Palaeomagnetic secular variation recorded by lavas from the Taupo Volcanic Zone, New Zealand
In order to understand the origin, temporal behaviour and spatial characteristics of Earth’s magnetic field, globally distributed records of the palaeomagnetic direction and absolute palaeointensity are required. However a paucity of data from the southern hemisphere significantly limits the resolution of global field models, particularly on short time-scales. In this thesis new, high quality palaeomagnetic data from volcanic materials sampled within the Taupo Volcanic Zone, New Zealand are presented, with a focus on the Tongariro and Okataina Volcanic Centre. New palaeomagnetic directions were obtained from 19 andesitic or rhyolitic lavas, of which 10 also produced successful palaeointensity results. Palaeointensity experiments were conducted using a combination of traditional Thellier-type thermal, and microwave techniques. Detailed magneto-mineralogical investigations carried out alongside these experiments helped to characterise the primary remanence carriers and to justify the reliability of the results. The study also revises the age controls and results from earlier palaeomagnetic studies on Holocene volcanic materials from the area. All new or revised data are summarized into a new data compilation for New Zealand, which includes 24 directions and ten palaeointensities dated between 1886 AD and 15,000 yrs BP. The new directional data reproduces the features of the most recently published continuous record from Lake Mavora (Fiordland, New Zealand), although with directions ranging in their extremes from 321° (west) to 26° (east) declination and -82 to -49° in inclination, the discrete dataset describes somewhat larger amplitude swings. With few exceptions, the new palaeointensity dataset describes a steady increase in the palaeointensity throughout the Holocene, from 37.0 ± 5.7 μT obtained from a pre-8 ka lava to 70.6 ± 4.1 μT from the youngest (≤ 500 yrs BP) flows sampled. A similar trend is also predicted by the latest global field model pfm9k. Furthermore, the data falls within the range of palaeointensity variation suggested by the Mavora record. The dataset roughly agrees with a global VADM reconstruction in the early Holocene (> 5000 yrs BP), but yields values significantly above the global trend in the late Holocene (< 1000 yrs BP) which supports the presence of significant non-dipolar components over the SW Pacific region in the time period, visible in global field models and from continuous PSV records. A comparison of the directional records with the Mavora Curve provided refinement of age estimates of five lava flows from the Tongariro Volcanic Centre, from uncertainties in the range of 2-3000 years. The new palaeomagnetic emplacement age estimates for these flows have age brackets as short as 500 years and thus highlight different phases of the young cone building eruptive activity on Ruapehu volcano.