Extensional fault kinematics within the Taupo Volcanic Zone, New Zealand: Soft‐linked segmentation of a continental rift system

Abstract

The Taupo Volcanic Zone (TVZ) is a young (<2 Ma) arc-backarc system in continental crust at the southern end of the Havre Trough characterised by intense volcanism and geothermal activity. An extensional fault belt (known locally as the Taupo Fault Belt) can be traced semicontinuously within the TVZ from Ruapehu in the southwest to the Bay of Plenty coast in the northeast. Contained within the belt are numerous active volcanic centres (andesitic, basaltic, rhyolitic) and geothermal systems. Kinematic studies demonstrate that normal faulting predominates within the fault belt, which we interpret as a distributed rift system (here termed the Ruaumoko Rift System), within which rift axes are locally defined by crudely symmetric dispositions of northwest-and southeast-dipping faults. The entire fault belt is partitioned along-strike into offset rift segments defined by association with discrete rift axes. Fault dips, where measurable, are generally steep (8 > 60°). Systematic fracture sets occur in all rock types in the TVZ and generally show a strong northeast-southwest trend, indicating a component of extensional strain distributed throughout the rock mass. Extension is predominantly orthogonal to the rift axes except in "accommodation zones" where interaction between offset rifting segments perturbs the stress field, locally enhancing permeability. The varying trends of extensional structures lie perpendicular to least principal stress trajectories in a locally heterogeneous stress field with normal dip-slip as the predominant fault mechanism.

Tectonics of the Taranaki region, New Zealand: Earthquake focal mechanisms and stress axes

“…A similar change was observed before and after the 1992 eruptions of Crater Peak vent, Mt Spurr, Alaska, from detailed analysis of earthquake focal mechanisms (Roman et al 2004). At Ruapehu this change was attributed to a magmatic intrusion along a northeast-southwest-oriented dike system that resulted in a northwest-southeast compressive stress, the reverse of the northwest-southeast extension normally expected in this area (Rowland & Sibson 2001). At Mt Spurr the stress change was strongly localised and did not extend as far as 10 km from the eruption site (Roman et al 2004), but at Ruapehu stress changes were observed over an area of at least 10 × 10 km (Gerst & Savage 2004).…”

“…These will tend to occur normal to the extension direction (Nakamura 1977), and vent alignment has been used in the southern Taupo Volcanic Zone for estimating the extension direction (Rowland & Sibson 2001). The overall SSE migration of the Taranaki volcanoes since 1.74 Ma is not compatible with a northwest-southeast S^ suggested by the orientation of the CEFZ.…”

The time scales of continental rifting: Implications for global processes

“…Transfer fault zone geometries may change, or be abandoned, during linkage (e.g., Morley and Nelson, 1990;Schlische, 1993;Cartwright et al, 1995;Densmore et al, 2007). The role of magmatism in segment linkage during the spe 500-11 1st pgs page 5 early rifting stages remains poorly understood owing to the general lack of detailed subsurface data (e.g., Ebinger et al, 1989;Rowland and Sibson, 2001), but clear patterns are seen in mature rift zones, as outlined below.…”

Evolution of the southern termination of the Taupo Rift, New Zealand

“…1). The tectonic structure has been defined as the Taupo Rift (Acocella et al 2002;Villamor & Berryman 2006), Taupo Fault Belt (TFB, Berryman & Villamor 1999, or Ruaumoko Rift (Rowland & Sibson 2001). At some latitudes along the Tauo Rift, such as near Ruapehu and to the south of Rotorua, the faults form a welldefined graben structure in the centre of the zone.…”