Geophysical Investigations of the Subsurface Structure of Tongariro Volcanic Centre, New Zealand

Abstract

The subsurface structure around Mt. Ruapehu and along Tama Lakes Saddle in Tongariro Volcanic Centre have been investigated using geophysical methods: particularly magnetotelluric (MT)sounding and gravity. Data from 60 new gravity sites across the Tama Lakes Saddle and surrounding Mt. Ruapehu have been added into the national gravity database to give detailed Bouguer anomaly and Bouguer residual anomaly maps for gravity modelling (2-D and 3-D). The modelling has revealed an uplifted region of high density beneath Mt. Ruapehu and Tama Lakes to the northeast. Topographic and galvanic effects on MT data have been analysed. The topographic effect of the two volcanoes (Mts. Ruapehu and Ngauruhoe) on the observed MT data across Tama Lakes profile is estimated to be small. To obtain the basic electrical structures beneath the Tama Lakes profile 2-dimensional MT inversion has been carried out using high frequency, low frequency and band averaged data. Compared with the high frequency inversion the low frequency inversion gives a slightly better fit to the low frequency ρa but the low frequency phase fits have little improvement. Although the high and low frequency inversions do show differences in structure a continuous conductive layer exists between TL11 and TL12, and a resistive layer is located below 1500 m throughout the Tama Lakes profile. The inversion result from band-averaged data gives a much smoother electrical structure than that from the other frequency inversions. The basic features still suggest the following three points: a near surface layer of high or intermediate resistivity; a conductive layer continuous through the Tama Lakes profile; a thick layer with gradually increasing resistivity beneath the conductive layer. A method for laboratory resistivity measurement of core samples has been developed which eliminates the effect of contact resistance on sample resistivity. Measurement of resistivity samples shows that dry andesites have extremely high resistivities (approximately insulators), dry greywacke ranges from 122,000 to 495,000 Ωm. Wet greywacke saturated with tap water has resistivity values around 400 Ωm. Resistivity values of wet andesites are divided into two groups: one group of porous andesite exhibits resistivities ranging from 500 to 600 Ωm; another of dense andesite ranging from 1200 to 2000 Ωm. Saline water can also substantially reduce the resistivities of the igneous rocks, especially for porous and fractured volcanic rocks. The resistivity values of saline fluid saturated andesite samples (1% salinity) drops 80% to 90% compared with tap water wetted samples. Saline fluid has little effect on dense greywacke samples. Three layers of subsurface structure of Tama Lakes profile were identified from the MT inversion result combined with the information from other geophysical surveys (gravity, magnetics and seismic). The first layer is interpreted as mixed pyroclastics & andesite with thickness varying from 300 - 400 m. The second layer is probably porous and fractured andesite with a thickness about 1300 m around Tama Lakes and 900 m at to the east and west. The third layer is greywacke basement. The conductive layer underneath the Tama Lakes profile is coincident with a high magnetic anomaly - the uplifted region of high density. It is interpreted as the fractured andesite saturated with saline fluid and/or mineralized. Along the Tukino Road profile two layers are inferred from the MT data and the gravity model. The first layer, from the surface to depths of about 1500 m, consists of mixed pyroclastics & andesites. The second layer is composed of two parts possible andesite with a thickness of about 2000 m and greywacke basement. A normal fault in the greywacke basement is found between TU2 and TU3. The structure beneath the Tukino Road is simpler than that beneath the Tama Lakes Saddle. There is no evidence of the existence of a magma chamber at shallow depth (less than 2 km) beneath Tama Lakes. This suggests that there is no connecting magma channel between Mt. Ruapehu and Mt. Ngauruhoe at shallow depth.