LATE PLEISTOCENE TO EARLY HOLOCENE FAULTING, SURFACE RUPTURE AND LIQUEFACTION NEAR MONTAGU, NORTHWEST TASMANIA

Along the northwest coast of Tasmania near Montagu, low relief beach ridges of probable MIS 5e age are cut by two sets of northeast striking faults. Lack of horizontal offset of the beach ridges indicates that motion on the faults is predominately dip slip, and differences in topographic elevation on either side of the faults give a maximum vertical displacement of ~3 m. This displacement is verified by borehole data on the underlying marine terrace strath surface, which is developed on Neoproterozoic and Cambrian bedrock. The modern topography along the fault scarps suggests reverse faulting, consistent with the modern northwesterly oriented maximum horizontal compressive stress.

The beach ridge sediments consist of up to ~ 7 m of well sorted, very fine sand with a basal shell layer and an iron-cemented hard pan 1-3 m below the surface. The beach ridges are now covered by a tea-tree swamp except where cleared for farming, so the water table is very shallow and approaches the ground surface in many places during wet years. These saturated marine sands experienced liquefaction during earthquake rupture of the faults, leading to extensive development of sand boils up to 5 m in diameter and 2-3 m high along the fault traces. Sand boils are mostly confined to the hanging wall in close proximity to the fault traces. The height of the sand boils may reflect the elevation of the potentiometric surface within the confined part of the beach ridge sediments below the iron-cemented hard pan. Excavations across the sand boils expose an offset paleosol with buried, in situ vegetation deformed into the down flow direction. Radiocarbon dating of the buried organic–rich sediments constrains the age of faulting to between 14.8 ka and 7.2 ka, latest Pleistocene to early Holocene. Mid – Holocene beach ridges adjacent to the fault zone with maximum radiocarbon ages of 5.25 ka are not offset by the faulting.