Normal Polarity Interval Research Articles

Paleomagnetism of the Siberian Flood Basalts of the Noril'sk Area: A Constraint on Eruption Duration

The volcanic sequence of the Noril'sk area, northern Siberia, provides the most complete section of early Siberian flood-basalt volcanism. Paleomagnetic measurements for more than 4000 samples of lava and tuff indicate that nearly all of this >3500-m-thick sequence was laid down during one interval of normal magnetic polarity. Lavas of the lower third of this sequence are cut by the ore-bearing Noril'sk-I intrusion, which has an age of 251 Ma, identical to that of the Permian-Triassic boundary. Thus, the normal-polarity interval represented by this sequence is inferred to be the first of the Triassic Period. Eruption of this enormous volume of material in a relatively brief period coincident with the earth's greatest mass extinction requires that all aspects of Siberian flood-basalt volcanism be evaluated carefully as possibly contributing to that catastrophe.

Hunting for azimuthal anisotropy beneath the Pacific Ocean region

Lateral variations in azimuthal anisotropy cause significant waveform anomalies in long‐period surface waves (ƒ < 15 mHz, T ≳ 70 s), as a result of coupling between fundamental branch Rayleigh and Love waves. These anomalies, termed “quasi‐Love” (QL) waves, have elliptical polarization, arrive slightly behind the Love wave but prior to the Rayleigh wave, and are observed on many propagation paths in the Pacific Ocean region. Our observations of quasi‐Love waves indicate the existence of strong lateral anisotropic gradients in the western Pacific Ocean, in particular, near Hawaii and seaward of the Tonga‐Kermadec, Kurile and Marianas‐Izu‐Bonin subduction zones. A lack of quasi‐Love generation beneath the Phillipine Sea suggests weak azimuthal anisotropy in the region. In the southwest Pacific Ocean, the long‐period quasi‐Love waveforms recorded at station SNZO (South Karori, New Zealand) can be fit well with a simple anisotropic Earth model with a 90° rotation in the orientation of the fast P velocity axis near the boundary between the Indo‐Australian and the Pacific plates. In this model, 6% P wave velocity anisotropy from the Moho to 210 km depth, with a NNE‐SSW fast direction (roughly parallel to the plate boundary), extends 500–1000 km east of the Tonga‐Kermadec trench, where it shifts to a WNW‐ESE orientation (parallel to fracture zones in the southernmost Pacific plate). This solution is nonunique, however, as a similar lateral variation of 2% S wave anisotropy at asthenospheric depths (100–300 km) generates similar waveform anomalies. We attribute the laterally varying anisotropy to either (1) flow variations in the asthenosphere, consistent with lateral shear detected in deep Tonga‐Kermadec seismicity, (2) variations in fossil spreading direction in the Cretaceous long normal polarity interval, (3) the disturbance of fossil anisotropy caused by the passage of the Louisville Ridge hotspot, and (4) lithospheric compression associated with continental collision along the Alpine Fault. The apparent location of the Love‐to‐Rayleigh “scatterer” favors options 1 and 3. Gradients in azimuthal anisotropy are inferred in the northwestern Pacific, associated with one or more of these mechanisms. A quasi‐Love wave observed on a path from Alaska to PPT (Papeete, Tahiti) is bandlimited to ƒ > 8 mHz, suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central Pacific.

Synopsis of paleomagnetic studies in the Kapuskasing structural zone

This paper summarizes results from paleomagnetic studies sponsored by Lithoprobe on the Kapuskasing structural zone (KSZ). Data from Archean rocks outside the KSZ indicate that the Wawa Subprovince has not been significantly rotated or translated (< 5°) relative to the Abitibi Subprovince. Results from the granulites and amphibolites indicate that the KSZ underwent several kilometres of uplift at ca. 2.51 Ga and then 10 ± 5° west-northwest tilt with several kilometres of further uplift between 2.04 and 1.88 Ga from thrust faulting on the Ivanhoe Lake fault zone. Localized chemical remagnetization occurred at 1.1 Ga along the west side of the Shawmere anorthosite. Paleomagnetic data from the 2.45 Ga Matachewan diabase dike swarm indicate that it was emplaced within one reversed to normal polarity interval of less than 5 Ma. Their polarity pattern indicates major north-trending faults with several kilometres of dip-slip displacement. Their remanence confirms that the Superior Province was deformed around the KSZ into an oroclinal flexure with 40° changes in trend between 2.04 and 1.88 Ga. Results from eight 1.1 Ga alkali syenite–carbonatite complexes show that the KSZ and adjacent subprovinces have undergone only minor uplift (< 6 ± 2 km) since emplacement. Also, these data refine the radiometric ages of some complexes, demonstrate that the use of superchrons to correlate Keweenawan units in the Midcontinental Rift is unsound, and show that Keweenawan magnetic field was symmetrical. Many specific conclusions that relate to a given unit or limited area were drawn in the KSZ paleomagnetic studies.

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

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.

Mechanisms of remanent magnetization acquisition in marl and limestone alternations. Case study: Upper Cretaceous (Chron 31–30), Sopelana, Basque Country

The marl and limestone (M/L) alternations of the Cretaceous/Tertiary section in Sopelana in the Basque Country provide a good example of two chemical magnetizations which have been acquired at different times. A detailed study was conducted on a core which contained an unusual paleomagnetic record in the vicinity of Chron 30 R. The magnetic polarity switches far more rapidly than expected from the reversal time scale. A 10 cm sampling interval provided, on average, six samples in each M/L couplet that was identified from a curve of carbonate content. The lithostratigraphic NRM pattern is related to the lithology. The intervals (three to four M/L couplets) along which the contrast in the carbonate content between the limestone beds and the marly layers is low ( < 20%) carry the expected NRM. In contrast, the intervals (two to three M/L couplets) along which there is a large carbonate content contrast ( > 20%) are characterized, in normal polarity intervals, by the expected NRM in the limestone beds and a reverse or blurred NRM in the marly layers. The dominant magnetic carrier in this complex zone is diagenetic hematite, whereas titanomagnetite was identified below this interval. The characteristics of the detrital minerals indicate an erosion of a mature landscape (dominance of weathered ilmenite), probably related to deformation in the Pyrenees. Diagenetic hematite is characterized by two families. The first (microgranular size, only tenths of a micrometre) is present in all the samples and is interpreted as a very early chemical remanent magnetization (CRM) acquired in the oxic zone that probably extended over a few decimetres below the sediment surface. The second is restricted to the marly layers of intervals with large carbonate content contrasts in the M/L couplets. The grains in this family are larger (very fine, some micrometres in size) and carry a reverse or blurred polarity in a normal polarity zone. It is proposed that this chemical remagnetization was acquired during the circulation of oxygenated fluids. The timing for this process is at least 2 m.y. after deposition, and more likely > 7 m.y. The second diagenetic family grew at least 90 m below the seafloor, and its distribution shows that the marly layers in intervals with large contrasts in the carbonate content between the marls and the limestones were preferential drains along which circulation was favoured. This suggests that, although the porosity was still high (40–60%), the diagenetic evolution in the M/L alternations was already being expressed by different physical properties. Limestone beds and marly layers in intervals with low carbonate content contrasts in the M/L couplets were impermeable to fluid circulation and the early diagenetic CRM signal has been preserved. In contrast, marls in intervals with a large carbonate content contrast in the M/L couplets were sites of later diagenesis and their early CRM was overprinted by another magnetization.

The Blake Geomagnetic Polarity Episode recorded in Chinese loess

A geomagnetic anomaly consisting of three clearly defined periods of reverse polarity separated by two short intervals of normal polarity has been documented in a loess section near Xining, western China. On the basis of previous studies of Chinese loess stratigraphy, we correlate this geomagnetic polarity episode with the Blake event. While more research is needed to improve our understanding of the exact process of remanence acquisition of loess deposits, this study shows that loess may provide records of fast geomagnetic events and lends further support to the global nature of the Blake geomagnetic event.

Formula omitted] age constraints on neogene sedimentary beds, Upper Ramparts, half-way Pillar and Canyon village sites, Porcupine river, east-central Alaska

40 Ar 39Ar ages of volcanic rocks are used to provide numerical constraints on the age of middle and upper Miocene sedimentary strata collected along the Porcupine River. Intercalated sedimentary rocks north of latitude 67°10′N in the Porcupine terrane of east-central Alaska contain a rich record of plant fossils. The fossils are valuable indicators of this interior region's paleoclimate during the time of their deposition. Integration of the 40 Ar 39Ar results with paleomagnetic and sedimentological data allows for refinements in estimating the timing of deposition and duration of selected sedimentary intervals. 40 Ar 39Ar plateau age spectra, from whole rock basalt samples, collected along the Upper Ramparts and near Half-way Pillar on the Porcupine River, range from 15.7 ± 0.1 Ma at site 90-6 to 14.4 ± 0.1 Ma at site 90-2. With exception of the youngest basalt flow at site 90-2, all of the samples are of reversed magnetic polarity, and all 40 Ar 39Ar age spectrum results are consistent with the deposition of the entire stratigraphic section during a single interval of reversed magnetic polarity. The youngest flow at site 90-2 was emplaced during an interval of normal polarity. With age, paleomagnetic and sedimentological data, the ages of the Middle Miocene sedimentary rocks between the basalt flows at sites 90-1 and 90-2 can be assigned to an interval within the limits of analytical precision of 15.2 ± 0.1 Ma; thus, the sediments were deposited during the peak of the Middle Miocene thermal maximum. Sediments in the upper parts of sites 90-1 and 90-2 were probably deposited during cooling from the Middle Miocene thermal maximum. 40 Ar 39Ar results of plagioclase and biotite from a single tephra, collected at sites 90-7 and 90-8 along the Canyon Village section of the Porcupine River, indicate an age of 6.57 ± 0.02 Ma for its time of eruption and deposition. These results, together with sedimentological and paleomagnetic data, suggest that all of the Upper Miocene lacustrine sedimentary rocks at these sites were deposited during a single interval of reversed magnetic polarity and may represent a duration of only about 40,000 years. The age of this tephra corresponds with a late late Miocene warm climatic interval. The results from the Upper Ramparts and Half-way Pillar sites are used to estimate a minimum interval of continental flood basalt activity of 1.1–1.5 million years, and to set limits for the timing and duration of Tertiary extensional tectonic activity in the Porcupine terrane. Our data indicate that the oroclinal flexure that formed before the deposition of the basalts at the eastern end of the Brooks Range was created prior to 15.7 ± 0.1 Ma.

Magnetostratigraphy and rock magnetism of the Ilerdian stratotype at Tremp, Spain

The first magnetic polarity stratigraphy of the Ilerdian stratotype locality—the Tremp section, Spain—has been established. In total, 315 samples from 66 levels have been demagnetized, both thermally and by alternating field. Various types of (rock) magnetic behaviors and (low- and high-temperature) magnetization components have been distinguished, and have been correlated with the three sampled rock types: clays, marls and fine-grained sandstones, and coarse-grained sandstones. The paleomagnetic directions have been classified according to their reliability in representing a primary magnetization. The magnetostratigraphy of the Tremp section is characterized by a long reversed polarity zone interrupted by a normal zone. We conclude that the normal polarity interval found in the Ilerdian stratotype may represent Chron 24.2. The lower and upper reversed intervals of the section are then respectively Chrons 24.1R and 24R.

Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments

Bedded pelagic limestones of early to mid Cretaceous age may record cyclic climate variability driven by changes in the Earth's orbit (“Milankovitch cycles”). If properly calibrated, the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy. The lower Cretaceous Maiolica Formation of northern and central Italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper M-series marine anomalies. The magnetic reversals divide measured sections into 0.5–2 Myr increments in which sedimentation rate (assuming continous deposition) can be measured to 15–25% accuracy. Paleomagnetically estimated sedimentation rates over a number of polarity zones establish, with one exception, a narrow range in periods of the carbonate cycles. Carbonate couplets have an estimated mean period of 23.5 kyr, and prominent modulations in bedding thickness (“bundling”) have a mean period of 117 kyr (Kent and Gradstein 1985 time scale). The close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing.Comparison of measured sections from different locations shows that bed-to-bed correlations are possible regionally, cosistent with the Milankovitch hypothesis. Episodes of Barremian “black shale” deposition occur in the troughs of the short eccentricity cycle, as roughly one half of the carbonate-marl couplet, or about 10 kyr duration. It is possible to estimate both the timing (relative to the top of reversed chron M-0) and duration of a period of unusual organic carbon-rich deposition (“Selli Horizon”) in the early part of the Cretaceous Long Normal Magnetic Polarity Interval by tracing the sedimentary cycles up from the Barremian strata.

A Middle Triassic paleomagnetic pole for North America

Two stratigraphic sequences were sampled through the early Anisian Anton Chico Member of the Moenkopi Formation in north-eastern New Mexico. Two polarities of magnetization are present: a normal-polarity interval succeeded by a reversed-polarity interval and followed by a short normal- and reversed-polarity couplet. Detailed thermal demagnetization (10 to 17 steps) was employed to separate magnetic vectors. The secondary magnetization is largely a direction similar to the present-day and/or axial-field directions. Demagnetization above 520°C reveals a near-horizontal characteristic magnetization. The lithology of the stratigraphic sequence is such that the reversed polarity is contained almost exclusively within coarse sandstone lithologies, and hence, the reversed-polarity characteristic magnetization direction is rarely completely separated from the secondary magnetization. Because of this, the paleopole was calculated from only the lower normal-polarity portion of the section. The pole, calculated from the samples of two localities, is located at 121.4°E, 43.2°N, (α 95 = 5.3). This Middle Triassic paleopole is in good agreement with published Triassic paleomagnetic poles for cratonic North America and statistically overlaps the Early Triassic paleopoles. The large-scale relative motion during the Triassic between the magnetic pole and the North American plate is constrained by this study to have begun after early Middle Triassic time; it suggests that as much as 10° of apparent-polar wander occurred between the late Anisian-Ladinian (late Middle Triassic) and the middle to late Carnian (early Late Triassic), a time interval of between 11 and 14 m.y.

The relationship between magnetic field strength and reversal frequency: Preliminary palaeointensity results from the cretaceous quiet zone

It has been predicted that the geomagnetic field strength will be at its highest during periods of low reversal frequency. Using basaltic lavas from Israel and India, which were erupted during the 35 Ma interval of normal polarity in the mid-Cretaceous (the Cretaceous Quiet Zone), we have obtained palaeointensity estimates. The mean virtual dipole moments from the two areas are about 75% of the present value. This suggests that there is no simple relationship between the time averaged strength of the dipole and the frequency of reversals.

Integrated magnetostratigraphy and biostratigraphy of the upper Pliocene-lower Pleistocene from the Monte Singa and Crotone areas in Calabria, Italy

The results of a detailed magnetostratigraphic and biostratigraphic study of late Pliocene to early Pleistocene marine marl sequences from the Monte Singa and Crotone areas in Calabria, Italy are presented. The magnetostratigraphy from the Monte Singa sequence ranges from below the Gauss/Matuyama boundary up to and including the lower Olduvai boundary. Normal polarities at a level corresponding to isotope stage 81 most probably represent the Réunion subchron. From the lower Olduvai boundary upward, a reliable magnetostratigraphy could not be established due to increased weathering of the marls, resulting in mainly secondary magnetizations. The magnetostratigraphy from the composite sequence of the Crotone area belongs to a large part of the Matuyama Chron and includes the Olduvai subchron. The position of the lower and upper boundaries of the Olduvai subzone could be established more precisely than from earlier results. Moreover, the upper boundary of the Olduvai subzone poses an ambiguity: a relatively long normal polarity interval representing the main Olduvai subchron and corresponding to a duration of 115 ka is followed by a short (30 ka) reversed subchron and the short (15 ka) normal Vrica subchron. Another option, and more in accordance with the duration of the Olduvai subchron in literature, would be to consider the complete N-R-N polarity succession with a total duration of 160 ka as representing the Olduvai subchron, implying that this Olduvai subchron has a short reversed interval in its upper part. Linear interpolation and extrapolation yield ages for the most important late Pliocene-early Pleistocene biostratigraphic datum levels. An age of 1.69 Ma is found for the Pliocene-Pleistocene boundary, using the conventional polarity time scale dated with radiometric results. However Hilgen [1], in correlating the sapropel groups and patterns to the precession curve of the Earth's orbit, obtained significantly different ages for the polarity transitions of the present study. According to this astronomically calibrated polarity time scale, the age of the Pliocene-Pleistocene boundary is 1.81 Ma.

Ordovician magnetostratigraphy: Llanvirn-Caradoc limestones of the Baltic platform

SUMMARY Palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the Baltoscandian Ordovician carbonates. After removing a ubiquitous PermoCarboniferous (287 f 14 Ma) remagnetization between 200 and 500 C, a LlanvirnCaradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550-580C. Three reversed (SE, down) and three normal (NW, up) antipodal polarity intervals have been recognized. A primary/early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons. Primary magnetizations are resident in detrital/biogenic or early diagenetically formed single- and pseudo-single domain magnetite phases and subordinate early diagenetic.pigmentary haematite. The recognition of a primary remanence within these well-dated Ordovician carbonates has the following important tectonic and magnetostratigraphic consequences. (1) Accurate time-calibration of the Baltic APW path implies that rapid counterclockwise rotation took place in late Tremadoc and Llandeilo times. The ArenigLlanvirn epochs are characterized by a 'still stand'. Baltica occupied intermediate to high Southerly latitudes during the early Ordovician (Tremadoc-Llanvirn). Systematic northward drift is recognized in post-Llanvirn times. (2) A time-calibrated Ordovician reversal stratigraphy is proposed. Presently available data suggest the geomagnetic field was predominantly reversely polarized during Tremadoc and Arenig times. Two normal polarity zones of short duration are identified within mid-Llanvirn and mid-Llandeilo strata. Discontinuities within the succession may mask other short-period events. Late Llandeilo to mid-Caradoc times were then characterized by a normal polarity field.

Latest pulse of Earth: Evidence for a mid-Cretaceous superplume

A calculation of Earth's ocean crustal budget for the past 150 m.y. reveals a 50% to 75% increase in ocean crust formation rate between 120 and 80 Ma. This in ocean crust production is seen both in spreading-rate increases from ocean ridges and in the age distribution of oceanic plateaus. It is primarily a Pacific Ocean phenomenon with an abrupt onset, and peak production rates occurred between 120 and 100 Ma. The pulse decreased in intensity from 100 to 80 Ma, and at 80 Ma rates dropped significantly. There was a continued decrease from 80 to 30 Ma with a secondary peak near the Cretaceous/Tertiary boundary at 65 Ma. For the past 30 m.y., ocean crust has formed at a nearly steady rate. Because the pulse is seen primarily in Pacific oceanic plateau and ridge production, and coincides with the long Cretaceous interval of normal magnetic polarity, I interpret it as a that originated at about 125 Ma near the core/mantle boundary, rose by convection through the entire mantle, and erupted beneath the mid-Cretaceous Pacific basin. The present-day South Pacific superswell under Tahiti is probably the nearly exhausted remnant of the original upwelling. How this superplume stopped magnetic field reversals for 41 m.y. is a matter of speculation, but it probably involved significant alteration of the temperature structure at the core/mantle boundary and the convective behavior of the outer core.

Paleomagnetic transition records of the Cobb Mountain Event from sediments of the Celebes and Sulu Seas

The Cobb Mountain (CM) event is a short normal polarity interval within the Matuyama Chron just before the Jaramillo Subchron. This event was found in the sediments of Site 767 in the Celebes Sea and Site 768 in the Sulu Sea during ODP Leg 124. Reversal depth and age relation analysis gave ages of 1.11 and 1.10 m.y. ago for the onset and termination of this event. We used discrete samples and high‐density pass‐through measurements to study the behavior of the geomagnetic field crossing this event. Although the two sites are very close to each other, the records of the transitional fields were found to be quite different. They are also very different from another CM event record from the North Atlantic Ocean. Based on the transition data we conclude that the transitional fields for the CM event were non‐axisymmetric. One brief reverse in direction, called rebound, was found within the onset reversal at Site 767. The absence of this feature at Site 768 raises an interesting question concerning the source for rebounds.

Steady Sedimentation and Lithologic Completeness, Bermejo Basin, Argentina

Late Miocene strata of the Bermejo basin, Argentina, (11-7 Ma) yield a magnetic polarity record of unusually high fidelity. Three sections, spaced along-strike over 20 km, are firmly correlated with the magnetic polarity time scale. All three sections demonstrate high accumulation rates (up to 0.88 mm/yr) and include an interval of normal polarity (extending from 7.84 to 7.81 Ma) that has yet to be resolved on the magnetic polarity time scale. The "polarity completeness" demonstrated by Bermejo strata is a result of high accumulation rates as well as environmental factors. Strata were deposited in an arid setting in which unconfined flows were the primary transporters of sediment. Shallow flow depths led to low basal shear stresses, and thus to little potential for erosion; these interpretations are corroborated by field observations. Steady sedimentation in the Bermejo basin is expressed by the low variance short-term accumulation rates have from the average long-term accumulation rate. Uniformity of sedimentation in the Bermejo basin is expressed by identical accumulation histories, even though time-transgressive formational boundaries and texture indicate that the depositional environments were not identical at all locations. These observations suggest that a change in accumulation rate does not necessarily produce a change in depositional environment and, conversely, a change in depositional environment is not necessarily accompanied by a change in accumulation rate.

Fission-track ages of tuff layers related to the Pliocene-Pleistocene boundary on the Boso Peninsula, Japan

Fission-track ages of zircon crystals from four tuff layers in the late Cenozoic sediment sequence of the Boso Peninsula,.Japan, are 1.6 ± 0.2 myr (the Kurotaki Formation), 5.5 ± 0.6 and 5.2 ± 0.5 myr (the uppermost part of the Amatsu Formation), and 11.5 ± 0.8 myr (the middle part of the Amatsu Formation). These ages provide numerical age constraints on magneto-biostratigraphy. The normal polarity interval in the lower part of the Kiwada Formation corresponds to the Olduvai polarity subzone. The boundary between the Pliocene and Pleistocene lies slightly above the Olduvai polarity subzone.

Paleomagnetic results from the Upper Cretaceous Maudlow and Livingston formations, southwest Montana

A paleomagnetic investigation of welded tuff units from the Late Cretaceous Livingston and Maudlow Formations was undertaken to refine the Late Cretaceous to Paleocene APWP for cratonic North America. After progressive AF and thermal demagnetization, the Maudlow Formation (46.1°N, 248.9°E) yields a paleomagnetic pole at 70°N, 208°E (11 sites, A95 = 9°, K = 22). Tectonic correction improves the directional grouping, and the change is statistically significant. Two normal polarity intervals bounding a single reversed polarity interval are present. We interpret the reversed interval to be Chron 33r, on the basis of: [1] stratigraphic correlation of the Maudlow Formation with the Campanian Sedan Formation located 40 km to the east, and [2] K‐Ar dates of 83 ± 2 Ma at the middle of the sampled section and 74.9 ± 1 Ma near the top of the sampled section.The Livingston Formation (45.5°N, 250.1°E) yields a high Hc magnetization with a negative fold test, suggesting a post‐folding secondary magnetization. The Livingston mean direction yields a paleomagnetic pole at 72°N, 218°E (10 sites, A95 = 4°, K = 107) and is clearly distinguishable from a co‐existing lower Hc component reflecting a recent viscous overprint. We interpret the high Hc magnetization as Late Cretaceous to Paleocene in age.Neither the Maudlow Formation pole nor the pole derived from the high Hc overprint in the Livingston Formation agree with previously published poles for either the Late Cretaceous or Tertiary, but they do lie on a simple continuation of the APWP between Late Cretaceous and Paleocene mean poles. Secondary magnetizations found in the Franciscan Complex of California also yield poles which lie on a proposed extension of the APWP. If the Franciscan secondary poles represent an overprint event associated with early Tertiary accretion, and if the Livingston secondary pole represents a deformational event related to Laramide uplift of the nearby Beartooth Range, then together these three poles suggest that the North American APWP may continue farther to the east and south to define a Cretaceous‐Tertiary cusp.

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part II: Early opening of the North Atlantic between the British Isles and Newfoundland

Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque. The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago.

The Paleomagnetic Field from Equatorial Deep-Sea Sediments: Axial Symmetry and Polarity Asymmetry

Paleomagnetic data from 89 equatorial deep-sea sediment cores indicate that the configuration of the time-averaged geomagnetic field depends strongly on polarity state but that it remains within 1 degree of axial symmery throughout the Pliocene and Pleistocene (last 5 million years). The relative magnitude of the nondipole field was greater by almost a factor of 2 during reverse than during normal polarity intervals. These results thus support earlier suggestions that there may be a standing (nonreversing) component of the geomagnetic dynamo.