Janna Riisager

New paleomagnetic pole and magnetostratigraphy of Faroe Islands flood volcanics, North Atlantic igneous province

Abstract A paleomagnetic sampling was carried out along four sections (altogether 86 lava flows, 548 samples) in the North Atlantic Igneous Province outcropping in Faroe Islands, Denmark. The four polarity zones in the 700-m-thick exposed part of the Faroes lower formation can be correlated with the geomagnetic polarity time scale as C26n–C25r–C25n–C24r. The seven lava flows erupted during C25n indicate a very low eruption rate in the upper part of the Faroes lower formation of ∼1/70 kyr. The Faroes middle and upper formations (composite thickness ∼2300 m) are all reversely magnetized corresponding to C24r. The eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field. The shift in eruption rate between the Faroes lower and middle formations and evidence that onset of the Faroes middle formation volcanism took place in C24r are of particular importance, placing onset of middle formation volcanism in close temporal relation to North Atlantic continental break-up and the late Paleocene thermal maximum. After grouping flows recording the same field directions, we obtained 43 independent readings of the paleomagnetic field, yielding a paleomagnetic pole with coordinates 71.4°N, 154.7°E (A95=6.0°, K=14, N=43); age 55–58 Ma. The pole is supported by a positive reversal test. Paleosecular variation, estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21.7°+3.9°/−2.8°, is close to expected for the given age and paleolatitude. Our new Faroes paleomagnetic pole is statistically different from the majority of previously published poles from the British and Faroes igneous provinces, and we suggest that these older data should be used with care.

Paleomagnetism of large igneous provinces: case-study from West Greenland, North Atlantic igneous province

Abstract We present new paleomagnetic and multi-model stereo photogrammetry data from lava sequences in the West Greenland part of the North Atlantic igneous province (NAIP). The joint analyses of paleomagnetic and photogrammetric data yield a well-defined paleomagnetic pole located at Lat=73.6°N, Long=160.5°E ( N =44, α 95 =6.2°, K =13.1; age ∼61–55 Ma), which is statistically indistinguishable from a pole recently obtained for the Eurasian part of the NAIP on Faroe Islands [Riisager et al., Earth Planet. Sci. Lett. 201 (2002) 261–276]. Combining the two datasets we obtain a joint NAIP paleomagnetic pole in Greenland coordinates: Lat=71.1°N, Long=161.1°E ( N =87, α 95 =4.3°, K =13.6; age ∼61–54 Ma). The results presented here represent the first study in which photogrammetry profiles were photographed at the exact same locations where paleomagnetic fieldwork was carried out, and a direct flow-to-flow comparison of the two datasets is possible. Photogrammetry is shown to be particularly useful because of (i) highly precise dip/strike measurements and (ii) detailed ‘field observations’ that can be made in the laboratory. Highly precise determination of the structural attitude of well-exposed Kanisut Mb lava sequences demonstrates that their apparently reliable in-field dip/strike measurements typically are up to ∼6° wrong. Erroneous dip/strike readings are particularly problematic as they offset paleomagnetic poles without affecting their confidence limits. Perhaps more important for large igneous provinces is the recognition of a variable temporal relationship between consecutive lava flows. We demonstrate how correct interpretation of paleosecular variation, facilitated by the detailed photogrammetry analysis, is crucial for the rapidly emplaced Vaigat Formation lavas. Inaccurate tectonic correction, non-averaged paleosecular variation and unrecognized excursional directions may, perhaps, explain why coeval paleomagnetic poles from large igneous provinces are often discordant. The difference between the joint NAIP paleomagnetitc pole and apparent polar wander path poles suggests that they may be less reliable than suggested by their confidence limits.

Thellier palaeointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications

Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4 × 10 22 Am 2 . Modest selection criteria imposed on all published 5–160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.

Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central

Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1°N, 171.2°E (A95 = 8.6°, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9° +6.5°/−3.5°, is close to expected, suggesting that the paleomagnetic pole represents a time-averaged field. Moreover, five new 40Ar/39Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine-grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second “pTRM tail” check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single-domain grains.

Paleomagnetic poles and paleosecular variation of basalts from Paraná Magmatic Province, Brazil: geomagnetic and geodynamic implications

A paleomagnetic study was carried out along two sections (altogether 35 lava flows, 300 samples) in the central Parana Magmatic Province (PMP), Brazil. The two sections, distanced ca. 200 km apart, yield statistically indistinguishable paleomagnetic poles. The combined paleomagnetic pole with coordinates −85.7 ◦ N, 197.9 ◦ E( A95 = 2.6 ◦ , N = 35) is statistically different from previously published paleomagnetic poles for other sections of PMP. We suggest that this angular difference, as well as differences between previously published poles, is caused by undetected local tectonic rotations not easily identified in the often-poorly exposed lavas of the PMP. A joint analysis of all published PMP paleomagnetic data indicate that paleosecular variation, estimated as the angular standard deviation (SF) of the virtual geomagnetic pole distribution, does not support the suggestion of anomalously high secular variation at low latitudes in the 110–195 Ma period [J. Geophys. Res. 96 (1991) 3923]. All SF estimates are, in fact, in better accordance with latitudinal dependence estimates derived from the curve for the 0–5 Ma period. Moreover, we find that all PMP paleomagnetic poles lie ∼10 ◦ away from the pole predicted by an assumed fixed hotspot reconstruction of South America. The PMP paleomagnetic poles, therefore, call for either true polar wander or motion of Indo-Atlantic hotspots.

Long-term variation of geomagnetic field strength: A cautionary note

Earths magnetic field is generated through magnetohydrodynamic processes within the fluid outer core. Several factors that may influence these processes, such as variations in heat flow across the core-mantle boundary, are believed to have varied through geologic time, triggering speculation that Earths magnetic field may have been significantly different in the geological past. Based on painstakingly difficult paleo-intensity experiments, it has, for example, been suggested that Earths dipole moment was much weaker during the Mesozoic, and perhaps related to different mantle convection [Thomas and Biggin, 2003]. The variable quality of published paleointensity estimates makes global compilations difficult, and extreme care must be taken not to interpret problematic data in terms of geodynamic features. In fact, recently available reliable paleo-intensity data suggest that the paleo-strength during early-middle Cretaceous may have been comparable or even higher than the present intensity and not “anomalously” low as suggested in previous studies [Tauxe and Staudigel, 2004]. The identification of any long-term features of virtual dipole moment (VDM) seems to rely heavily on the criteria for accepting paleo-intensity data, which casts serious doubts on the quality and geomagnetic significance of the majority of existing data. In a recent analysis, Thomas and Biggin [2003]—based on so-called self-consistency selection criteria—concluded that the relatively low intensity during the Mesozoic is a real phenomenon.