ABSTRACT The problem of how map-view curves (variously named salients, recesses, arcs, oroclines, virgations, festoons, bends, oroflexes, and syntaxes) in fold-thrust belts originate has caught the attention of geologists for more than 200 years. This chapter reviews the advances in understanding curves. Early geologists recognized that by understanding curve formation, one might gain insight into the process of orogeny. In recent decades, researchers have proposed several geologically reasonable models to explain curve formation; no single explanation can work for all curves. The majority of curving fold-thrust belts can be called “basin controlled,” in that their presence reflects the architecture of the predeformational sedimentary basin from which the curve formed. Factors such as depth to detachment, rock strength, detachment strength, and detachment slope all affect the width of a fold-thrust belt for a given amount of hinterland displacement, as predicted by critical-taper theory. Therefore, along-strike variation in these factors leads to the inception of thrust belts that vary in width along strike, and thus have curved traces. However, not all curved thrust belts are basin controlled. Other causes for curve formation include interaction of a thrust belt with foreland obstacles or promontories, hinterland collision of an indenter, interaction with subsequent strike-slip faults, and warping of the downgoing (underthrust) plate. Not all curve-forming processes lead to “oroclinal” bending of a fold-thrust belt, in that not all curves involve rotation of segments of the thrust belt around a vertical axis. Thus, not all curves are oroclines, where the term “orocline” specifically refers to a mountain belt bent in plan. Basin-controlled curves and curves formed in front of indenters generally initiate with a curved trace, whereas curves formed in response to interactions with foreland obstacles or with strike-slip faults involve oroclinal bending.

Yucca Mountain has been proposed as the site for the nation's first geologic repository for high-level radioactive waste. This chapter provides the geologic framework for the Yucca Mountain region. The regional geologic units range in age from late Precambrian through Holocene, and these are described briefly. Yucca Mountain is composed dominantly of pyroclastic units that range in age from 11.4 to 15.2 Ma. The proposed repository would be constructed within the Topopah Spring Tuff, which is the lower of two major zoned and welded ash-flow tuffs within the Paintbrush Group. The two welded tuffs are separated by the partly to nonwelded Pah Canyon Tuff and Yucca Mountain Tuff, which together figure prominently in the hydrology of the unsaturated zone. The Quaternary deposits are primarily alluvial sediments with minor basaltic cinder cones and flows. Both have been studied extensively because of their importance in predicting the long-term performance of the proposed repository. Basaltic volcanism began ca. 10 Ma and continued as recently as ca. 80 ka with the eruption of cones and flows at Lathrop Wells, ∼10 km south-southwest of Yucca Mountain. Geologic structure in the Yucca Mountain region is complex. During the latest Paleozoic and Mesozoic, strong compressional forces caused tight folding and thrust faulting. The present regional setting is one of extension, and normal faulting has been active from the Miocene through to the present. There are three major local tectonic domains: (1) Basin and Range, (2) Walker Lane, and (3) Inyo-Mono. Each domain has an effect on the stability of Yucca Mountain.