# The early Miocene rotation of the Corso-Sardinian block. New paleomagnetic constraints for the end of the motion

### Details

Serval ID

serval:BIB_5DF4AB26FCFC

Type

**Article**: article from journal or magazin.

Collection

Publications

Fund

Title

The early Miocene rotation of the Corso-Sardinian block. New paleomagnetic constraints for the end of the motion

Journal

Bulletin de la Société Géologique de France

ISSN-L

0037-9409

Publication state

Published

Issued date

2001

Peer-reviewed

Oui

Volume

172

Pages

275-283

Language

english

Notes

ISI:000169935300003

Abstract

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.

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.

Create date

11/12/2012 15:25

Last modification date

03/03/2018 16:37