Hervé Perroud

Paleozoic evolution of the Armorica plate on the basis of paleomagnetic data

New paleomagnetic data support the contention that Armorica and Gondwana formed a coherent block until Ordovician time. However, by Early Devonian time, Armorica collided with the North American–northern European assembly to form the Old Red Continent, and the collision itself was responsible for the Acadian orogeny, not for the earlier Taconic orogeny as postulated previously. The Carboniferous collision of Gondwana with the Old Red Continent subsequently formed Pangaea and produced the Appalachian-Hercynian orogeny.

A paleomagnetic study of Tertiary formations from the Kyrgyz Tien‐Shan and its tectonic implications

As part of a general investigation of Cenozoic deformation in Central Asia, we studied the paleomagnetism of Tertiary red beds and lava flows from intermontane basins in the Tien-Shan region of Kyrgyzstan. We collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them, thermally or with alternating fields. For most sites, there are magnetic components with unblocking temperatures higher than 600°C. We infer that the magnetic carriers are mainly hematite and magnetite. For most localities, the high-temperature component appears to predate tectonic folding. For all localities, mean inclinations are shallower than expected from apparent polar wander paths. Inclination anomalies range from 16°±5° for the Issyk-Kul basin, to 26°±7° for the Fergana basin. If due to changes in latitude, these anomalies imply at least 2000 km of northward displacement of the Tien-Shan during the Tertiary, for which there is no tectonic evidence. We thus consider that the paleomagnetic reference directions cannot be directly compared with our Tertiary data. We explored other possible reasons for this anomaly, which has also been reported from other parts of the Alpine belt, but we could not find a satisfactory explanation. Absolute rotations cannot be accurately determined, because of problems with the reference direction. Nevertheless, the mean declination for the Fergana basin lies counterclockwise by 20°±11° from the mean declination of the Issyk-Kul basin. This result is consistent with the counterclockwise rotation inferred for the Fergana basin from the pattern of Cenozoic faults and folds. It suggests a Cenozoic right-lateral displacement of 110±60 km on the Talas-Fergana fault.

Paleomagnetic evidence for Cenozoic block rotations in the Tadjik depression (Central Asia)

This paper presents results of a paleomagnetic study of Oligo-Miocene red beds of the Tadjik depression in Central Asia. We sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high-temperature components appear to predate folding of upper tertiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the western margin of the depression. However, they are shallower by about 30° than the inclination predicted from the reference apparent polar wander path. This appears to indicate a 23° difference in latitude, which is incompatible with paleogeographic reconstructions for the Tertiary. A sound interpretation of this anomaly would require a better-constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significantly rotated, counterclockwise with respect to those measured on the Turan platform. The eastern part of the depression is a domain of large rotation (52°±13° to 46°±15°), whereas smaller amounts of rotation have occurred in the western part (27°±14° to 14°±15°). The similarity between Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression. Paleomagnetic and structural data suggest that block rotations in the Tadjik depression are associated with indentation of the Pamirs into stable Asia. At a larger scale, observed rotations are compatible with a model of regional sinistral wrenching, along a strip running from the Gulf of Oman to Lake Baikal.

Strain removal applied to paleomagnetic directions in an orogenic belt: the Permian red slates of the Alpes Maritimes, France

Abstract Detailed investigations of relationships between strain and directions of characteristic remanent magnetization (ChRM) have been conducted on the Permian red series of the Alpes Maritimes, France. The analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations: in most sites, the mean direction of the strain shortening axis is close to the expected Permian paleomagnetic direction. In this case we observe a large scatter of the ChRM directions within each site, which is interpreted as an increase of an initial dispersion due to strain. Moreover this dispersion more or less increases with strain intensity. In other cases, where the angle between the shortening axis and the paleomagnetic direction is high, the whole population is deflected towards the cleavage ( XY ) plane. These observations lead us to assume a material line-like behaviour of the ChRM vectors and consequently an attempt has been made to recalculate these directions by strain removal. These calculations give positive results at two levels: (1) a statistically significant improvement in the clustering of the within-site distributions, and (2) after tilt correction, using unstrained bedding planes, a significant improvement in the clustering of the between-site distribution. The formation mean direction, after strain removal, is in good agreement with known Permian results for stable Europe.

Paleomagnetism of Cretaceous red beds from Tadzhikistan and Cenozoic deformation due to India-Eurasia collision

Abstract We have carried out structural and paleomagnetic studies in the Tadzhik depression in order to evaluate the main features of the Alpine tectonics of this area. About 340 cores from 43 sites of Lower Cretaceous red beds were sampled from four different localities in the basin and adjacent ranges. A well-defined component of magnetization (A) of normal polarity with high unblocking temperatures up to 650–670°C was isolated from all the sites. Another component of magnetization (B) with unblocking temperatures between 650 and 680°C was isolated from only fifteen sites; this component is bipolar. The fold test is positive for both components. We believe that component A was acquired during the Cretaceous long interval of normal polarity. Comparison with Eurasian reference data shows significant counterclockwise rotation of a locality close to the Pamir wedge ( R = 51 ± 5°) and another counterclockwise rotation from the inner part of the basin ( R = 15 ± 5°). No significant rotations are observed at the two other localities on the periphery of the Tadzhik basin.

Paleomagnetism of the Silurian volcanism at Almaden, southern Spain

The Silurian volcanism of the Almaden mining district, southern Spain, has been the object of a very detailed paleomagnetic study. The 175 cores (25 sites) sampled all along the eastern termination of the Almaden syncline were subjected to magnetic susceptibility measurements, thermomagnetic cycles, progressive alternating field and thermal demagnetizations as well as microscopic observations. The magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms, including small cubic distributed grains and large elongated crystals. The observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases, showing a complete inversion of the susceptibility tensors. This anisotropy has been related to the emplacement of the flows on a gentle slope dipping ∼ 10° toward the southwest, implying that their volcanic source was located to the northeast. The paleomagnetic analysis has revealed two characteristic components of magnetization: the first (direction D=109°, I=+11°, α95=9°, pole position at 11°S, 69°E, dp=5°, dm=9°) yields a negative fold test and is interpreted as a Late Paleozoic partial overprint; its declination is 46° away of the expected Late Paleozoic field direction, which confirms that rotations are still occurring after the main folding phases of the Variscan orogeny; the second (direction D=62°, I=−36°, α95=14°, pole position at 8°N, 118°E, dp=9°, dm=13°) is prefolding as evident from the significant clustering when applying appropriate tectonic corrections, and we interprete it as the original magnetization of Silurian age. The compilation of all Paleozoic paleomagnetic results from Spain reveals that the Iberian Meseta has been moving along with Gondwana at high drift rates throughout Ordovician and Silurian times.

Secondary magnetizations from the Clinton-type iron ores of the Silurian Red Mountain Formation, Alabama

From 27 (out of a total of 29) sites a characteristic pre-folding magnetization has been obtained with D = 150 °, I= +20 °, ags=3.5 °, and paleopole at 38°N, 132°E. However, we conclude from a bedding-error test and a conglomerate test, as well as from descriptions of the hematite as a replacement mineral, that the magnetization is a (late) post-depositional chemical remanent magnetization. The age constraints on the magnetization, between Middle Silurian and Early Permian, can be refined by a comparison with the apparent polar wander path for cratonic North America; this comparison suggests a Late Carboniferous age for the magnetization and the hematite, which constitutes the principal component of the iron ores. Similar Late Paleozoic remagnetizations have been noted in other Appalachian and mid-continent formations and suggest a widespread, but as yet ill-defined mechanism for the remagnetization. It is tempting to correlate this event with the early phases of the Alleghenian orogeny in Carboniferous times and with possible fluid migrations resulting from the tectonism.

Hydrocarbon seepage dating through chemical remagnetization

Abstract A magnetostratigraphic study of Permian red sandstones from the Van Dyck well, offshore the Netherlands, revealed an anomalous magnetization, with normal polarity and a high inclination (+47°), interpreted as a Jurassic chemical remagnetization. An extensive investigation of the magnetic mineralogy indicated that, in addition to the main carrier (hematite), minor magnetic phases were present (magnetite, maghemite or pyrrhotite). This last phase is unstable with temperature and has a Curie point of 350°C. As an interpretation, late hematite precipitation (200–150 Ma ago) is proposed, following an initial dissolution of the original hematite and precipitation of magnetite and/or other low coercivity minerals, possibly in association with seepage of reducing fluids such as hydrocarbons. This model allows the proposition of a Jurassic age for the end of the hydrocarbon migration, giving new constraints to the origin of the fossil products in this basin.