ISSN:
1365-246X
Quelle:
Blackwell Publishing Journal Backfiles 1879-2005
Thema:
Geologie und Paläontologie
Notizen:
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 40Ar/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 0.95 Ma.Transitional or intermediate palaeodirections were obtained from Ahuroa ignimbrite (1.18 ± 0.02 Ma) and Mamaku ignimbrite (0.22 ± 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 40Ar/39 Ar ages (0.33 ± 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. 40Ar/39Ar ages of 0.77 ± 0.03 Ma for the reversely magnetized Rahopeka ignimbrite and 0.71 ± 0.06 Ma for the overlying normally magnetized Waiotapu ignimbrite bracket and constrain the age of the Maluyama-Brunhes transition.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1111/j.1365-246X.1996.tb05645.x
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