Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopSTRUCTURE OF THE LAPSTONE MONOCLINEThe Lapstone Structural Complex varies along its length with monocline(s), high-angle faults andminor structures such as thrusts, minor folds, tectonic breccias and joints (Branagan & Pedram 1990,1997). An east-facing monocline is well developed along much of the Complex and this is shown ina west-east cross section at Upper Kurrajong where the central limb of the monocline consists ofseveral segments with different dips to the east (Figure 2). Monoclines, such as in the ColoradoPlateau, have been attributed to contractional deformation along thrust faults at depth (Bump &Davis 2003) but they are also related to deformation in rock strata overlying steeply dippingextensional faults in underlying basement (Withjack & Callaway 2000).Figure 2 West to east cross section through the Lapstone Structural Complex at KurrajongHeights. Wg = Wianamatta Group (underlying units not labelled). Vertical scale = horizontalscale. Cross section is constrained by the contact between the Wianamatta Group and theunderlying Hawkesbury Sandstone as shown on the Penrith 1:100 000 Geological Sheet(Clarke & Jones 1991). The cross section has been drawn using the kink-band method ofconstruction (Suppe 1985).The steepest dips on the monocline associated with the Lapstone Structural Complex occur belowthe Hawkesbury Lookout. Here, strata west of the monocline are flat and gradually increase in dipup to ca 15º to the east before crossing a fault onto the steeply dipping near vertical central limb(Branagan & Pedram 1990, figure 7). A cross section is shown along a west-east line through themonocline at Hawkesbury Lookout in Figure 3. The zone of ductile deformation at the surfaceassociated with the central limb of the monocline is replaced at depth by a moderately west-dippingthrust fault in the lower part of the Sydney Basin and presumably the underlying basement withapproximately 500 m of net slip.BASEMENT CONTROLIt has been suggested that the Lapstone Structural Complex was controlled by reactivation of thewestern margin of the Budawang Syncline (the Eden-Comerong-Yalwal Rift) in the underlyingLachlan Fold Belt especially where this was intersected by elements of the east-trending LachlanLineament (Branagan & Pedram 1990). The structure of the Budawang Syncline exposed at theShoalhaven River and Ettrema Creek has been described by Cooper (1992); the western margin ofthe Syncline is locally faulted and other faults occur within the structure although no evidence hasbeen found to indicate that any of these faults have been reactivated in the present-day stress field.South the Sydney Basin, Ordovician turbidites and black shales dominate the Lachlan Fold Belteither side of the Budawang Syncline. These rocks are complexly deformed with multipledeformation and are also affected by common late brittle structures (Fergusson 1998; Fergusson &Frikken 2003). Moderate to intense deformation with widespread folding, cleavage development andnumerous faults occur in the Lachlan Fold Belt to the west of the Sydney Basin (Glen 1992). Faultsin this region are west dipping and include structures such as the Wiagdon Fault Zone, MulwareeFault, Yarralaw Fault and other structures (Fergusson & VandenBerg 1990). The Lake George Fault,15