Segmentation and fault–monocline relationships in the Lapstone Structural Complex, Sydney Basin, New South Wales

Abstract The 95 km-long Lapstone Structural Complex is a segmented structural association consisting of north-trending faults and monoclines in the western Sydney Basin between the Cumberland Basin to the east and lower Blue Mountains to the west. It has developed in a compressional regime. At depth, the Lapstone Structural Complex is most likely a deeply penetrating, west-dipping thrust fault that is seismogenic in the brittle middle crust. This structure has propagated upwards into the overlying Sydney Basin in the top ∼3 km of the crust and formed a suite of distinct fault and fault–monocline structures, including: (a) in the north, inferred imbricate faults at depth in the lower Sydney Basin dying out upwards into a well-displayed, major east-facing monocline (central limb has an overall consistent <20°E dip); (b) in the middle part of the complex, an east-facing monocline with dips increasing from west to east formed as a fault-propagation fold; and (c) as found in the south, a single thrust fault (Bargo and Nepean faults). The complex has a probable late Cenozoic age and has played a role in landscape development as shown by topography, uplifted river gravels and knick points along lower order streams. Therefore, it has formed late in the uplift history of the southeastern Australian highlands postdating uplift in the Cretaceous and uplift associated with basaltic eruptions in the Cenozoic. KEY POINTS The Lapstone Structural Complex is an association of segmented faults and monoclines formed at the tip of a deep-seated thrust fault. The largest structure in the complex is a gently east-dipping monocline formed above an inferred imbricate fault system in the lower Sydney Basin succession. Much of the middle and southern parts of the complex are single faults and a fault-propagation monocline related to a thrust fault at depth. The Lapstone Structural Complex is of late Cenozoic age and is most likely seismogenic.