The paleomagnetic investigations carried out in the 70's on
Oligo-Miocene volcanics of Sardinia have demonstrated that the island
was turned by 35-30 degrees clockwise from 33 Ma up to 3-1-20.5 Ma and
rotated counterclockwise in a few million years [De Jong et al., 1969,
1973; Bobier et Coulon, 1970; Coulon et al., 1974; Manzoni, 1974, 1975;
Bellon rr nl.. 1977: Edel et Lortscher, 1977; Edel, 1979, 1980]. Since
then, the end of the rotation fixed at 19 Ma by Montigny er al. [1981]
was the subject of discussions and several studies associating
paleomagnetism and radiometric dating were undertaken [Assorgia er
al., 1994: Vigliotti et Langenheim, 1995: Deino et al., 1997; Gattacceca
rt Deino, 1999]. This is a contribution to this debate that is hampered
by thr important secular variation recorded in the volcanics. The only
way to get our of this problem is to sample series of successive flows
as completely as possible, and to reduce the effect of secular variation
by the calculation of means.
Sampling was performed north of Bonorva in 5 pyroclastic flows that
belong to the upper ignimbritic series SI2 according to Coulon rr nl.
[1974] or LBLS according to Assorgia et al. [1997] (fig. I).
Ar-40/Ar-39 dating of biotites from the debris flow (MDF) has yielded an
age or 18.35 +/- 0.03 Ma [Dubois, 2000]. Five of the investigated
sites are located beneath the debris flow ITV, TVB, TVD, SPM85, SPM86),
one site was cured in the matrix of the debris flow (MDF) and one in 4
metric blocks included in the flow (DFC). Another site was sampled in
the upper ash flow (PDM) that marks the end of the pyroclastic activity,
just before the marine transgression. According to micropaleontological
and radiometric dating this transgression has occurred between 18.35 and
17.6 Ma [Dubois, 2000].
After removal of a soft viscous component, the thermal demagnetization
generally shows a univectorial behaviour of the remanent magnetization
(fig. 2a). The maximum unblocking temperatures of 580-620 degrees (tab.
I) and a rapid saturation below 100 mT (fig. 3) indicate that the
carrier of the characteristic magnetization is magnetite. The exception
comes: from the upper site PDM in which were found two characteristic
components, one with a normal polarity and low unblocking temperatures
up to 350 degreesC and one with a reversed polarity and maximum
unblocking temperatures at 580-600 degreesC of magnetite. After
calculation of a mean direction for each flow, the mean << Al >>
direction 4 degrees /57 degrees (alpha (95) = 13 degrees) computed with
the mean directions for the 5 flows may be considered as weakly affected
by secular variation. But the results require a more careful
examination. The declinations are N to NNW beneath the debris flow. NNW
in the debris flow. and NNE (or SSW) above the debris flow, The
elongated distribution of the directions obtained at sites TVB and TVD.
scattered from the mean direction of TV to the mean direction of MDF is
interpreted as due to partial overprinting during the debris How
volcanic episode, The low temperature component PDMa is likely related
to the alteration seen on thin sections and is also viewed as an
overprint. As NNE/SSW directions occur as well below (mean direction <<
B >> : 5 degrees /58 degrees) as above the debris flow (PDMb : 200
degrees/-58 degrees). the NNW directions (<< C >> : 337 degrees /64
degrees) associated with the debris flow volcanism may be interpreted as
resulting from a magnetic field excursion. According to the polarity
scale of Cande and Kent [1992, 1995] and the radiometric age of MDF,
the directions with normal polarity (TV, TVB, TVD, SPM85. SPM86a. MDF.
DFC) may represent the period 5En. while the directions with reversed
polarity PDMb and SPM86b were likely acquired during the period 5Dr.
Using the mean << Al >> direction, the mean << B >>, or the PDM
direction (tab. I). the deviation in declination with the direction of
stable Europe 6.4 degrees /58.7 degrees (alpha (95) = 8 degrees) for a
selection of 4 middle Tertiary poles by Besse et Courtillot [1991] or
7 degrees /56 degrees (alpha (95) = 3 degrees) for 19 poles listed by
Edel [1980] can be considered as negligible.
Using the results from the uppermost ignimbritic layer of Anglona also
emplaced around 18.3 Ma [Odin rt al.. 1994]. the mean direction << E
>> (3 degrees /51.5 degrees) leads to the same conclusion. On the
contrary, when taking into account all dated results available for the
period 5En (mean direction << D >> 353 degrees /56 degrees for 45 sites)
(tab. II). the deviation 13 degrees is much more significant. As the
rotation of Sardinia started around 21-20.5 Ma. the assumption of a
constant velocity of rotation and the deviations of the Sardinia
directions with respect to the stable Europe direction locate the end of
the motion between 18.3 and 17.2 or 16.7 Ma (fig. 4). During the
interval 18.35-17.5 Ma, the marine transgression took place. At the same
period a NE-SW shortening interpreted as resulting from the collision of
Sardinia with Apulia affected different parts of the island [Letouzey
et al., 1982]. Consequently, the new paleomagnetic results and the
tectono-sedimentary evolution are in favour of an end of the rotation at
17.5-18 Ma.