Abstract:
The Mangatete sinter, located ~20 km south of Rotorua, is centred on two quarries cut into the footwall of the Whirinaki Fault. The studied quarries, and surrounding 2km x 1.5km region, exhibit evidence of widespread, vigorous, geothermal activity from ~18ka to ~8ka (ages based on AMS 14C dates on opaline sinter and fossil wood). Extensive debris flow breccia throughout the study area contains sinter fragments of distinctive, quartzose stromatolitic textures, and many burnt logs and branches at its base (indicating hot flow), suggesting that hydrothermal activity occurred over a more extensive region, and may have been manifest for much longer. The Mangatete sinters record evidence of geothermal and associated paleoenvironments. These include warm water streams, deep and shallow thermal pools, sinter apron terraces, waterlogged fens, and silicified soils that either were the substrates upon which sinter formed or were positioned in distal, geothermally influenced settings at the margins of hot water discharge. Microbial fabrics include palisade, shrub, domal stromatolitic, plant-‐rich, tufted/wavy laminated, and packed fragmental textural types, the last of which has a modern analogue in the outflow of Hot Water Creek, Waimangu geothermal area, and a Jurassic equivalent in sinters of Argentinean Patagonia. The Mangatete region is structurally governed by SW-‐NE trending faults of the Taupo Fault Belt, but there is evidence of additional NW-‐SE structural lineaments in the study area, likely generating cross-‐faulting and increased permeability beneath the quarries. This hypothesis explains why the fossil geothermal activity is evidenced at this particular location. Electrical resistivity measurements beneath Mangatete are inconsistent with similar measurements beneath currently active geothermal surface expressions. This contrast implies that there have been changes in the pattern of deep hydrothermal convection cells. A cell must have been below Mangatete when it was a vigorous geothermal field, but it has since ceased. In the Mangatete area it appears that surface geothermal activity required both near surface faulting to provide a fluid conduit, and also a iii magmatic intrusion into the underlying crust to supply sufficient heat to generate upwelling hot fluids beneath a given near-‐surface area of appropriate permeability. Thus, the initiation and cessation of geothermal activity at Mangatete is considered to be due to an ephemeral magmatic intrusion that generated, but did not sustain, a deep hydrothermal fluid convection cell