Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopThis contribution briefly reviews existing geologic information for the northern LSC and presentsnew structural data from a preliminary traverse through Wheeny Gap. The implications of the newdata for landscape evolution and seismic hazard assessment are discussed.GEOMORPHIC CHARACTER AND STRUCTURE OF THE LAPSTONE STRUCTURAL COMPLEXThe range front scarp relating to the LSC rises monotonically from zero near Bargo to a relief ofapproximately 400 m immediately south of the Colo River (Fig. 2). The expression of the rangefront south of the confluence of the Nepean and Warragamba rivers is sharp and linear, and isdefined over much of its length by a line of cliffs developed in Hawkesbury Sandstone. Thisexpression reflects the emergence at the surface of the range front fault, the Nepean Fault, asevidenced by exposures in major cross-cutting drainage lines (Branagan and Pedram 1990). Furthernorth the east-facing scarp is broader (up to 2 km) and is underlain by a flexure called the LapstoneMonocline (e.g. Bryan 1966; Branagan and Pedram 1990). West-facing scarps are prominentlydeveloped a couple of kilometres to the west of the monocline (e.g. David 1896, 1902).Interpretation of seismic reflection profiles acquired south of the Grose River suggest that high anglereverse faults with a geometry suggestive of a significant wrench component underlie these westfacing scarps in the north, and the east facing scarps in the south (Herbert 1989). Herbert (1989)further contends that in contrast to the continuous and sinuous nature of the faults as mapped (c.f.Bryan 1966), they are in fact composed of much shorter, discontinuous, straighter en echelonsegments; namely the Kurrajong, Burralow, Grose, Frasers, Yellow Rock and Glenbrook faults(Branagan and Pedram 1990).Figure 2: Scarp-parallel topographic profiles along the LSC. “Residual” represents valuesfrom the scarp front minus those 5 km east. The residuals are depressed south of the NepeanRiver as some deformation is accommodated on the Oakdale Fault, west of the range frontNepean Fault. Location of profiles is shown on Fig. 1.The nature of the faulting which gave rise to the Lapstone Monocline is the subject of debate.Sedimentary units thin to the west across the Lapstone Monocline, indicating that the structure wasactive during Permian and Mesozoic sedimentation and rifting, and hence may be underlain by an33
Seismic Hazard in SydneyProceedings of the one day workshopeast-dipping normal fault (e.g. Bryan 1966; Branagan and Pedram 1990; Pickett and Bishop 1992;Australian Oil and Gas Corporation Ltd 1966). Pickett and Bishop (1992) argue that monoclinedevelopment was complete by the time of the Early Jurassic intrusion of the Norton’s BasinDiatreme, and that the high angle reverse faulting, at least in part, postdates this. While more recentpalaeomagnetic evidence requires that the majority of flexure post-dates the mid-Cretaceous(Schmidt et al. 1995), the hypothesis that monocline development and high angle reverse faultingare temporally separate remains plausible.Herbert (1989) implies that monocline development is related to displacement on the high anglewest-dipping reverse faults imaged in seismic reflection profiles. This structural relationship issupported by Fergusson (this volume) who relates the structures exposed in the central section of theLapstone Monocline, at the Hawkesbury Lookout, to a 50° west-dipping reverse fault at depth. Analternative interpretation of the main physiographic features is presented by Berryman et al. (1999),who envisage the high angle reverse faults of Herbert (1989) linking into a low angle master reversefault at depth. The existence of this master fault is inferred from a spatial clustering of earthquakehypocentres at depth to the west of the LSC (cf. Gibson, this volume). Berryman et al. (1999)postulate that a shallowing of the underlying master fault dip relative to the Nepean Fault accountsfor the monoclinal shape of the range front north of Penrith, and that both monocline and westfacingscarps are a consequence of post mid-Miocene movement on this blind thrust system.AGE OF MAJOR DISPLACEMENT ACROSS THE LAPSTONE STRUCTURAL COMPLEXBlue Mountains basalts cap relatively flat hilltops, suggesting that they may be remnants of aninitially continuous subhorizontal sheet and predate river incision (Carne 1908; Wellman 1979). Areconstruction of the ca. 20 Ma palaeotopography west of the Kurrajong Fault based upon the subsurfaceelevation of a number of basalt flows suggests that the landscape had a low relief(predominantly
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