| Popis: |
Fault-controlled fluid flow is a well-recognised feature of hydrothermal systems. Yet, the role of extinct lava domes in governing fluid flow is less established. Here we present fluid-flow models for the spatial distribution and origins of permeability within the two thermal areas (Ohaaki West (OHW) and Ohaaki East (OHE)) of the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand. We develop conceptual fluid-flow models using detailed mapping of surface heat and mass flow gradients and structural and volcanic/stratigraphic data. Heat and mass flow models based on extinct lava domes are of two main types: (i) a ‘volcanic conduit’ model where hydrothermal fluids ascending from an underlying upflow exploit remnant permeability structures associated with dome growth (dyke margins, conduit shear zones and eruptive vents) and discharge at the surface of OHW, and; (ii) a dome and regional fault, which collectively produce a channel-barrier fluid-flow system within surficial calcite-cemented alluvium and ash fall deposits that mantle the dome (OHE). These models show that silicic lava domes localise fluid flow through either remnant eruptive structures, or by providing islands of locally competent and low-porosity rock that favour the development of high-permeability fault and fracture systems. In each model, hydrothermal sealing and cementation of otherwise unconsolidated and low strength materials, mantling the domes, play an important role in permeability generation. From these observations and recent mass and heat flow surveys of other fields, we argue that the Quaternary dome complexes of the TVZ, overlying long lived (≥ 0.15–0.5 Myr) hydrothermal systems, may constitute a major pathway of magmatic heat and mass release to the atmosphere long after eruption and dome formation. |
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