Structural Controls on Regional and Reservoir Scale Hydrothermal Flow

Abstract

The central Taupo Volcanic Zone contains over twenty high-temperature (>250°C) geothermal systems. Upwelling plumes are distributed semi-regularly across an area of ~2000km2, much as one would expect in the case of porous flow above a regional-scale hotplate positioned at depths of 6- 8km. However, the central TVZ is not uniformly porous, comprising a layered Quaternary cover sequence of varying material properties that overlies a more competent basement of equivocal rock type. Furthermore, the region is dissected by numerous faults, and in places is punctured by dikes. Structures have a strongly preferred NE-SW orientation, and are offset every ~20km by transverse accommodation zones, a consequence of the rifting process. We argue that the stratigraphy of the central TVZ interplays with rift architecture to influence the style of fault-fluid interactions at the regional and reservoir scale. Upwelling plumes are localised where rift architecture enhances vertical permeability deep within the convective regime. Focused flow through faults and fractures well-oriented for reactivation (i.e., steeply dipping NE- and NW-trending structures) likely predominates within basement rocks. In contrast, porous flow through fault-bounded compartments characterises the flow regime in the reservoir. Connected gaping fractures over vertical intervals exceeding 500m are not favoured at this structural level. In fact, the highest rates of focused flow are observed to discharge from hydrothermal eruption vents.