Drainage modification associated with the northern Lapstone Structural Complex, New South Wales, Australia

The Lapstone Structural Complex (LSC) comprises a series of north-trending faults and monoclinal flexures forming the eastern margin of the Blue Mountains Plateau, ~50 km west of the Sydney CBD. The LSC is considered a potential source of large earthquakes, however its evolution, and in particular its tectonic history is not well constrained. The LSC is bounded to the west by the Kurrajong Fault System (KFS), a series of en echelon reverse faults downthrown to the west. Streams crossing the LSC oversteepen by about 2-5 times over these faults. This study aims, through longitudinal profile analysis of 18 streams crossing the LSC coupled with field observation, to determine whether the oversteepening can be attributed to a lithological change at the faults, or tectonically-induced disequilibrium. Two approaches are used. Firstly, plots of log slope versus log distance (DS plots) are produced for each of the streams. As a result of noise in the topographic data, these results are inconclusive in demonstrating either situation. Secondly, an area-slope relationship, defined by A0.4S (where A = area and S = slope), is plotted against downstream distance. This factor is derived from the stream incision law, dz/dt = KAmSn, where K is assumed to be constant, and m and n are positive constants relating to erosional processes, and basin hydrologic and geometric factors. The analysis shows that in all but two streams, values for A0.4S are at a maximum over the LSC. Peak A0.4S values of about 0.2 are estimated to be equivalent to vertical incision rates of about 70 m/Ma. A0.4S varies with lithology; however the lithological effect is demonstrated to be of similar magnitude or smaller than the apparent structural control exerted by the LSC. All streams with catchment areas less than 100 km2 have developed swamps upstream of faults on the LSC. Sediment accumulated in these swamps is generally 0.5-4 m thick, but reaches 14 m in Burralow Swamp. In Blue Gum Creek and Burralow Swamps, the sedimentary sequence includes an organic clay layer indicative of low-energy depositional conditions. Previous radiocarbon dating and pollen analysis suggests the sediment is of Pleistocene age. The elevation of the clay layer is similar to that of bedrock downstream of the faults, consistent with damming related to from tectonically induced uplift.