Ferruginous thermal spring complexes, northwest Tasmania: evidence that far-field stresses acting on a fracture mesh can open and maintain vertical flow in carbonate terrains

Far-field stress changes in the southern Australian plate since 5 Ma have produced significant areas of uplift and seismicity. In northwest Tasmania, there is evidence that this stress reorientation to maximum horizontal NW–SE stress has influenced meteoric-derived thermal (15–20°C) discharge patterns of confined karstic aquifers, by placing pre-existing NW-trending faults/fractures into a dilated state or a critically stressed state. Previous studies have shown that spring discharge has operated continuously for at least 65,000 years, and has transported large volumes of solutes to the surface to be deposited as mounds of calcite-goethite-silica up to 7 m high. The thermal spring chemistry at one site, Mella, is consistent with descent to at least 1.2–1.5 km, although the hinterland within 50 km is less than 500 m elevation. Thermal spring chemistry is consistent with most of the deep water–rock interaction occurring in low-strontium Smithton Dolomite. While some of this water is discharged at springs, some instead intersects shallow zones of NE-fracture-controlled rock (2 × 4 km in area) with karstic permeability where, although confined and subject to a NE-directed hydraulic gradient, it circulates and cools to ambient temperature, with only minor mixing with other groundwaters.