Niels Abrahamsen

Early Permian Pangea ‘B’ to Late Permian Pangea ‘A’

The pre-drift Wegenerian model of Pangea is almost universally accepted, but debate exists on its pre-Jurassic configuration since TedIrving introd ucedPangea ‘B’ by placing Gond wana farther to the east by V3000 km with respect to Laurasia on the basis of paleomagnetic data. New paleomagnetic data from radiometrically dated Early Permian volcanic rocks from parts of Adria that are tectonically coherent with Africa (Gondwana), integrated with published coeval data from Gondwana and Laurasia, again only from igneous rocks, fully support a Pangea ‘B’ configuration in the Early Permian. The use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for Pangea ‘B’. The ultimate option to reject Pangea ‘B’ is to abandon the geocentric axial dipole hypothesis by introducing a significant non-dipole (zonal octupole) component in the Late Paleozoic time-averaged geomagnetic field. We demonstrate, however, by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from Gondwana as well as Laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for Pangea ‘B’ in the Early Permian. We therefore regard the paleomagnetic evidence for an Early Permian Pangea ‘B’ as robust. The transformation from Pangea ‘B’ to Pangea ‘A’ took place during the Permian because Late Permian paleomagnetic data allow a Pangea ‘A’ configuration. We therefore review geological evidence from the literature in support of an intra-Pangea dextral megashear system. The transformation occurred after the cooling of the Variscan mega-suture andlasted V20 Myr. In this interval, the Neotethys Ocean opened between India/Arabia and the Cimmerian microcontinents in the east, while widespread lithospheric wrenching and magmatism took place in the west around the Adriatic promontory. The general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right-lateral shear couple between Gondwana and Laurasia during the Permian. Transcurrent plate boundaries associated with the Pangea transformation reactivated Variscan shear zones andwere subsequently exploitedby the opening of western Neotethyan seaways in the Jurassic.

Forested Arctic: Evidence from North Greenland

Shallow-water marine sediments of the Kap Kabenhavn Formation at lat 82°30′ N, Perry Land, North Greenland, contain an abundance of well-preserved remains of terrestrial vegetation and invertebrate fauna indicative of a mosaic of forest tundra environments having similarities to present conditions in Labrador. The presence of foraminifera faunas, marine molluscs, and one mammal species (supported by studies of paleomagnetism and amino acid diagenesis) suggests an age of ∼2 Ma, at the Pliocene-Pleistocene transition. At this time, the arctic tree line was located 2500 km to the north of its present position in the northwest Atlantic region, and forest tundra vegetation existed in lowland areas bordering on an Arctic Ocean without perennial sea-ice cover.

Paleomagnetic evidence for Neogene tectonic rotations in the northern Apennines, Italy

Abstract Paleomagnetic analysis was carried out in the northern Apennines on Eocene to Pliocene Epiligurian units. Five Early Miocene and two Middle Miocene sites yielded dual polarity site-mean directions which show signs of clustering after correction for bedding tilt. These likely primary magnetizations, in conjunction with data from the literature, give an overall mean Late Oligocene–Middle Miocene paleomagnetic pole which shows a large and significant counterclockwise rotation of 52° (±≈8°) with respect to the Africa reference paleopoles (or a similar amount of rotation with respect to the coeval Europe reference paleopole). However, this paleopole falls close to the roughly coeval paleopole for Corsica–Sardinia, which is here calculated by averaging data from the literature. Three additional Early Miocene sites from an area west of Parma affected by Pliocene tectonics yielded site-mean directions which pass the fold test and are rotated counterclockwise by a lesser amount than the rest of the Miocene sites. Most of the remaining sites bear paleomagnetic directions acquired after tilting during a recent phase of remagnetization. We suggest that the large-scale rotation observed in the northern Apennines was associated with the motion of the Corsica–Sardinia block within the general context of the Africa–Europe relative motions. A compilation of published data from the central Apennines also shows a differential rotation of the northern relative to the southern Umbria belt which occurred after the motion of Corsica–Sardinia and may have been due to pivoting of the northern Umbria belt against a deep-seated lineament during the non-rotational opening of the Tyrrhenian Sea.

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.

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 evidence for a Neogene two-phase counterclockwise tectonic rotation in the Northern Apennines (Italy)

Paleomagnetic directions have been determined for a new collection of Early Oligocene and Late Miocene‐Pliocene Epiligurian clastic sediments from the frontal portions of the northern Apennines. These results are combined with Cenozoic data from the literature to evaluate whether rotations of units in this region are related to the OligoMiocene Corsica‐Sardinia rotation and/or to younger phases of deformation of the Apennine chain. When Corsica/Sardinia moved counterclockwise oV the coast of France, the Ligurian units located at its front were presumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the Adria/Africa margin. We propose that about 24 out of a total of 52° of rotation observed in the Epiligurian units can be related to the Oligo-Miocene motion of the Corsica‐Sardinia block, in partial agreement with previous conclusions, and the remaining 28° to the Pliocene tectonic phase at the Apennine chain front, which may have (re)activated thrust planes in the Adria/Africa succession below the Ligurian wedge.

A multi-proxy study of Pliocene sediments from Île de France, North-East Greenland

Abstract A multi-technique approach has been used to study a Pliocene shallow water marine deposit, designated the Ile de France Formation, in North-East Greenland. The sequence is correlated on the basis of 87Sr–86Sr ratios in shells and palaeomagnetic studies with the Gauss normal polarity chron, which is dated to between 2.60 and 3.58 Ma years BP. This dating is in accordance with amino acid epimerisation and evidence from dinoflagellates, foraminifers and molluscs. Sediments, marine molluscs and foraminifers show that the sequence was deposited on the inner shelf, below storm wave base. Seawater temperatures were much higher than today, as demonstrated by the occurrence of a number of southern extra-limital species. The same applies to air temperature, and the few remains of land plants may indicate a forested upland with Picea and Thuja. A number of extinct taxa are present, including Nucula jensenii that is erected as a new species.

Palaeointensity of West Greenland Palaeocene basalts: asymmetric intensity around the C27n–C26r transition

Abstract Palaeomagnetic sampling of the lower part of the Palaeocene flood basalts of West Greenland, covering a composite succession of 1.6 km and close to 350 individual lava flows, revealed two polarity zones and a 200-m zone of intermediate directions. 40 Ar / 39 Ar ages indicate that the transition is C27n–C26r. Following palaeodirectional and rock magnetic analysis, 102 samples from 64 flows from the pre-transitional normal polarity zone, the zone of intermediate directions, and the post-transitional reverse polarity zone were chosen for Thellier palaeointensity experiments. Altogether, 83 samples yielded reliable palaeointensity estimates. The mean intensity of the pre-transitional C27n field is 16.1±6.9 μT (17 flows), which is only twice the mean transitional field strength, 9.9±3.7 μT (10 flows). The post-transitional C26r field intensity is generally high with a mean 43.3±15.0 μT (21 flows). The asymmetric palaeointensity around the C27n–C26r transition with a high post-transitional field resembles the only other detailed absolute palaeointensity studies of geomagnetic transitions recorded in volcanic sequences in Hawaii (∼4 Ma) and Steens Mountain (∼15.5 Ma). These new data, therefore, suggest that a strong post-transitional field might be a systematic feature of geomagnetic reversals, probably indicating a prolonged period of an unusual state of the geodynamo following polarity transitions.

Archaeomagnetic intensity in Denmark

Abstract A palaeointensity study has been carried out on archaeological material from 15 sites in Denmark and one in Norway. The experiments were performed using the Thellier & Thellier double-heating method on 91 samples from the 16 sites. A number of samples have been corrected for anisotropy. All the corrections showed that the anisotropy factor was negligible as the correction factors for the palaeointensity typically was less than 5% with a few exceptions. Values between 50 and 73 μT were found for 13 out of 16 sites, dating from 20 BC to 1980 AD. There appears to be a maximum in the palaeo-field intensity just before 1500 AD in Denmark.

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.