CONICET | Buscador de Institutos y Recursos Humanos

Crustal extension, normal faulting and recent quarrying at Mangatete, Taupo Volcanic Zone (TVZ), New Zealand, illuminate a rare spatial and temporal window on a dynamic Late Quaternary geothermal system (~2 km2). Detailed geological mapping, stratigraphic logging, AMS 14C dating, and textural and mineralogical analyses were used to construct a complex history of hydrothermal, volcanological and tectonic activity from ~32 to 2 ka. Evidence of extinct, surface hydrothermal manifestations include in situ siliceous sinters distributed on normal fault terraces, a hydrothermal eruption breccia (HEB) containing acid-etched sinter blocks, another HEB that was bathed in silicifying thermal fluids, and sinter clasts that were entrained in a hot debris flow. Preserved sinter textures typical of near-neutral pH, alkali chloride spring discharge include plant-rich, palisade, tufted bubble mat, and domal stromatolitic. In addition, a packed fragmental sinter fabric is confirmed herein to have developed in point bars along thermally fed streams. Moreover, four unusual siliceous sinter fabrics ? vuggy, spongy, scalloped, and arcuate wavy layered ? are inferred to have formed from local acidic thermal springs, possibly associated with paleo-fumaroles. The oldest sinters at Mangatate constitute blocks in the northern area dated at ~32 and 19 ka, which formed in alkali chloride hot springs, but were post-depositionally altered by acid sulfate steam condensate overprinting and then dismembered by a hydrothermal eruption. Acidic hot spring discharges also were localized in northern areas. Another change in paleo-hydrology produced further geothermal activity to the west and south ~18-3 ka, but with a distinct break in sinter ages from ~17.2-8.5 ka, corresponding to relative quiescence in fault movement between the Rotorua and Rotoma eruptions. The location and ages of these unaltered, post-8.5 ka, alkali chloride sinters and silicified paleosols indicate an overall westward migration of spring discharge. Abundant charcoal and burnt wood in ash at the base of a locally extensive volcaniclastic debris flow, containing sinter blocks of a range of ages and mineralogical maturity, suggest its emplacement around 2 ka. Structurally these younger, in situ and ex situ sinters occur along SW-NE striking faults that dissected the topography into stepped terraces. Inferred cross faults, i.e. structural lineaments orthogonal to the regional trend of the TVZ, likely served as fluid pathways at depth through the Mesozoic graywacke basement, initiating and maintaining hydrothermal activity at Mangatete for about 30 k.y. Within the larger Ngakuru Graben (~95 km2), the ages and distributions of sinters and hydrothermal eruption breccias, including those at Mangatete, demonstrate that significant hydrothermal activity occurred for more than 60 k.y., in an area lacking such activity today. This suggests that the ductile basement graywacke underlying the active TVZ has sufficient thermal capacity to buffer and distribute underlying heat source(s) to act as a uniform ?hot plate,? but it also requires periodic intrusions of dikes or sills to supply and maintain heat to the overlying geothermal systems. Such intrusion(s) beneath the Ngakuru Graben over several thousand years likely accounted for the widespread and long-lived paleo-hydrothermal activity evident in the region.

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