Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopThere is an apparent conflict between this evidence for recent activity and the palaeomagnetic dataindicating that folding related to the Lapstone Monocline occurred prior to 8 ± 5 Ma (Bishop et.al.,1982 with ages recalculated by Pillans, 2003). Given that the Kurrajong Fault System and theLapstone Monocline form part of the same structure (Herbert, 1989; Fergusson, this volume),deformation on the one might be expected to be accompanied by deformation on the other. Onepossibility, which is testable by careful field observation, is that the blind fault underlying theLapstone Monocline (see Gibson, this volume) reached the surface in the Pliocene and deformationsince then has been accrued by sliding along the fault rather than by folding. Alternatively, therecent active period may have been the first since the late Tertiary (cf. Branagan & Pedram, 1990) orMesozoic (cf. Pickett & Bishop, 1992; Schmidt et al., 1992).Given the very low uplift rates postulated for the LSC, passive exhumation (cf. Pickett and Bishop1992) is likely to have played a significant role in the development of the contemporary landscape.In particular, the fact that the Tertiary Rickabys Creek Gravels overly Wianamatta Group shales onthe Cumberland Plain, and Hawkesbury Sandstone on the face and top of the Lapstone Monocline,implies that erosional lowering of the landsurface had stripped shale from many areas west of thesyn-depositional warp (proto-Lapstone Monocline) by the time of gravel deposition, but not to theextent that significant relief was generated [as indicated by structure within the gravels suggesting abraided stream depositional environment, Bishop & Hunter (1990)]. However, we demonstrate asignificant tectonic imprint in the geomorphology of streams crossing the LSC, as distinct from thelithologic control that would be expected to dominate in a scenario only involving passiveexhumation. Additional research, guided by new erosion rate data obtained on streams withcatchment areas significantly smaller than the Grose River, is required to determine definitivelywhether this tectonic imprint relates only to the last active period identified in the sediments ofBurralow Swamp, or to periodic activity throughout the late Tertiary.Intuitively, the prominent ‘V-notch’ valley profiles of streams between the Kurrajong Fault Systemand the Lapstone Monocline suggest a recent tectonic history extending beyond the last activeperiod. Furthermore, it is tempting to speculate that the apparent associations between the LSC andMiocene volcanics (Pedram, 1983; Branagan and Pedram, 1990; Van Der Beek et al., 2001) relate tolate Tertiary development of the complex, synchronous with the establishment of the current crustalstress field, and a pulse of tectonic activity recorded in all southeast Australian basins (Dickinson etal., 2002; Sandiford et al., 2004). A terrestrial analogue to this pulse of deformation is seen in theLake George Fault, which has accumulated up to 200 m of throw since the late Miocene (Abel,1985).CONCLUSIONSThe results of morphometric analysis of drainage lines crossing the northern Lapstone StructuralComplex show indicators of geologically recent, and potentially ongoing tectonic deformation.Stream long-profiles oversteepen as they cross major faults, valley cross sections narrow to ‘Vnotches’where streams enter the upthrown side of fault blocks, and several streams have beenpartially dammed, forming swamps and lakes. While these indicators are consistent with a model ofperiodic (perhaps temporally clustered) deformation of the Kurrajong Fault System, near to thethreshold conditions of uplift/incision, additional erosion rate data is required to determine therelative contribution of passive exhumation of syn-depositional basin structure in forming thepresent landscape.The important question to ask relating to seismic hazard is whether faults of the LSC are still withinan active period, or whether we are now in a quiescent period that might last many hundreds ofthousands to a million years or more (cf. Crone et al., 2003). Earthquake epicenters located at depthto the west of the LSC (Gibson, this volume) potentially indicate that the most recent active periodhas not yet finished.28