Jérôme Surmont

New paleomagnetic constraints on the Cenozoic tectonic evolution of the North Arm of Sulawesi, Indonesia

Abstract Extensive paleomagnetic sampling (34 sites) was carried out in Cenozoic formations of the North Arm of Sulawesi (between 120 and 124°E). Consistent results obtained between 120 and 122°E suggest the existence of a unique post-Miocene clockwise rotation of about 20–25° of the western part of the North Arm. This rotation probably occurred during the northward drift of the North Arm along the Palu-Matano sinistral transcurrent fault. Between 122.5 and 124°E, both clockwise and counterclockwise rotations with various amplitudes (from ±6° to 85°) are encountered, and these very likely correspond to individual microblock rotations due to pervasive deformation along shear zones. This result is consistent with the complex fault system of the Gorontalo and Kotamobagu shear zones that have been recognized on radar imagery and by SPOT image analyses.

CHANGES OF THE GEOMAGNETIC FIELD VECTOR OBTAINED FROM LAVA SEQUENCES ON THE ISLAND OF VULCANO (AEOLIAN ISLANDS, SICILY)

Abstract A combined geochronologic ( K Ar ) and palaeomagnetic study has been conducted on five volcanic sections (70 flows) on the island of Vulcano. The sections span the interval from 15 ± 2 to 135 ± 4 ka, with large hiatuses. Rock magnetic investigations indicate that the natural remanent magnetisation (NRM) is carried by titanomagnetites in the four oldest sections, with a contribution from high coercivity magnetic minerals in the youngest section. Palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion, at around 110 ka, with virtual geomagnetic poles (VGPs) that lie over eastern Siberia. These directions may represent a volcanic record of the termination of the Blake event. Only about 15% of the samples (from 19 flows) proved suitable for palaeointensity (Thellier and Thellier) determinations. These measurements indicate that the geomagnetic field has varied at Vulcano between 16 and 53 μT in the explored time interval. When combined with previous results obtained at Mt. Etna for the period from 140 to 60 ka (Tric et al., 1994, Phys. Earth Planet. Inter. , 85: 113–129), these results document a broad low at around 115 ka. The palaeointensity values from Vulcano and Etna are consistent with global values, suggesting the presence of a dominant dipole-field contribution. These values, on the other hand, are significantly smaller than those recently obtained from La Reunion in the southern hemisphere. This suggests that large, long-lived, non-dipolar components of the geomagnetic field may be present at La Reunion in the interval from 130 to 90 ka. These observations are consistent with the fact that the VGP angular dispersion, calculated from non-excursional data, is significantly smaller at Vulcano than at La Reunion.

Late Cainozoic rotation of the Peruvian Western Cordillera and the uplift of the Central Andes

Abstract A palaeomagnetic study was carried out in the region of Lima (Peru), on 55 sites (564 cores) of middle Cretaceous to Late Miocene age, distributed along three northeast-southwest transverses which cross the Western Cordillera of the Central Andes. Over the entire studied area, the results consistently yield westerly declinations corresponding to post-Cretaceous and post-Eocene counterclockwise rotations with respect to stable South America, of 16° ± 6° and 13.7° ± 8.1°, respectively, when compared to the ill-defined reference poles of this continent. These new results are in good agreement with previous ones from the Central Peruvian Andes and support the model recently proposed for the uplift of the Andean Cordillera.

Geomagnetic field intensity between 70 000 and 130 000 years B.P. from a volcanic sequence on La Réunion, Indian Ocean

Abstract A detailed paleointensity study was made of a sequence of 70 successive lava flows of the Piton des Neiges volcano on the island of La Reunion (Indian Ocean). Radiometric dating brackets the age of this sequence between 130 ± 3 ka and 72 ± 3 ka. Rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the Natural Remanent Magnetisation (NRM). Over 350 samples were used for paleointensity determinations carried out with the Thellier method in vacuum or in an argon atmosphere. Of these, 89 samples yielded reliable results, with within-flow scatter often lower than 20%. These results indicate that the geomagnetic field intensity has varied at La Reunion between 13 and 65 μT during the period of time explored. The average value, 42 μT, is higher than the present field at La Reunion (35 μT). The results from the upper part of the section are consistent with previous results obtained for the 82–98 ka period also at La Reunion [1] and document a broad low around 95 ka. not associated with large directional changes. On a larger geographic scale, the paleointensity values from La Reunion are significantly higher than those obtained from Mount Etna [2]. Precise comparison is, however, difficult because of the lack of detail in the Etna results. In the lower part of the section, a marked intensity low, coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the Blake event.