Structural and Hydrothermal Inferences from a Magnetotelluric Survey across Mt. Ruapehu, New Zealand

摘要

This thesis explains the electrical conductivity structure of Mt. Ruapehu. To identifyhydrothermal or volcanic components of the volcano, data from 25 magnetotelluricsites are analyzed.Data collected are first analyzed in the time domain prior to conversion into the frequencydomain. Here, data are remote referenced, and the impedance tensors, tippers,apparent resistivity and phase values are calculated. These components are then analyzedto identify major features within the data. The new phase tensor ellipse methodis applied to identify influential features and determine the dimensionality of data.This analysis indicates where it is appropriate to apply 1 or 2 dimensional inversionschemes.Dimensionality analysis led to 1-D modelling of the determinant impedance at eachsite; and limited 2-D profiles across the Tongariro Volcanic Centre boundaries. Thesemodels are used to create a simple 3-D structural model of the volcano that is thenforward modelled. The results of the 3-D forward modelling indicate that the dominatingfeatures of the volcano's electrical structure have been identified in the previousmodels.Crater Lake is the only possible hydrothermal system on Mt. Ruapehu identifiedin this study. It is also very unlikely that any large coherent bodies of magma existin the near surface. However, a second thin conductor laying somewhere between 10and 30 km deep beneath the eastern flank may contain 13% melt and is the probabledriving heat force beneath the volcano.The structure of Mt. Ruapehu can be split into seven layers.A resistive surface layer (100 ohm m) of young volcanic debris within the TongariroVolcanic Centre that is up to 500 m thick near the crater. A conductive layer (10 - 30 ohm m) of wet, fractured and altered volcanic debrisunderlaying the younger debris throughout the Tongariro Volcanic Centre. A layer of Tertiary sediment under the Tongariro Volcanic Centre that extends tothe south and west. This layer is electrically indistinguishable from the previouslayer and extends to approximately sea level.A resistive layer (400 ohm m), and consistent with greywacke basement covers theentire field area. A second conductive layer (20 ohm m) is identified under the eastern flank of thevolcano somewhere between the depths of 10 and 30 km. This layer is likely tobe the heat and magma source driving the volcanic activity. A surrounding resistive layer extends beyond and below the second conductivelayer mentioned above. This surrounding layer is electrically similar to thegreywacke above. A very high resistivity layer (7000 ohm m) is identified below 80 km deep, and maybe associated with the land/sea boundary or subduction zone to the east.