Structure and neotectonics of the Blackstone Hill Antiform, Central Otago, New Zealand

Abstract

Blackstone Hill is a northeast‐trending ridge of schist basement rising between the Ida Burn and Manuherikia River valleys. Deformation of young sediments and uplift of strath river terraces cut into both sides of the ridge indicate that its rise is an ongoing, late Cenozoic phenomenon. Deformation of poorly exposed cover sediments at the ridge margins appears to be relatively localised and complex. In the schist basement, a new generation of fractures has developed to accommodate late Cenozoic deformation. The basement also displays a prominent lineation and foliation of Mesozoic age. Lineation defined by quartz rodding trends c. 020°. The orientation of foliation varies and defines the Blackstone Hill Nappe, a large synmetamorphic fold of Mesozoic age with a hinge trending c. 167°. The Otago Peneplain, the regional unconformity between schist basement and overlying sedimentary cover, was a subhorizontal erosion surface before the onset of ridge formation. It is now warped around the late Cenozoic Blackstone Hill Antiform hinge, which trends 035°, rather than offset discontinuously along a range‐scale fault. The antiform is box‐shaped, asymmetric, and open, although the peneplain may be steeply tilted at depth. Data from a strath terrace system indicate that, on the flanks of the antiform, the peneplain has been progressively tilted during folding. Drainage patterns incised into the peneplain suggest that the southeastern flank of the antiform began rising before the northwestern flank. These observations are consistent with a fault‐propagation fold model for the Blackstone Hill Antiform, whose geometry is best explained by reverse motion along two steep faults at depth. Variations in basement fabric related to the Mesozoic Blackstone Hill Nappe have mechanically influenced the development and topographic expression of the antiform.