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| Detailed Reference Information |
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Tanaka, H., Turner, G.M., Houghton, B.F., Tachibana, T., Kono, M. and McWilliams, M.O. (1996). Palaeomagnetism and chronology of the central Taupo Volcanic Zone, New Zealand. Geophysical Journal International 124(3): 919-934. |
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The central Taupo Volcanic Zone (TVZ) of New Zealand is a region of intense Quaternary silicic volcanism, active since 1.6 Ma. We report palaeomagnetic measurements from 59 distinct volcanic units sampled at 98 sites in the TVZ. These are mainly rhyolitic ignimbrites and lava domes, with a few basaltic, andesitic, and dacitic lavas. Most have new K/Ar or (super 40) Ar/ (super 39) Ar ages. The remanent magnetizations are generally stable to both thermal and alternating-field demagnetization, and well-determined mean palaeodirections were obtained for all sites. Our findings suggest that the Taupo, Whakamaru, Maroa, Reporoa, Rotorua and Okataina volcanic centres were magnetized during the Brunhes normal chron. Kapenga is an older volcanic centre, where activity commenced around 0.89 Ma and extended into the Brunhes. Mangakino volcanic centre is significantly older and was active from 1.6 to 095 Ma. Transitional or intermediate palaeodirections were obtained from Ahuroa ignimbrite (1.18 plus or minus 0.02 Ma) and Mamaku ignimbrite (0.22 plus or minus 0.01 Ma). The former almost certainly corresponds to the Cobb Mountain Event. The latter is significantly older than the Blake Event, and probably corresponds to the recently reported Pringle Falls Summer Lake magnetic episode. Multiple sites from the Whakamaru ignimbrite have indistinguishable (super 40) Ar/ (super 39) Ar ages (0.33 plus or minus 0.01 Ma) and glass composition, but divergent palaeomagnetic directions. This contrast suggests that either (1) the different sites were formed during a phase of extremely violent activity, lasting up to a few hundred years, during which geomagnetic secular variation was recorded; or (2) that they were formed in a single eruption and rotation during subsequent extensional tectonism has caused divergence of the palaeodirections. (super 40) Ar/ (super 39) Ar ages of 0.77 plus or minus 0.03 Ma for the reversely magnetized Rahopeka ignimbrite and 0.71 plus or minus 0.06 Ma for the overlying normally magnetized Waiotapu ignimbrite bracket and constrain the age of the Matuyama-Brunhes transition. |
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Keywords
absolute age; Ar-40/Ar-39; Ar/Ar; argon; Australasia; Cenozoic;, dates; demagnetization; igneous rocks; ignimbrite; isotope ratios;, isotopes; magnetization; New Zealand; noble gases; North Island;, paleomagnetism; pyroclastics; Quaternary; radioactive isotopes;, stable isotopes; Taupo volcanic zone; volcanic rocks; volcanism, 24, Quaternary geology |
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