Salients, recesses, arcs, oroclines, and syntaxes - A review of ideas concerning the formation of map-view curves in fold-thrust belts

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.