In volcanic rift zones, surface faulting from tectonic faults or from dike intrusions can be difficult to discriminate because they have similar geomorphic expression. At the Tongariro graben, near the southern end of the Taupo Rift, New Zealand, crustal extension over the last 350 k.y. has been accommodated by a combination of magma intrusion and tectonic faulting. Normal faults prevail along this 30-km-wide, NNE-oriented graben, with vents of the Tongariro volcanic complex lying parallel to and overlapping the graben axis. This study quantifies the geological extension at the Tongariro graben (7 ± 1.2 mm/yr since 20 ka) and the relative contributions from tectonic faulting and dike intrusion. Field observations were used to interpret fault geometry and activity. To discriminate between tectonic faulting and that associated with dike intrusion (volcano-tectonic), theoretical fault dislocations were modeled from dike intrusion for likely fault-dike spatial relationships and compared to measured displacements. Most of the mapped faults are tectonic in origin. The calculations indicate that the rift-bounding normal faults (National Park and Upper Waikato Stream faults) and the intrarift inward-dipping faults (Waihi and Poutu faults) accommodate 78%–95% (5.8–7.0 mm/yr) of the total extension across the graben (tectonic extension), while dike intrusions could accommodate only 5%–22% (0.4–1.6 mm/yr; magmatic extension), from which 4%–5% is associated with volcanic eruptions and the remainder with deep arrested dikes. These results help to refine the seismic and volcanic hazards of the region and raise questions on the spectrum of volcano-tectonic interactions possible in similar continental rifts worldwide.

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