Peter Riisager

Detecting multidomain magnetic grains in Thellier palaeointensity experiments

The Thellier technique is the most commonly used method for absolute palaeointensity studies. It is based on Thellier’s laws stating that partial thermoremanent magnetisations (pTRMs) are independent and additive, which however only are true for non-interacting single domain (SD) grains. For multidomain (MD) grains, individual pTRMs are not independent and Thellier palaeointensity experiments on rocks with a significant proportion of MD grains may therefore lead to erroneous palaeofield estimates. By introducing a “pTRM-tail check” in the Coe version of the Thellier technique [J. Geophys. Res. 72 (1967) 3247], we present a simple method to detect and discard such rocks. The usefulness of the pTRM-tail check is exemplified by Thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy. Miocene baked sediments from French Massif Central are shown to be ideal for Thellier studies with the remanent magnetisation residing in thermally stable SD grains. For Palaeocene–Eocene basaltic lavas from Faeroe Islands, 64% of otherwise acceptable palaeointensity estimates are discarded due to the failure of pTRM-tail checks indicating the presence of significant concentrations of MD grains.

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.

Paleomagnetic paleolatitude of Early Cretaceous Ontong Java Plateau basalts: implications for Pacific apparent and true polar wander

Abstract We present paleomagnetic data from basaltic pillow and lava flows drilled at four Ocean Drilling Program (ODP) Leg 192 sites through the Early Cretaceous (∼120 Ma) Ontong Java Plateau (OJP). Altogether 270 samples (out of 331) yielded well-defined characteristic remanent magnetization components all of which have negative inclinations, i.e. normal polarity. Dividing data into inclination groups we obtain 5, 7, 14 and 15 independent inclination estimates for the four sites. Statistical analysis suggests that paleosecular variation has been sufficiently sampled and site-mean inclinations therefore represent time-averaged fields. Of particular importance is the finding that all four site-mean inclinations are statistically indistinguishable, strongly supporting indirect seismic observation from the flat-lying sediments blanketing the OJP that the studied basalts have suffered little or no tectonic disturbance since their emplacement. Moreover, the corresponding paleomagnetic paleolatitudes agree excellently with paleomagnetic data from a previous ODP site (Site 807) drilled into the northern portion of the OJP. Two important conclusions can be drawn based on the presented dataset: (i) the Leg 192 combined mean inclination (Inc.=−41.4°, N =41, k =66.0, α 95 =2.6°) is inconsistent with the Early Cretaceous part of the Pacific apparent polar wander path, indicating that previous paleomagnetic poles derived mainly from seamount magnetic anomaly modeling must be used with care; (ii) the Leg 192 paleomagnetic paleolatitude for the central OJP is ∼20° north of the paleogeographic location calculated from Pacific hotspot tracks assuming the hotspots have remained fixed. The difference between paleomagnetic and hotspot calculated paleolatitudes cannot be explained by true polar wander estimates derived from other lithospheric plates and our results are therefore consistent with and extend recent paleomagnetic studies of younger hotspot features in the northern Pacific Ocean that suggest Late Cretaceous to Eocene motion of Pacific hotspots.

Rates of volcanic deposition, facies changes and movements in a dynamic basin: the Nuussuaq Basin, West Greenland, around the C27n-C26r transition

Abstract The initial stages of Palaeogene volcanism in the Nuussuaq Basin in West Greenland were characterized by eruption of basaltic and picritic magmas through sediments of Cretaceous to early Paleocene age into a marine or lowlying coastal environment. Recent magnetostratigraphic work has recognized the C27n-C26r transition (estimated duration less than 10 ka and here assumed to be 5 ka) as a c. 170 m thick zone within a succession of thin picritic lava flows. Multimodel photogrammetry combined with chemical and lithological analysis of the volcanic rocks has allowed detailed 3D analysis of the facies variation within this narrow time window. Subaerial lavas flowed eastwards over a more than 40 km wide front. On northern Disko they covered an existing lava plateau and buried a subaerial landscape of dipping Cretaceous sandstones, while on Nuussuaq they flowed into an up to 700 m deep marine embayment and formed prograding hyaloclastite fans passing into fine-clastic mass flows. With a progradation rate of 0.5–1 m a−1 the palaeogeography of the basin changed considerably during the short time interval. In addition to substantial basin subsidence, the volcanic facies changes have also preserved a record of synvolcanic differential movement of extensional fault blocks. The following parameters are estimated for the volcanism within the Nuussuaq Basin during the C27n-C26r transition: Production rate c. 0.042 km3 a−1, productivity c. 1.2 × 10−3 km3 a−1 km−1 (rift), volcanic aggradation c. 33 m ka−1, subsidence c. 25 m ka−1. If the volcanism evolved continuously at this high aggradation rate, all of the Vaigat Formation could have erupted in 70 ka. However, the complex geological record indicates a much longer total duration, and the volcanism must have had an intermittent character.

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 evidence from Cape Verde Islands basalts for fully reversed excursions in the Brunhes Chron

Abstract In this study we present paleomagnetic data from two lava sequences on Santo Antao, Cape Verde Islands: the Tarrafal and Agua Nova profiles from which 63 and 43 lava flows were sampled, respectively. Previous 40Ar/39Ar ages have constrained the two profiles to the Brunhes Chron, which is in accordance with the normal polarity of the majority of the flows. Some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45° from the geographic North Pole are interpreted to represent geomagnetic excursions. Based on interpretation of the directional data three excursions are recorded in the Tarrafal profile (T-I, T-II, and T-III) and four in the Agua Nova profile (AG-I, AG-II, AG-III, and AG-IV). Both 40Ar/39Ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the Tarrafal profile (T-I) is the same as one recorded in six flows in the Agua Nova profile (AG-I). This excursion, which passes the reversal test with classification C, is the first geomagnetic excursion in the Brunhes Chron to pass the reversal test. The 40Ar/39Ar ages suggest that this excursion could represent chryptochron C1n-1 (0.493–0.504 Ma), the only chryptochron or ‘tiny wiggle’ identified in the marine magnetic anomaly record for the Brunhes Chron. The remaining excursions may not yet be unambiguously correlated between the two profiles. Another excursion recorded by two flows in the top of the Tarrafal profile (T-III) also displays completely reversed polarity.

Modelled palaeolatitudes for the Louisville hot spot and the Ontong Java Plateau

Abstract Formation of the Ontong Java Plateau (OJP), a large igneous province in the western Pacific, has been attributed to a rising plume head in the initial stage of the Louisville hot spot, approximately 120–125 Ma ago. However, the Neal et al. plate reconstruction suggests that the plateau formed approximately 9° north of the current location of this hot spot at 51°S. The magnetization of the plateau’s basement records a palaeolatitude of approximately 25°S which further increases the discrepancy with the plume-head model. Modelling the motion of the Louisville hot spot for the last 120 Ma yields a possible southward motion of up to about 6°. True polar wander (TPW) models also shift the predicted palaeolatitudes of the plateau farther north. Taking into account both hot-spot motion and TPW, formation of the OJP by the Louisville not spot remains a possibility.

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.