FIELD AND MODELING CONSTRAINTS ON THE FORMATION OF MAP-VIEW CURVES IN FOLD-THRUST BELTS

The problem of how map-view curves (including salients, recesses, arcs, oroclines, virgations, festoons, bends, oroflexes, and syntaxes) in fold-thrust belts originate has caught the attention of geologists for over 200 years. 19th century researchers suggested that the formation of curves was evidence for horizontal translation of crustal slices during orogeny. In recent decades, researchers have proposed several geologically reasonable models to explain curve formation — no one explanation can work for all curves. Structural analysis of curves in 5 examples around the world, analysis of published descriptions of 15 other examples, as well as sandbox modeling of curve formation indicates that the majority of curved fold-thrust belts are “basin-controlled” in that their presence reflects along-strike variations in the width of fold thrust belts that in turn reflect along-strike variation in depth to detachment, rock strength, detachment strength, and detachment slope—i.e., characteristics of the pre-deformational basin—as predicted by critical-taper theory. 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, warping of the downgoing (underthrust) plate, and overprinting of non-coaxial shortening. Of note, not all curve-forming processes lead to "oroclinal" bending of a fold-thrust belt, meaning that not all curves involve differential 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 that has been bent in plan during or subsequent to its formation. For example, thrusts in basin-controlled curves, as well as in curves formed in front of indenters, can initiate with a curved trace, while curves formed in response to interactions with foreland obstacles or with strike-slip faults display oroclinal bending. But even where oroclinal bending is not involved, map-view syn-tectonic simple shear in thrust sheets along the margins of curves may result in internal rotation of paleomagnetic declination, leading to a false impression that oroclinal bending has occurred.