Thermoviscous remagnetization in some Appalachian limestones

Experimental evidence shows that blocking temperature“relaxation time theory for single“domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in Devonian limestones of New York State. Thermoviscous remagnetization at moderately elevated burial temperatures during the late Paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks. More generally, these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite“bearing rocks

Further paleomagnetic evidence for oroclinal rotation in the central folded Appalachians from the Bloomsburg and the Mauch Chunk Formations

Renewed paleomagnetic investigations of red beds of the Upper Silurian Bloomsburg and the Lower Carboniferous Mauch Chunk Formations were undertaken with the objective of obtaining evidence regarding the possibility of oroclinal bending as contributing to the arcuate structural trend of the Pennsylvania salient. These formations crop out on both limbs of the salient and earlier, but less definitive paleomagnetic studies on these units indicate that early acquired magnetizations can be recovered. Oriented samples were obtained from nine sites on the southern limb of the salient and eight sites from the northern limb in the Bloomsburg. The natural remanent magnetizations are multivectorial, dominated by a component (B) with a distributed spectrum of unblocking temperatures ranging up to 670°C, and a component (C) with a higher and very discrete distribution of unblocking temperatures. The B component is uniformly of reverse polarity, shows a statistically significant synfolding character, and represents a Late Paleozoic remagnetization. The C component passes fold tests with normal and reverse polarity site means. The C component directions from the southern limb (345.1°/-31.6°) and the northern limb (359.3°/-29.7°) are significantly different in declination (14.2°±10.4°) but not in inclination (1.9°±9°). Samples were also analyzed from seven additional sites in the Mauch Chunk on the southern limb of the salient. Inclusion of these new data gives a revised estimate of the difference between southern and northern limb mean directions of prefolding magnetizations in the Mauch Chunk of 23.3°±12.5 in declination and 4.8°±11° in inclination. Paleomagnetic data from the Bloomsburg, Mauch Chunk, and revised results recently reported for the Upper Devonian Catskill Formation together indicate 22.8°±11.9° of relative rotation, accounting for approximately half the present change in structural trend around the Pennsylvania salient. The oroclinal rotation can be regarded as a tightening of a less arcuate depositional package that developed across a basement reentrant, to achieve a curvature closer to that of the earlier zigzag continental margin outline

Paleomagnetic Evidence for Post-Devonian Displacement of the Avalon Platform (Newfoundland)

The possibility that the Avalon Platform, where the Avalonian lithotectonic belt is best developed, was involved in late Paleozoic displacement was tested by paleomagnetic study of red sandstones of the Upper Devonian Terrenceville Formation of eastern Newfoundland. Two magnetization directions were identified by thermal demagnetization analysis of 60 oriented samples from 10 sites: a high blocking temperature, thermally discrete A component of normal and reversed polarity, and an intermediate blocking temperature, thermally distributed B component of reversed polarity. The B component (D = 185.9°, I = -3.3°, a_95 = 7.2° for N = 8 sites) is interpreted as a postfolding secondary magnetization and gives a paleomagnetic pole position (latitude = 43.6°N, longitude = 117.1°E) near Early to Late Permian paleopoles for North America. The A component (D = 181.6°,I = 28.0°, a_95 = 10.1° for N = 9 sites) is interpreted as the characteristic magnetization possibly dating from near the time of deposition of the Terrence ville Formation. The corresponding paleomagnetic pole position (latitude = 27.4°N, longitude = 123.5°E) falls within a group of Late Devonian-early Carboniferous paleopoles obtained from the Acadia displaced terrain, encompassing the coastal areas of New England and the Canadian maritimes which form another part of the Avalonian belt. These paleopoles are systematically offset by 15° to 20° in latitude from coeval pole positions obtained from cratonic North America. Thus the Avalonian belt of the northern Appalachians, which is thought to represent a remnant of a Precambrian and early Paleozoic microcontinent on tectonostratigraphic considerations, appears to correspond to a late Paleozoic displaced terrain on the basis of paleomagnetic evidence

Volume II: Paleomagnetism and Confirmation of Drift (H.R. Frankel)

This is the story of the formative years – the decade of the 1950s – of paleomagnetism as a scientific discipline in conjunction with a focus on the big questions of the day – the origin of the geomagnetic field, polar wander, continental mobility. The exposition is meticulously documented with referral to primary published literature and enlivened by extensive referral to real-time correspondence and retrospective views based on the author’s interviews and written exchanges with many of the principals dating back to the early 1980s. Some of the themes that emerge from the account is the everimportance of serendipidity and the ability of top scientists to identify tractable aspects of a big problem, adjust the scope and direction of the research as needed, and recognize applications to seemingly oblique problems

Post-depositional Remanent Magnetisation in Deep-sea Sediment

Sediments of various lithologies cored from most parts of the deep ocean floor have been found to contain a record of the past behaviour of the Earth's magnetic field, especially the sequence of reversals over at least the past 5 m.y. (ref. 1). The mechanism by which these sediments acquired their natural remanent magnetism (NRM) is, however, still poorly understood. We have recently conducted experiments that indicate that post-depositional remanent magnetisation (ref. 2) is a viable mechanism of magnetisation of deep-sea sediments

Paleocontinental Setting for the Catskill Delta

Paleomagnetic data confirm geological evidence for tropical paleolatitudes for deposition of the Catskill Delta and related Old Red deposits in Europe. A tectonic model constrained by paleomagnetic data suggests that the Catskill deposits are the product of the Acadian orogeny of the Northern Appalachians as a complex continent-continent collision between Armorica (Hercynian Europe), Laurentia (cratonic North America) and possibly Gondwana, with the Traveler terrane (central New England and New Brunswick) rotated and compressed in between

Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic paleoclimate

Several recent analyses of paleomagnetic data support the concept of Pangea, an assemblage of most of the world‘s continents that was mobile in terms of large-scale internal deformation and with respect to paleolatitude. The main feature of internal deformation involved the transformation from a Pangea B—type configuration in the late Paleozoic, with northwestern South America adjacent to eastern North America, to a more traditional Pangea A—type configuration in the early Mesozoic, with northwestern Africa adjacent to eastern North America. Pangea B thus seems to coincide in time with extensive low-latitude coal deposition and high southern-latitude Gondwana glaciations, whereas Pangea A coincides with generally drier conditions over the continents and no polar ice sheets. Although the configuration of Pangea may have been more stable as an A-type configuration in the Early and Middle Jurassic prior to breakup, the paleomagnetic evidence suggests that there was appreciable latitudinal change of the assembly. Such changing tectonic boundary conditions emphasize the practical importance of age registry of paleoclimate data in making valid comparisons with model results. A simple zonal climate model coupled with the geocentric axial dipole hypothesis for establishing paleolatitudes in precisely controlled paleogeographic reconstructions can explain many of the climate patterns in both the late Paleozoic and the early Mesozoic, but it cannot explain the presence or absence of continental ice sheets

Grain-size-dependent paleointensity results from very recent mid-oceanic ridge basalts

We report paleointensity analyses on a suite of four samples from two axial zero-age mid-oceanic ridge basalt flows from the East Pacific Rise. Paleointensity experiments have been performed on several profiles, each consisting of a batch of small (millimeter scale) subsamples going from the rapidly cooled glassy margin to the interior. The Coe version of the Thellier double-heating procedure was used with back checks performed after every other heating step. Most of the samples show very good behavior, i.e., constant ratio between the lost and acquired magnetization through a large temperature range (quality factor usually above 10) and positive checks, which lead to unambiguous paleointensity determinations. Paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of Earth's magnetic field intensity. However, results found within the crystalline part of the samples show values that are considerably (up to 50%) higher than expected. These variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern. An extensive study of magnetic properties allows us to link this incongruent behavior to (1) the presence of multidomain effects and (2) the cooling rate difference between laboratory experiments and in situ cooling

Multiple remagnetizations of lower Paleozoic limestones from the Taconics of Vermont

The natural remanent magnetizations in some upper Cambrian and middle Ordovician limestones from the Taconics of Vermont (~44°N 287°E) are multicomponent and can be attributed to early Paleozoic, late Paleozoic and Cretaceous-early Tertiary remagnetizations. A northwesterly and up magnetization found at 2 sampling localities, characterized by maximum unblocking temperatures of about 350°C, has directions prior to tilt correction corresponding to Ordovician reference poles for North America and is likely to represent a Taconic remagnetization. A shallow, southerly magnetization found at a third locality, with unblocking temperatures that range to about 500°C, has directions corresponding to a Permian (Kiaman and Alleghenian) remagnetization. Common to all 3 sampling localities is a low (up to 300°C) unblocking temperature overprint similar in direction to Cretaceous to Paleocene reference poles

Viscous Remanent Magnetization in Basalt Samples

Remanent magnetization measurements were made on four specimens of fresh, fine-to coarse-grained basalt from Site 321 and seven specimens of fresh medium-grained diabase at Hole 319A. The natural remanent magnetizations of these rocks were very unstable, characterized by median destructive fields of less than 100 oe in every sample and large directional changes during partial demagnetization. The samples were able to acquire large viscous remanences (VRM) in the laboratory in a 1.0-oe field. Moreover, the intensity of VRM acquired in the presence of the NRM was twice that acquired under similar conditions but after AF demagnetization. Whether acquired from the demagnetized or NRM state, the VRM acquired in 500 hr amounted to a very large fraction of the NRM intensity, particularly at Hole 319A. These results suggest that the large vertical component of magnetization observed in samples from this site may be in part a VRM acquired while drilling the hole in the presence of the drill pipe ambient field