UNDERSTANDING THE TECTONIC EVOLUTION OF YUCCA MOUNTAIN

In judging the suitability of Yucca Mountain, Nevada, as a site for a high-level nuclear waste repository, questions concerning the history of deformation and the potential for earthquakes and fault slip required consideration of the tectonic evolution of the mountain. The depositional and deformational histories of the mountain are relatively simple -- it is composed of a layered sequence of tuffs deposited between 14 and 11.4 Ma, down-faulted into Crater Flat basin to the west chiefly prior to 9 Ma, undergoing as much as 20° vertical axis rotation near the south end of the basin during the process. However, the causes and evolution of this tectonic deformation are only generally understood, and the uncertainty is reflected in differing intepretations of regional and local strain phenomena. The search for understanding is aided by consideration of conceptual tectonic models and studies of regional strain. Four kinds of models have been considered: a caldera model, a strike-slip pull-apart basin model, a detachment model, and a half-graben model. Based on detailed site characterization studies, the detachment and half-graben models are presently considered viable; they are adequate for explaining the structural evolution of the mountain within its regional tectonic setting, and they do not require unique, site-specific mechanisms to account for local deformation. However, the causes and rates of Plio-Pleistocene basaltic volcanism (as recent as 80 ka) near Yucca Mountain are not well explained and remain a problem for both models. Ongoing studies of volcanism and investigation of the structure of the southern margin of Crater Flat basin present challenges to the tectonic models and help to frame additional criteria for testing the models.