Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopNew field observations pertaining to the ageand structure of the northern LapstoneStructural Complex, and implications for seismichazardDAN CLARK 1 AND ANDREW RAWSON 21. GEOSCIENCE AUSTRALIA, GPO BOX 378, CANBERRA ACT 26012. NSW DEPT. OF ENVIRONMENT AND CLIMATE CHANGE, C/O FACULTY OF SCIENCEAND AGRICULTURE, CHARLES STURT UNIVERSITY, LEEDS PDE, ORANGE PO BOX 883NSW 2800ABSTRACTA structural transect along a ridge entering Wheeny Gap from the west provides important insightinto the nature of faults comprising the northern Lapstone Structural Complex (LSC). Shearedsandstone outcropping along the ridge east of the Kurrajong fault scarp suggests that the KurrajongFault is a steeply east-dipping reverse fault, in accordance with the interpretation of seismic profilesfurther to the south. A previously unrecognised west-dipping reverse fault (Wheeny Gap Fault) withat least several tens of metres of displacement was observed in a cliff face on the northern side ofWheeny Gap, several hundred metres east of the Kurrajong Fault. Relatively recent activity issuggested on the Wheeny Gap Fault as it laterally displaces the cliff face formed during the passageof a knickpoint up Wheeny Creek relating to initial relief generation across the Lapstone Monocline.Earthquake hypocentres recorded over the last few decades occur predominantly at depth to thewest, and have been used to suggest the presence of a blind west-dipping reverse fault, into whichthe Kurrajong and Wheeny Gap faults link at depth. We present an evolutionary model for the LSCbased upon this architecture which reconciles evidence for late Cenozoic uplift across the LSC andthe observation that the Rickaby’s Creek Gravels overlie shale on the Cumberland Plain andsandstone on the Lapstone Monocline. This model suggests that the findings of a major seismichazard assessment of the Sydney Basin, which concludes that magnitude M W 7.0 and greaterearthquake events might be expected on the LSC on average every 15-30 ka, should be treated withcaution. This expectation of regular recurrence must be tempered by the possibility that a large partof the relief relating to the LSC may have formed in the late Miocene or earlier, and evidence fromother Australian intraplate faults suggesting that large earthquake occurrence is markedly temporallyclustered.Keywords: Lapstone Structural Complex, reverse fault, seismic hazard, seismicityINTRODUCTIONFaults of the Lapstone Structural Complex (LSC - Mauger et al. 1984) underlie the eastern rangefront of the Blue Mountains west of Sydney over a distance of at least 100 km, and perhaps as muchas 160 km (e.g. Branagan 1969; Branagan and Pedram 1990). More than a dozen major faults andmonoclinal flexures have been mapped in the region where relief relating to the complex is mostprominent, between Bargo and the Colo River north of Mountain Lagoon (Fig. 1). While thesestructures have been known for more than 100 years (Branagan, this volume and references therein)very little is known about their subsurface geometry and faulting history, particularly in theCenozoic. The latter uncertainty stems in a large part from a paucity of readily dateable Cenozoicunits associated with faulting (e.g. Wellman and McDougall 1974; Bishop et al. 1982; Rawson1990; Pickett and Bishop 1992).31
Seismic Hazard in SydneyProceedings of the one day workshopFigure 1: Colour-draped and hill-shaded SRTM DEM (90m) image of the Lapstone StructuralComplex west of Sydney. The region covered by this article is indicated with a box, as is thearea of Fig. 3. Illumination is from the west. Red arrows highlight scarp segments. Blackarrows indicate the extents of the profiles in Fig. 2.Over the last few decades the most significant concentration of earthquake hypocentres in theSydney Basin has occurred at depth west of the LSC (Gibson, this volume). This relationship hasbeen used to suggest that the surface expression of the LSC reflects displacement across a low anglereverse fault, emergent at the surface south of Penrith, and underlying the Lapstone monocline to thenorth. Based upon this model, a recent study of seismic hazard in the Sydney Basin identified theLSC as a potential source for large and damaging earthquakes, and estimated a recurrence of 15-30ka for events exceeding M W 7.0 (Berryman et al. 1999). As this analysis was necessarily basedupon limited geochronological and structural data the results must be treated as highly uncertain. Inparticular, the authors contend that deformation resulting in the development of the LapstoneMonocline postdates the 18.8 Ma (Wellman and McDougall 1974) extrusion of the Green ScrubBasalt, which appears to have been truncated by the Kurrajong Fault (Branagan and Pedram 1990)but might just as plausibly have been emplaced against an extant fault scarp. Alternative viewscontend that fold development was largely complete by the Early Jurassic (Pickett and Bishop1992), the Late Jurassic (Herbert 1989), post-mid Cretaceous (Schmidt et al. 1995), the EarlyTertiary (Branagan and Pedram 1990; van der Beek et al. 2001), and the Late Tertiary (Bishop et al.1982 with age recalculated by Pillans 2003). Furthermore, seismic reflection data acquired alongseveral lines across the Nepean Fault (the southern range bounding fault) have been interpreted tosuggest steep westerly dips, in the range of 50-70°, to at least 2.5 km depth (Herbert 1989).32
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Landslides in the Sydney Basin - Geoscience Australia

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