Review of Structure and Basement Control of the Lapstone Structural Complex, Sydney Basin, Eastern New South Wales

In the western Sydney Basin, the Lapstone Structural Complex is a major north-trending association of monoclines and faults that forms the frontal ridge of the Blue Mountains Plateau. At Kurrajong Heights, the Lapstone Structural Complex is dominated by an eastfacing monocline with a gently dipping central limb containing several different homoclinal segments. At the Hawkesbury Lookout section, strata are steeply dipping to near vertical along the main east-facing monocline. The Lapstone Structural Complex has been related to either steep east-dipping extensional faulting or to moderate to steep west-dipping contraction faults. Strike-slip displacement may also have played a role in its development. The Hawkesbury Lookout section is interpreted in the subsurface as a moderately west-dipping thrust fault. The historical development of the Lapstone Structural Complex has been difficult to resolve although palaeomagnetic data from the southern part are indicative of Late Cretaceous to Cenozoic deformation. Neotectonic activity may also have occurred along the structure. Basement to the Sydney Basin is the eastern Lachlan Fold Belt, which includes moderately west-dipping faults that may have been reactivated as thrust faults in the presentday stress regime. These structures provide a potential analogue for a basement-controlled fault that has generated the Lapstone Structural Complex. INTRODUCTION This account presents a brief review of the Lapstone Structural Complex in the western Sydney Basin (Figure 1). Two cross sections are shown for Kurrajong Heights and Hawkesbury Lookout and these complement the structural outline of the Lapstone Structural Complex given by Branagan and Pedram (1990, 1997). The Lapstone Structural Complex is a significant feature but its understanding has been limited by various contradictory data on its historical development and also by its overall classification as one of contractional, extensional and/or strike-slip association. Additionally, it has been considered controlled by an underlying basement structure and this is considered herein with reference to potentially active faults in the Lachlan Fold Belt to the west and southwest of the Sydney Basin. BACKGROUND The Lapstone Structural Complex is an association of east-facing monoclines, high-angle faults, and fracture zones to the west of Sydney (Branagan & Pedram 1990, 1997). Its development was considered syndepositional during the Permian and Triassic periods by Pickett and Bishop (1992) as units thicken immediately to the east of the complex (see also seismic interpretation in Australian Oil & Gas Corporation Ltd 1966). An upper limit to tilting associated with monocline development was based on the cross-cutting Jurassic Nortons Basin diatreme near Wallacia which is apparently undeformed (Pickett & Bishop