Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopFigure 2 Coastal, West Head and Berowra Waters Lineaments (after Mauger et al, 1984; andNorman, 1986).The strike-slip faults and related structures, (including strongly developed shear zones, and possiblythe breccia features) have attracted some attention in recent years as they have been encounteredduring various large engineering projects in the region and have required special treatment.Slickensides patterning on the Mooney strike slips, and a similar fault at Normanhurst appearssurprisingly fresh (Branagan & Packham 2000), but no evidence is known of any movements onthese features, although such cannot be discounted. Norman (1986) and Whitehouse and Branagan(1998) discussed aspects of the “Coastal Lineament’ of Mauger et al (1984), the West HeadLineament and related structures (Figure 2), and more recently Ochs et al (2004) have produced acoloured map of the Sydney CBD showing mapping by Branagan, 1985; Norman, op. cit., withadditional more recent mapping. For further discussion on Sydney faulting see Branagan, 1977;2000a, and 2000b; Branagan and Norman, 1984; Branagan et al., 1987; Branagan and Mills, 1990).THE LAPSTONE STRUCTURAL COMPLEXAs mentioned earlier this structure marking the eastern edge of the Blue Mountains has been thefocus of attention by geophysicists as the most likely site for a reasonably large earthquake toaffect the Sydney region. Cotton (1921) presented information on the last reasonably largeearthquake here, in 1919, and there are some additional notes on this earthquake in his personalpapers (University of Sydney Archives).It is not proposed to discuss the geology of this region here. It has been covered to aconsiderable extent in Pedram, 1983; Rawson 1989; Branagan & Pedram, 1990, 2000), whichalso list many of the earlier papers.My only concern is to comment on the suggestion put forward by a number of speakers at therecent symposium on Sydney Earthquakes, and also in some papers in this volume (e.g.Fergusson, 2005) that the major movement on the Lapstone structure has been a thrust(variously suggested as high angle, and low angle) from the west. This has been based to some9
Seismic Hazard in SydneyProceedings of the one day workshopdegree on the suggested pattern of reverse faulting in the north-south striking faults in theLachlan Foldbelt to the west, and on the not too detailed seismic records obtained during coaland oil exploration in the region, (Herbert, 1989)..Figure 3. The Lapstone Structural Complex and its associated faults to the north and Southseem more consistent with normal down warping to the east, rather than thrust deformationfrom the west [after Bembrick et al, 1973].While the classical explanation of the formation of the originally named Lapstone Monoclinerequired an uplift of the Blue Mountains in the Late Pliocene (The Kosciusko Uplift) (Andrews,1910), Branagan (1975) suggested an alternative – down dragging of the Cumberland Basin duringthe period of sea-floor spreading at the beginning of the Tertiary. Herbert (op. cit.) attributed theorigin of the faulting to a late Triassic movement, during reactivation of underlying LachlanFoldbelt deformation, including some wrench faulting (see also Harrington, 1982).Although the author has recognised overturning of the steep Mt. Riverview Fault (east of theHawkesbury Lookout) and also of the Nepean Fault at Bents Basin (Branagan & Pedram, 1990) hebelieves these are very localised events. One problem with the west-dipping thrust suggestion is thatit seems only to be looking at the north-south fault forming the main scarp of the edge of the BlueMountains. I believe we also have to take into consideration the associated faults and flexures which10
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Landslides in the Sydney Basin - Geoscience Australia

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