ALONG-STRIKE VARIATION IN THE GEOMETRY AN ACTIVE FAULT-PROPAGATION FOLD; A BOUGUER GRAVITY MAP OF THE IDA BASIN, CENTRAL OTAGO, NEW ZEALAND

In central Otago, New Zealand, active reverse faults in schist basement are associated with fault-propagation folding of the Cretaceous Otago peneplain surface. Along-strike variations in the geometry of the folds and underlying reverse faults relate to variations in the mechanical behavior of basement rocks. We present new Bouguer gravity data and detailed cross-sections of the geometry of the deformed peneplain beneath the Ida Basin, a NE-SW elongate valley in Central Otago, New Zealand, in order to investigate along-strike variations in fault displacement and fold style. A -15 mgal Bouguer anomaly results from the density contrast between Late Cenozoic Ida Basin sediments and Mesozoic Otago Schist basement. Using 2D gravity modeling and geologic constraints, we present a series of NW-SE cross sections through the Basin. Underneath the NW side of the Ida Basin, the peneplain is folded in a strongly deformed zone at the tip of the Blackstone Fault, and the peneplain's orientation varies dramatically along strike. In the NE, the peneplain is overturned, and the fault-propagation fold is a tight and gently inclined anticline-syncline pair. In the SW, the peneplain is vertical, and the fault-propagation fold is an open and steeply-inclined anticline-syncline pair. Along-strike variation in the geometry of the fault-propagation fold associated with the Blackstone Fault may be caused by variation in the depth of the fault tip, mechanical anisotropy in the basement, or variation in crustal strength associated with a neighboring fault system.