Modeling Stress and Deformation Near Intersecting Misoriented Normal Faults in the Taupō Rift, Aotearoa New Zealand: A New Approach to Target Geothermal Permeability?

Intersecting normal faults that depart from ideal Andersonian orientations in a rapidly extending rift, subject to both orthogonal and rotational opening as well as magmatic activity, generate complex patterns of stress interaction. We investigate these processes in the Wairakei Geothermal Field, the oldest and largest electricity producer within the Taupō Rift, Aotearoa New Zealand. There, boreholes show pervasive fracturing both near faults and in intervening blocks. We developed three-dimensional forward finite element models (FEM) with the Adeli open-source code which accounts for elasto-visco-plastic behaviour (pressure dependent Drucker-Prager plasticity and temperature dependent viscosity). We simulate far-field extension applied on a simplified crustal scale, synthetic fault system consistent with the structural settings. Three steeply dipping (70°) pre-existing faults are set mechanically weaker than the surrounding bedrock. One fault aligns with Andersonian strike, while two intersecting faults are misoriented by –15° and +30°.

Modeled fault displacements and stress rotations broadly agree with paleoseismic slip rates and with the limited but clear stress rotations observed in geothermal boreholes. Preliminary results provide indicators to explain enhanced crustal permeability and the exceptionally productive Wairakei Geothermal Field : zones of strain localisation where fracturing concentrates; stress ratio reflecting how faults behave kinematically with respect to the applied regional stress field; domains undergoing stress rotations and creating conditions where fractures of various orientations become optimally oriented for slip and dilation, most pronounced in domains within 1 km of the pre-existing faults.

Alternating boundary conditions between orthogonal and oblique rift extension (representing rotational rift opening or nearby magma deflation) further enhances the opening of fractures of different orientations at different times. We also tested the influence of the main faults dip and relative strength on resulting slip and dilation tendencies patterns.

This approach provides new insights into stress evolution in magmatic rifts, with implications for seismic and volcanic hazard assessment and for improving the targeting of permeable zones in geothermal reservoirs.