Paleomagnetism and Paleointensity of the 1.1 Ga Old Diabase Sheets from Central Arizona

Fabio Donadini, Lauri J. Pesonen, Kimmo Korhonen, Alex Deutsch, Stephen S. Harlan

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Eighty-five oriented samples (33 sites) from diabase sheets (ca. 1100 Ma) were collected from
Central Arizona. Detailed rock magnetic, petrological, paleomagnetic, and paleointensity analyses were
performed. The rock magnetic studies, as well as thermal and alternating field demagnetization
treatments of the specimens indicate that the primary natural remanent magnetization (NRM) is carried
mainly by low-Ti magnetite. Other phases like maghemite are also present and are responsible for some
of the secondary components.
In general paleomagnetic measurements isolate two distinct magnetic polarities, which show a
clear asymmetry consistent with previous studies performed in Central Arizona as well as in the Grand
Canyon and in the Lake Superior regions. Two of the sites in the present study also reveal directions that
appear to be consistent with two distinct symmetric events.
The asymmetry, almost entirely in inclination, is of the same sense as the Lake Superior one, but
appears larger with a very steep (upward) reversed inclination and a moderately shallow (downward)
normal one. The normal component has a mean direction of D = 274.8°, I = 38.7° (N = 13 sites, k = 30.4
and α95 = 7.6°). The reversed AR2 component has a mean direction D = 193.7°, I = -78.7° (N = 5 sites, k
= 24.7 and α95 = 15.7°). The angle between the two directions is about 126°, indicating an inclination
offset of about 30°. The present study reveals at one site a steep downwards inclination (site DF, N=1,
D=332.6°, I=69.4°, k=145.5, α95 =3.5°) that is antipodal to the reversed component. At another site
(SD) we also observed a reversed component (N=1, D=95.8°, I=-35.9°, k=60.7, α95 =7.8°) that is
antipodal to the normal component. Although further data showing these trends are required, the current
set suggests that there may have been two symmetric reversal events, separated by a rapid apparent polar
wander. A similar conclusion was drawn recently on a study of the Mamainse Point volcanic sequence
(Swansson-Hysell et al., 2009).
The paleointensity results indicates that the field was weak during the emplacement of the dykes
(normal polarities field: 15.0 ± 5.9 μT, reversed polarities field: 9.5 ± 5.5 μT). Although the paleofield
during the normal period appears higher than during the reversed one, the scatter in the mean values
indicates that the two means are indistinguishable.
Sidor (från-till)3-30
Antal sidor28
StatusPublicerad - 2011
MoE-publikationstypA1 Tidskriftsartikel-refererad


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