Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy |
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Authors: | Elena Zanella Alex Cicchino Roberto Lanza |
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Institution: | 1.Dipartimento di Scienze della Terra,Università di Torino,Torino,Italy;2.ALP—Alpine Laboratory of Paleomagnetism,Peveragno,Italy;3.Department of Earth Sciences,Durham University,Durham,UK |
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Abstract: | This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic
density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization
(NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra.
The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature
component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the
lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and
the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that
the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling
within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the
base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence.
This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second,
thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results
show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes
and that magnetic fabric data are essential to unravel its complex nature. |
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