Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopReview of structure and basement control of theLapstone Structural Complex, Sydney Basin,eastern New South WalesChristopher L. FergussonSchool of Earth and Environmental Sciences, University of Wollongong, NSW 2522, AustraliaABSTRACTIn the western Sydney Basin, the Lapstone Structural Complex is a major north-trending associationof monoclines and faults that forms the frontal ridge of the Blue Mountains Plateau. Palaeomagneticdata from the southern part of the Lapstone Structural Complex indicates a Late Cretaceous toCenozoic timing of deformation. At Upper Kurrajong, the Lapstone Structural Complex isdominated by an east-facing monocline with a central limb containing several different homoclinalsegments. At the Hawkesbury Lookout section, strata are steeply dipping to near vertical along themain east-facing monocline and the subsurface structure is interpreted as a moderately west-dippingthrust fault. The Lapstone Structural Complex can be regarded as related to either a west-dippingthrust at depth or formed from steep east-dipping extensional faulting. Seismicity is consistent withthe first alternative. The eastern Lachlan Fold Belt is basement to the Sydney Basin and includesmoderately west-dipping faults that may also have been reactivated as thrust faults in the presentdaystress regime. These structures provide an analogue for a structure in the basement thatcontrolled development of the Lapstone Structural Complex.INTRODUCTIONThis account presents a brief review of the structure of the Lapstone Structural Complex in thewestern Sydney Basin with two cross sections shown across the structure at Upper Kurrajong andthe Hawkesbury Lookout. These cross sections complement the structural outline of the LapstoneStructural Complex given by Branagan and Pedram (1990, 1997). Additionally, the issue ofbasement control of the Lapstone Structural Complex is considered with reference to potentiallyactive faults in the Lachlan Fold Belt to the west and south of the Sydney Basin.BACKGROUNDThe Lapstone Structural Complex consists of an association of east-facing monoclines, high-anglefaults, and fracture zones that form the frontal ridge of the Blue Mountains Plateau in the Permian toTriassic Sydney Basin to the west of Sydney (Branagan & Pedram 1990, 1997). Development of theLapstone Structural Complex has been related to the Late Permian – Early Triassic Hunter-BowenOrogeny (Pickett & Bishop 1992). This argument was based on the inference that the JurassicNortons Basin diatreme postdated local development of the monocline near Wallacia and fromsuggestions that stratigraphic units thicken from west to east as they cross the Lapstone StructuralComplex and it is therefore a syndepositional feature. Eastward thickening of units begins well tothe west of the Lapstone Structural Complex and therefore this structure may not have been activeduring deposition (Mayne et al. 1974). Palaeomagnetic data indicate that the Lapstone StructuralComplex has affected the widespread mid-Cretaceous overprint magnetisation in the Sydney Basinand therefore that the structures formed later than mid Cretaceous (Schmidt et al. 1995). This isconsistent with an Early Tertiary age for the Lapstone Structural Complex advocated by Branaganand Pedram (1990) and also with studies of sub-basalt topography and modelling of uplift in theBlue Mountains (van der Beek et al. 2001). Numerous earthquakes have occurred in the BlueMountains and indicate that the Lapstone Structural Complex may still be active (Brown & Gibson2004) despite the lack of tilting of Miocene magnetisation along the monocline at Lapstone (Bishop13