jones (since 1 January 1991), Postdoctoral Fellow, Ingrid Ramos, Secretary. Administrative Activities The General task continued to coordinate and oversee activities of the research tasks. Administrative activities are principally oversight of budgets and preparation of monthly reports, and coordination and collation of research reports and reviews required by the contract in a timely manner. Dr. Wesnousky has also represented NWPO at a number of meetings with the NRC and other federal agencies during the last year. Technical Activities Research activities conducted by the general task have focused on the tectonics of the Yucca Mountain region. Dr. Peizhen Zhang continued neotectonic studies of the Panamint Valley. Research conducted by Dr. Jones has focussed more on the seismological and tectonic framework of the entire lithosphere. Because Yucca Mountain lies near the boundary between two very different extensional regimes, general tectonic study of both regions will improve understanding of the Yucca Mountain site. In addition to the research projects actually conducted over this period, preparations have been made for using earthquakes in the NTS/Yucca Mountain area to construct three-dimensional velocity structures; these structures will also improve the confidence in the location of earthquakes in the area. Field investigations in the region suggest the possibility of conducting in the next year neotectonic investigations along faults to the west of Yucca Mountain that will improve understanding of the overall distribution of deformation in the Basin and Range. 0.1 The first project represents the completion of a project initiated by Dr. Jones. In this experiment, seismometers were deployed in the high Sierra Nevada of southern California in 1988. Teleseisms and regional earthquake arrival times recorded by this network were used to examine the crustal and upper mantle structure beneath the southern Sierra. The results, presently in a manuscript in review, have proven quite controversial: While extension in the upper crust has accomodated over 250 km of motion in the Basin and Range, it appears from this work (when placed in context for the entire region) that the downward continuation of that deformation actually lies under the Sierra Nevada to the west. This deformation is inferred to have warmed and thinned the anti-buoyant mantle lithosphere, thus causing the Sierra Nevada to rise. Such a model has important implications in the Yucca Mountain area, because Death Valley's deformation lies only a few miles to the southwest. Understanding the lithosphere-scale tectonics of the region should improve the framework for systematically examining the Yucca Mountain site; this work complements the earlier study of Dr. Zhang (see last year's report), who described, without providing a tectonic explanation, the evolution of faulting in the Death Valley area through time. A continuation of the seismological investigation into the Basin and Range toward Yucca Mountain is planned. The second project is something of an outgrowth of the first; Dr. Jones, because of the above described work, was invited to participate in an international conference on the tectonics of rifting held in early November, 1990. The authors of presentations were invited to write papers for a proceedings volume; the four workers who represented the Basin and Range (Wernicke, Farmer, Walker, and Jones) combined to write a single paper that attempts to integrate geological, geochemical, and geophysical observation. Dr. Jones has been responsible for the geophysical study and the overall compilation and preparation of the manuscript. Although the paper contains considerable review, the geophysical section explores the variation in the style of deformation through the Basin and Range by taking seismic velocity profiles of the crust that have been obtained in the past few years and converting them into density structures. Armed with the density of the crust, one can infer how much of the variation in elevation seen through the Basin and Range is due to variations in density in the crust; what remains is probably due to variations in the mantle. Results of this study that bear on Yucca Mountain directly are that to the north, extensional deformation in the mantle probably lies under the central part of the Basin and Range, while to the south, deformation in the upper mantle lies under the western flank of the Basin and Range. This implies that a major lithospheric boundary lies near the Yucca Mountain area; this boundary might be responsible for the diffuse band of seismicity that crosses the Basin and Range at this latitude. A third project is a continuation of work undertaken with Drs. Leslie Sonder (Dartmouth College) and Steven Salyards (New Mexico State University), which in turn was inspired by earlier work of Nelson and Jones (1987). Paleomagnetic samples have been gathered in Miocene sediments near Lake Mead in order to understand the mechanics that accompany the creation of "oroflexes," which are great bends