Insights Into Faulting and Volcanic-Tectonic Interactions: Case Studies From the Central North Island, New Zealand

Insights into processes of active faulting and volcanic-tectonic interactions in the Taupō- Reporoa Basin, Kaingaroa Plateau and surrounding areas are presented to highlight a region previously thought to be largely inactive tectonically. Located between the Taupō Rift and the North Island Dextral Fault Belt (NIDFB), the Taupō-Reporoa Basin and Kaingaroa Plateau are collectively referred to here as the Central Block. Using a Light Detection and Ranging-derived Digital Elevation Model (DEM) built between 2011 and 2015, linear traces have been identified and inferred as scarps related to possible active faults. This study examines three facets of these DEM-inferred faults through: (a) a remote kinematic analysis of the scarp lineaments, (b) a paleoseismic trenching study of one example accompanied by a Ground Penetrating Radar (GPR) survey, and (c) consideration of volcanic-tectonic interactions related to the Taupō 232 CE eruption.

Remote mapping and kinematic analysis were carried out on the DEM compilation through examination of previously inferred DEM scarps and comparisons with the New Zealand Active Fault Database and GNSS-derived velocity vectors. Comparison of fault strike orientations and velocity vectors corrected for vertical components allowed for small changes in orientation and motion of the faults to be reviewed. Overall, (a) the Central Block faults are orientated ENE (050°) parallel to the Ōkataina and Whakatāne sections of the Taupō Rift and (b) changes in fault strike and oblique motion on the NIDFB occur across a 070° kink line in the axial ranges. Proposed controls of the Central Block faulting include: continuation of the Taupō Rift through biaxial extension, the rift jump, the NIDFB (helping to accommodate margin-parallel motion) and a spatially variable combination of the Taupō Rift and NIDFB motions.

Paleoseismic trenching, ground penetrating radar traverses and fault profile tool applications were carried out on the newly named Parekarangi Fault in the Taupō-Reporoa Basin and demonstrated Holocene movement as recently as 232 CE (during the Taupō eruption). Moment magnitude calculations imply that the fault is capable of an estimated Mw 6-6.5 earthquake, with a mean of Mw 6.37. Between data collected through paleoseismic trenching, ground penetrating radar and fault profile tools (net slip-rate), an estimated average extension rate for the Central Block of 0.06 ± 0.01 mm/yr is calculated.

Lastly, the study presents evidence of volcanic-tectonic interactions on the DEM-inferred faults within the central block, leading to a compilation of evidence for syn-eruption fissuring, slumping and faulting during the Taupō eruption. The examples are interpreted as representing shaking of approximately MMIS 6 and MMIS 7, allowing a rough isoseismal intensity map to be compiled, using the Edgecumbe 1987 ML 6.3 earthquake as a proxy. In addition, the possibility of a secondary control of Central Block faulting is presented, with the interpretation of syn-eruption faulting on Central Block faults, ~50 km from Taupō caldera.