Lapstone Structural Complex – Sydney Basin structures
In the western Sydney Basin, the Lapstone Structural Complex (Figure 1) is a major north-trending association of monoclines and faults that forms the frontal ridge of the Blue Mountains Plateau. Palaeomagnetic data from the southern part of the Lapstone Structural Complex indicates a Late Cretaceous to Cenozoic timing of deformation. At Upper Kurrajong, the Lapstone Structural Complex is dominated by an east-facing monocline with a central limb containing several different homoclinal segments (Figure 2). At the Hawkesbury Lookout section, strata are steeply dipping to near vertical along the main east-facing monocline and the subsurface structure is interpreted as a moderately west-dipping thrust fault (Figure 3). The Lapstone Structural Complex can be regarded as related to either a west-dipping thrust at depth or formed from steep east-dipping extensional faulting. Seismicity is consistent with the first alternative. The eastern Lachlan Fold Belt is basement to the Sydney Basin and includes moderately west-dipping faults that may also have been reactivated as thrust faults in the present-day stress regime. These structures provide an analogue for a structure in the basement that controlled development of the Lapstone Structural Complex.
Figure 1. Sydney Basin and major structures in the adjoining Lachlan Fold Belt. Lapstone Structural Complex is marked along with locations of Figures 2 and 3. Faults in Lachlan Fold Belt: CSF = fault on western margin of Cockbundoon Syncline, LGF = Lake George Fault, QF = Queanbeyan Fault, MF = Mulwaree Fault, YF = Yarralaw Fault, WFZ = Wiagdon Fault Zone.
Figure 2 West to east cross section through the Lapstone Structural Complex at Kurrajong Heights. Wg = Wianamatta Group (underlying units not labelled). Vertical scale = horizontal scale.
Figure 3 West to east cross section through the Lapstone Structural Complex at the Hawkesbury Lookout (note that stratigraphy is omitted but the surface exposure is in the Hawkesbury Sandstone). Vertical scale = horizontal scale.
References
Fergusson C. L. 2006. Review of structure and basement control of the Lapstone Structural Complex, Sydney Basin, eastern New South Wales. Sydney Basin Symposium. In: Hutton A. and Griffin J. (eds), Proceedings of the Thirty Sixth Sydney Basin Symposium, Advances in the study of the Sydney Basin, School of Earth and Environmental Sciences, University of Wollongong, pp. 45?50. (pdf)
Memarian H. and Fergusson C. L. 2003. Multiple fracture sets in the southeastern Permian-Triassic Sydney Basin, New South Wales. Australian Journal of Earth Sciences 50, 49–61.
