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Salients, Recesses, Arcs, Oroclines, and SyntaxesA Review of Ideas Concerning the Formation of Map-view Curves in Fold-thrust Belts
Department of Geology, University of Illinois, Urbana, Illinois, U.S.A.
This paper incorporates results from the work of my graduate students at the University of Illinois. I draw on publications and dissertations by Juliano Macedo, Tim Paulsen, Scott Wilkerson, and Oswaldo Araujo, and from Master's theses by John Tabor and Dave McEachran, and wish to thank them sincerely for many years of fruitful interactions. I am also very grateful to Thomas Flttmann and Fernando Alkmim, who have shared their thoughts concerning curved fold-thrust belts. Finally, I wish to thank the authors of numerous published studies (notably those of Steve Boyer and Gautam Mitra) from which I have extracted ideas, and Bob Hatcher, who emphasized the relationship between basin thickness and salients. The title is a modification of one used by Sarwar and DeJong (1979). I am grateful for reviewer's comments by Jacques Malavieille. The contents of this manuscript were presented at the Thrust Tectonics '99 meeting, generously organized by Ken McClay. The manuscript was submitted for publication in 1999.
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.
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