Fergusson, Chris1 and Hatherly, Peter2
1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Lavender Bay, Sydney, Australia
The eastern margin of the Blue Mountains uplift is well defined by the Lapstone Structural Complex (LSC) with elevations typically of 150 to 200 m but locally up to 600 m at Kurrajong Heights. While it has been suggested that the LSC is related to normal faulting associated with passive margin development, this suggestion is inconsistent with a west-dipping reverse fault with minimal fault damage development (the Bargo Fault) we have found in the southern part of the LSC. We prefer the alternative view that the LSC consists of east-facing monoclines and reverse faults that dip both east and west. The underlying controlling structure could be a west-dipping thrust and the surface expression is due to the formation of a triangle zone. Further support for our interpretation is found in the northern part of the LSC where there are gravels (Rickabys Creek Gravel) and clay (Londonderry Clay) which are draped along the front of the monocline. Their position indicates that they were folded during development of the LSC. Thus the gravels do not occur in a series of terraces as was previously considered. Unfortunately no direct age of the Rickabys Creek Gravel and the Londonderry Clay has been determined but their unconsolidated state is consistent with a Neogene age thereby constraining formation of the LSC to the Neogene. Ongoing seismic activity associated with the LSC indicates neotectonic activity along the structure. The higher parts of the Blue Mountains are at elevations of over 1000 m and overall rock units of the Sydney Basin slope to the east at an angle of ca 1.5°. We consider that the latest phase of uplift in the Blue Mountains was associated with formation of the LSC and other structures including the Mt Tomah Monocline. Given earlier phases of uplift associated with formation of the eastern highlands it is problematic to identify the limits of this younger phase of Blue Mountains uplift particularly in the west where it apparently merges with the Central Tablelands. The enduring debate about the cause of the uplift of the Great Dividing Range of eastern Australia has become increasingly centred on the importance of mantle upwelling. We consider that the younger phase of uplift of the Blue Mountains associated with formation of the LSC was related to Australia’s setting west of the Southwest Pacific convergent margin rather than mantle upwelling.
Peter Hatherly is a retired geophysicist who has taken an interest in the structural and geomorphological evolution of the Blue Mountains. He has published relevant papers on the results of high resolution seismic reflection surveys, analysis of longitudinal stream profiles and mapping of semi-consolidated gravels above river systems.