ABSTRACT Asymmetric anticlines with steeptooverturned forelimbs areacommon element of fold-and-thrust belts, but typically their origin must be interpreted on the basis of incomplete knowledge of their geometry and kinematics. Many such folds are interpreted to be fault-propagation folds, but their known characteristics fit as well or better with interpretation as a detachment fold or a thrust-truncated example of either fold type. A fault-propagation fold forms by propagation of a ramp tip, so a ramp on which displacement decreases upward to a tip is consistent with this fold type. Fault-propagation fold models generally assume that hinges migrate with respect to the rock, especially in synclines, and that limbs, especially backlimbs, do not rotate with fold growth. A detachment fold forms above a décollement that may have a fixed tip or a propagating tip or that may extend beyond the limits of the fold. Nonparallel thickening in the anticlinal core and lack of a ramp are characteristic of a detachment fold. Detachment-fold models assume either fixed or migrating hinges and either fixed or rotating limbs, although rotating limbs and at least a fixed anticlinal hinge seem best supported by natural examples. Truncation and displacement of a preexisting fold by a thrust fault modifies fold geometry and makes it more difficult to determine a fold’s origin. A ramp results from truncationof the forelimbof an existing anticline, so a ramp does not, in itself, rule out a detachment-fold origin. An anticlinal forelimb in the hanging wall may be steepened either by displacement over a convex-upward bend in the underlying thrust or by the thrust being folded into an antiform. Thrust truncation of an existing anticlinal fore-limb may result in a footwall syncline, but most fault-propagation fold models require either an abandoned ramp tip or significant strain within the forelimb to account for a footwall syncline. Origin as a detachment fold is possible if a footwall syncline is present, especially if an abandoned ramp tip is absent, or if the anticlinal core is internally thickened. The structural style and mechanical stratigraphy of a region may provide additional useful information for determining fold origin. Knowledge about a specific fold may be insufficient to determine its origin, but other, less ambiguous examples in an area may indicate the most likely possibilities to consider. Detachment folds are likely where a competent unit overlies a much less competent unit, and fault-propagationfoldsmaybe more likely in evenly layered rocks that have relatively high competency but weak layer interfaces. The originof asymmetric map-scale folds in the northeastern Brooks Range of Alaska is difficult to determine on the basis of their geometry alone, especially because most of them have been truncated by thrust faults. The presence of thickened anticlinal cores, the absence of ramp tips, the presence of remnant uncut detachment folds, a mechanical stratigraphy characterized by a competent unit over an incompetent unit, and a transition from unbroken detachment folds to thrust-truncated asymmetric folds, together suggest that the thrust-truncated folds originated mainly as detachment folds rather than as fault-propagation folds.