We present new Bouguer gravity data and detailed cross-sections of the geometry of the deformed Otago peneplain surface beneath the Ida valley, a northeastsouthwest elongate basin in Central Otago, New Zealand. Taking the strong regional gravity gradient into consideration, a-15 mgal Bouguer anomaly results from the density contrast between Cenozoic Ida Basin sediments and Mesozoic Otago Schist basement. Using gravity modelling and geologic constraints, we present a series of northwest-southeast crosssections through the basin. Underneath the southeast side of the Ida Basin, adjacent to the flank of North Rough Ridge, the peneplain surface dips gently. Underneath the northwest side of the Ida Basin, adjacent to the flank of Blackstone Hill, the peneplain surface is strongly deformed by a faultpropagation fold associated with the tip of the Blackstone Fault. Along this margin, the orientation of the peneplain surface varies dramatically along-strike. In the north, the peneplain surface is overturned, and the fault-propagation fold is a tight and gently inclined anticline-syncline pair. In the south, the peneplain surface is upright, and the faultpropagation fold is an open and moderately inclined anticlinesyncline pair. Three possible explanations for the along-strike variation in fold geometry are: (1) the dip of the fault varies dramatically along-strike, (2) the fault tip at the northeast end of the study area has an unexpectedly shallow depth, and (3) mechanical anisotropy of the basement schists exerts a strong influence on fold shape.