Rift architecture and Caldera volcanism in the Taupo Volcanic Zone, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The Taupo Volcanic Zone (TVZ) is investigated to determine the interaction of regional structure and volcanism. A three-tiered approach is employed involving (i) analysis of rift geometry and segmentation in Modem TVZ(<300 ka) from remote sensing and digital topographic data; (ii) fault kinematic data collected along the length of TVZ; and (iii) combining new and existing volcanological data for TVZ. Modem TVZ is a NNE-SSW trending intra-arc rift zone, subject to dextral transtension, and characterised by a segmented axial rift zone with a number of offset and variably oriented rift segments. These segments are subject to varying degrees of extension, and a general correlation exists between the amount of extension and the volume and style of volcanism in each segment. Segments with the highest degrees of extension correspond to the Okataina and Taupo Caldera Complexes in the central rhyolitic zone of Modem TVZ, while segments with a higher degree of dextral transtension correspond to the volumetrically-subordinate andesitic extremities. The influence of the structural framework on the shape and formation of calderas in Modem TVZ has been inferred from remote sensing and ground-based structural analysis. Detailed analysis of caldera structure and geometry in Modem TVZ indicates that caldera evolution is largely a function of caldera location relative to the axial rift zone. Calderas peripheral to the rift are simple, single-event structures, while those located within the axial rift zone are multiple-event caldera complexes with geometries dictated by their coincidence with rift faulting. These results show that in Modem TVZ the type, volume, and spatial distribution of magmatic activity is strongly influenced by rift structure and kinematics. The inter-relationship between rift geometry and caldera-complex development is particularly clear at the intra-rift Okataina Caldera Complex (OCC). OCC is located at a step-over in the rift where local rotation of the extension direction accompanies the development of a major transfer zone. Three main collapse events are spatially concentrated in a zone of orthogonal extension within the transfer zone. The 28 x 22 km OCC is elongate parallel to the extension direction, with a complicated topographic margin largely controlled by regional faulting. Major embayments occur on each side of OCC where it is intersected by adjacent rift segments. These are contiguous with two intra-caldera dome complexes forming two overlapping linear vent zones, which transect the caldera complex. The development of volcanism at OCC records the progressive interaction between offset rift segments and the propagation of overlapping rift segment axes. As rift propagation proceeded, a diffuse zone of volcanism progressively concentrated in the centre of the transfer zone then divided into two spatially restricted eruptive centres as through-going faults became established. Field investigations at OCC reveal a major revision to the eruptive stratigraphy that has implications for the development of the caldera and for hazard assessment in northern TVZ. Kawerau Ignimbrite is a partially welded pumice-rich ignimbrite that fills Puhipuhi Basin on the eastern side of the caldera complex and forms a thick terrace in and around the Kawerau township area. Within Puhipuhi Basin it is ~100 m thick, exposed on clear-felled knolls and locally forms jointed bluffs in thickest sections where it is valley ponded. Originally mapped as Kaingaroa Ignimbrite, it was subsequently considered distinct and renamed Kawerau Ignimbrite by Beresford & Cole (2000) with an accepted age of 240 ka. In Puhipuhi basin the Kawerau Ignimbrite overlies both the ~280 ka Matahina and ~65 ka Rotoiti ignimbrites and also the older tephras of the 43-31 ka Mangaone Subgroup. Whole-rock and glass geochemistry tie the ignimbrite specifically to the 33 ka Unit I eruptive phase of the subgroup, vastly increasing the eruptive volume of that unit and implying caldera collapse in this recent phase of OCC activity. Two pumice compositions are identified, reflecting eruption of two distinct magma bodies. Vertical variation in the ignimbrite records rapid depletion of a subordinate dacitic magma such that pumices of this composition are rare beyond proximal exposures. Lithic and pumice size distribution data indicate a source within OCC to the west of Puhipuhi basin. The residual volume of the ignimbrite is <15 km3, but estimates of the original volume approach 50 km3 when intra-caldera volumes are considered. Kawerau Ignimbrite thus represents the largest eruption from OCC in the last 65 ka since the Rotoiti event, and is the youngest partially-welded ignimbrite in TVZ.