Present Earth's Field Research Articles

A paradigm shift in magnetic data interpretation: Increased value through magnetization inversions

Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.

Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis

ABSTRACTPaleomagnetic sampling and measurement of a boulder accumulation on Little Beecroft Head on the Illawarra coastline of New South Wales was undertaken to evaluate potential emplacement mechanisms. This deposit is of central importance in the Australian Megatsunami Hypothesis (AMH) debate, but to date, there has been no unequivocal determination of its provenance. The most likely emplacement mechanisms are by slow collapse during denudation of overlying strata, storm wave overwash or a combination of these. Characteristic Remanent Magnetisation (ChRM) directions were obtained from 15 individual boulders and the in situ bedrock platform on which they currently rest. The in situ Permian bedrock has a normal polarity mean ChRM direction of D/I = 1.6°/–66.7° (α95 = 5.2°; k = 33.9) that is statistically indistinguishable from the Present Earth Field direction at the site. The magnetisation is most likely due to Cenozoic/recent weathering, which is common in surficial rocks throughout the Sydney Basin. ChRM directions for the boulders are stable but scattered, although not random, and the mean boulder direction is indistinguishable in geographic (i.e. current in situ) coordinates, at the 5% significance level, from the mean direction of the in situ bedrock. Further statistical tests confirm that the scatter in the mean directions of the boulders and the in situ bedrock is different, at the 5% significance level, with the boulder mean being more scattered. At an individual boulder level, some blocks have mean ChRM directions that are statistically indistinguishable from the mean in situ rock ChRM direction, whereas others are distinguishable at the 5% significance level.These results indicate that the boulders were magnetised prior to emplacement but were not moved far from their original positions during emplacement. The emplacement age is constrained to the last ca 780 000 years. These observations strongly support the hypothesis that the Little Beecroft Head boulder deposit was emplaced by a non-catastrophic mechanism, namely slow collapse during denudation of pre-existing cliff material or overtopping from severe storms, which occur regularly on the east coast of New South Wales. Even if a catastrophic wave were responsible, the results constrain the age of that event to be older than 780 000 years. Therefore, the results presented here are not supportive of the AMH as it currently stands. Further paleomagnetic work, on similar deposits along the Illawarra coastline and from elsewhere in Australia, is needed to evaluate the interpretations presented here.

Paleointensity determinations from the Etendeka province, Namibia, support a low‐magnetic field strength leading up to the Cretaceous normal superchron

Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (∼135Ma), which add to the sparse Southern Hemisphere data set. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long-term stability associated with the onset of the Cretaceous Normal Superchron at ∼121 Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetization (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room-temperature susceptibility during the experiments. Following these checks, a total of 64 individual samples from five sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 × 1022 Am2. This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron.

Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan

ABSTRACTWe use temperature-dependent viscous remanent magnetization to estimate the emplace-ment age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The boulders consist of the hermatypic coral Porites , and the time of their deposition by tsuna-mis has been established using radiocarbon dating. Recently deceased corals at reef edges around Ishigaki Island record the Earth’s current magnetic field (present Earth field, PEF) as a remanence parallel to the field in the skeleton. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magne-tization was partially overprinted in the boulder parallel to the PEF. The results of thermal demagnetization indicated that the boulders were rotated at least once, and their emplace-ment ages determined from L. Neel’s relaxation theory for single-domain magnetite agree well with the radiocarbon ages, although there are traces of multidomain magnetites. New applica-tion of Neel’s theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual tsunamigenic coral boulders in this area.

The Apparent Polar Wander Path of the Tarim block (NW China) since the Neoproterozoic and its implications for a long-term Tarim–Australia connection

In order to better understand the kinematic history of the Tarim block after the breakup of the Rodinia supercontinent, paleomagnetic studies have been carried out on Neoproterozoic strata from the Quruqtagh and Yecheng areas in the northeastern and southwestern Tarim block (NW China), respectively. Totally, 547 sedimentary and volcanic samples were collected from 64 sites. Magnetic mineralogical studies show that titanium-poor magnetite and hematite are the principal magnetic remanent carriers. For the Quruqtagh area, the directional analyses reveal that the stable and primary magnetic remanences have been isolated from the ca. 635Ma Tereeken Formation cap carbonate and the 615Ma Zhamoketi basaltic andesite, providing two Neoproterozoic paleomagnetic poles at: λ=27.6°N, φ=140.4°E, dp=8.8°, dm=11.1° for the Tereeken Formation and λ=−4.9°N, φ=146.7°E, dp=3.0°, dm=5.2° for the Zhamoketi formation. Nevertheless, all other Neoproterozoic formations from this area show magnetic directions close to either the Present Earth Field (PEF) or the Late Carboniferous–Early Permian ones. Samples from the Yecheng area yield high anisotropy degree values and near vertical magnetic fabrics indicating the absence of reliable Neoproterozoic magnetic remanence. Combined these two newly obtained paleomagnetic poles with the previously published ones, an Apparent Polar Wander Path (APWP) from the Neoproterozoic to the Present has been built for the Tarim block. The comparison between the APWPs of Tarim and Australia implies a long-term Australia–Tarim connection and/or same kinematic evolution from at least the Neoproterozoic until the Middle Silurian. The Tarim block probably began to break away from northwestern Australia since the Late Ordovician and the final separation occurred at the Late Silurian–Early Devonian.

Testing Continental Drift: Constructing the First Palaeomagnetic Path of Polar Wander (1954)

We describe the discovery that the natural remanent magnetisation (NRM) of certain rock formations in Britain that are Eocene or older have directions that differed significantly from the Earth's present field and from one another. In 1954 the first author, a third year research student in the Department or Geodesy and Geophysics (DG&G) at Cambridge University, observed that the poles corresponding to these old geomagnetic field directions fell consecutively on a path beginning in the Proterozoic in Arizona, swooped across the Pacific Ocean to the coast of eastern Asia and from there northwards to the present north geographic pole by the middle Tertiary. This was the first path of apparent polar wander (APW) based on the NRM of rocks. This path ought to have been reproducible in all continents had they not moved. Thus was born the first successful physical test of Wegener's Theory of continental drift. The work had its origins several years earlier with the construction in the later 1940s of a very sensitive magnetometer by P. M. S. Blackett at Jodrell Bank, a field station of the University of Manchester. In the summer and autumn of 1951, the second author, a recent geology graduate from Cambridge University, showed that Blackett's instrument, which had been made for an entirely different purpose, was well suited to measuring the NRM of rocks (palaeomagnetism). In the following years Blackett-type magnetometers became the means by which the British path of APW was observed. Although Creer's path was based only on nine poles we show that subsequent work fully justifies its use as a starting point for the successful global test of Wegener's theory that was carried out in the following years. Since then, palaeomagnetic poles and the APW paths derived from them have become the basis for constructing the general frame of reference for palaeogeographic maps, for mapping the past distribution of continents, oceans and orogenic belts, the location of ancient climatic zones and many other applications.

A case study of the internal structures of gossans and weathering processes in the Iberian Pyrite Belt using magnetic fabrics and paleomagnetic dating

In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps, i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans. To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci: (1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF) direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities. No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans, the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion.

Paleomagnetism of the Precambrian Eastern Sayan rocks: Implications for the Ediacaran–Early Cambrian paleogeography of the Tuva-Mongolian composite terrane

The Tuva-Mongolian Precambrian composite terrane is located within the complex Central Asian fold belt, which separates the Siberian portion of the Eurasian continent from other incorporated continental blocks. This terrane is one of the key elements that must be accurately modeled to reconstruct the Neoproterozoic–Paleozoic tectonic history of Eurasia. The reconstruction of the terrane's paleoposition after breaking up of supercontinent Rodinia relatively to other continental blocks has fundamental importance, as it is one of the missing blocks in most present day reconstructions. We present a paleomagnetic study of rocks from several formations of the Eastern Sayan region south of Siberian platform (representative location λ = 52.0°N, ϕ = 100.5°E), within the Tuva-Mongolian composite terrane. Sections of siltstone, fine grained sandstone and associated sills were sampled at several localities in this complex region. These units have been dated as Precambrian to Early Cambrian in age. The resulting collection was taken from three formations, representing a total of 33 sites, collected from this previously unsampled and remote region. Generally 6 to 11 samples per site were collected. Stepwise thermal demagnetization was completed using between 10 and 18 heating steps to up to temperatures of 680 °C. Principal component analysis of the stepwise thermal demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may correspond to the present day Earth's magnetic field. The higher unblocking temperature magnetic component (B), was observed in the Dunzhugur Formation (B DF, N = 8 sites) and the Bokson Formation (B BF, N = 11 sites from 5 localities). The B component differs significantly from component A, and is recorded by sites of downward and upward directed magnetic inclinations in the Bokson Formation. Component B DF is most likely a result of the regional remagnetization of the sediments and sills during multiple tectonic events in the area. Component B BF yields a positive fold test and one site has reversed polarity direction. A virtual geomagnetic pole calculated from component B BF, after rotating along a small circle, is coincident with other Ediacaran to Early Cambrian aged poles reported from nearby Mongolia and Siberia. This observation supports the earlier interpretation of Kravchinsky et al. (2001) postulating an adjacent position of the Tuva-Mongolian composite terrane and the Siberian continent in Ediacaran–Early Cambrian times.

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew–Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic host-rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising >10 vol% of the rock. The pyrrhotite-rich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain <10 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1–20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1–2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).

Paleomagnetic and rock magnetic study of the Mesoproterozoic sill, Valaam island, Russian Karelia

A paleomagnetic study of the Mesoproterozoic Valaam monzodioritic sill (U-Pb age ∼1458 Ma) was undertaken in an attempt to provide a new well dated paleomagnetic pole for Baltica and to test the proposed joint drift history of Baltica and Laurentia from 1.83 to ca. 1.26 Ga. Two components of natural remanent magnetization (NRM) were isolated in the sill with alternating field and thermal treatments. The first component is a low coercivity component of viscous origin with a direction similar to that of the Earth's present field at the sampling site. The second one (component A) with a high unblocking temperature/coercivity has a remanent magnetization direction of D = 43.4°, I = −14.3°, α 95 = 3.3°, corresponding to a paleomagnetic pole of Plat. = 13.8°, Plong. = 166.4°, A 95 = 2.4°. Rock magnetic and scanning electron microscopy (SEM) studies revealed that the main carrier of the magnetization A is medium sized primary titanomagnetite, with varying Ti-content. This component is interpreted to be of primary origin and it provides a new well dated (1458 +4/−3 Ma) pole for Baltica. This result supports the idea that Baltica and Laurentia drifted together during the interval from 1.76 to 1.26 Ga forming the core of the supercontinent Hudsonland (Columbia, Nuna).

The Lower Palaeozoic apparent polar wander path for Baltica: palaeomagnetic data from Silurian limestones of Gotland, Sweden

SUMMARY Well-dated and undeformed Silurian (Lower Wenlock) limestones from Gotland, southern Sweden, yield two stable remanence components following detailed thermal and alternating-field demagnetization studies. (1) A low blocking-temperature/coercivity magnetization, termed L, delineated below 250dC/10mT, is oriented parallel to the present Earth's field (Dec. 001, Inc. +67, n= 4 sites, a95= 16d). (2) A higher blocking-temperature/coercivity magnetization, termed H, unblocked between 250-400dC/10-35mT, has a NNE declination and shallow negative inclination (Dec. 025, Inc. -19, n= 5 sites, a95= 5d). This H component direction compares favourably with a previous result from Gotland based upon blanket cleaning. Given a lack of evidence for subsequent geological heating (Conodont Alteration Index = 1-1.5), or pervasive palaeomagnetic overprinting, the H palaeopole is regarded as reliable and primary/early diagenetic in origin (19dS, 352dE, dp/dm 3/5). The only other well-constrained Mid-Silurian pole from Baltica, that from the Ringerike Sandstone of the Oslo district, is in excellent agreement with the Gotland data. These combined poles resolve previous problems regarding the shape and time-calibration of Silurian apparent polar wander relative to Baltica.

Palaeomagnetism of Prince Edward Island

Summary This paper describes the magnetic properties of the bedrock of Prince Edward Island situated in the Gulf of St. Lawrence on the eastern coast of Canada. The intensities and direction of magnetization of 60 samples have been measured. The mean declination of these samples is almost opposite to the declination of the present Earth's field at the location and the mean inclination differs from that of the Earth's field by 66°. The pole position, calculated from these data, is located in the Yellow Sea.

Age and paleomagnetic signature of the Alnø carbonatite complex (NE Sweden): Additional controversy for the Neoproterozoic paleoposition of Baltica

A paleomagnetic investigation of the Alnø carbonatite complex dikes was undertaken in an attempt to refine the apparent polar wander path for Baltica. The currently available paleomagnetic database for the Ediacaran–Cambrian segments of the Baltica APWP is marked by disparate (and often supposedly) coeval poles. This has led to remarkable conclusions about rapid rates of drift or true polar wander. Our study shows that the Alnø Complex (584 ± 7 Ma) is coeval with the Fen Carbonatite Complex (583 ± 15 Ma) via 40Ar/ 39Ar dating of biotite and K-feldspar. We identify three components of remanent magnetization in the Alnø Complex. The first is a low unblocking/coercivity component with a mean declination of 51.2° and inclination of +70.2° ( k = 22, a95 = 8.3°). A paleopole calculated from this direction falls at 62.7°N, 101°E. The high coercivity and unblocking components show a large spread in both declination and inclination. A somewhat artificial grouping of shallow–intermediate vectors (inclinations less than 60°) yields a mean direction with a declination of 108.1° and an inclination of 10.5° ( k = 5.3, a95 = 32.1°). This direction is statistically indistinguishable from that obtained by Piper [Piper, J.D.A., 1981. Magnetic properties of the Alnøn complex. Geol. Foren. Stock. Forhandlingar, 103, 9–15 (Part 1)] and yields a paleomagnetic pole at 3.5°N, 269°E. Conversely, the remaining high unblocking/coercivity components (inclinations > 60°) yields a second grouping with a mean declination of 28° and inclination of 76.8° ( k = 24; a95 = 16.1°; Fig. 5E). This direction is indistinguishable from the present Earth's field at the site. We conclude that the Alnø Complex poles (and indeed many of the Ediacaran poles for Baltica) should be viewed with scepticism when used for paleogeographic/geodynamic models.

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

A paleomagnetic study of subsurface core samples from dolomitized carbonates of two producing reservoirs in the Upper Ordovician Trenton Formation, collected from four wells in southwestern Ontario yielded a paleomagnetic direction of D = 152.3°, I = − 12.3° ( N = 49, α 95 = 8.7). This characteristic remanent magnetization (ChRM) direction was azimuth-corrected by aligning the viscous remanence magnetization (VRM) with the present Earth's magnetic field direction. A drilling-induced magnetization (VRMdi) was present in less than half the specimens sampled in this study. In addition, where the VRM correction could not be made, a paleolatitudinal arc calculated from the inclination-only mean of I = − 9.0° ( N = 34, α 95 = 3.0°) intersected the apparent polar wander path in the Late Permian–Early Triassic. These paleodirections are similar to the paleomagnetic directions observed in Ordovician Trenton carbonates from the Michigan Basin and New York State, U.S.A., suggesting a related regional late Paleozoic remagnetization.

Magnetic minerals in the Martian crust

Using rock magnetism and thermal modeling, we evaluate the candidate minerals responsible for strong magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions of Mars' southern highlands. We assume an early global dynamo field similar in strength to the present Earth's field, enduring about 500 Myr after accretion and core formation, and a basaltic crust containing no more than 4–7 weight% of magnetic minerals. Thermal evolution models with a wide variety of initial crustal thicknesses, distributions of radioactive elements, and thermal expansion coefficients all yield similar thermal histories for the crust: warming in the first ∼1000 Myr (due mainly to radioactive heating) followed by monotonic cooling for the remainder of Mars' history. Primary thermoremanent magnetization (TRM) acquired by intrusive and extrusive bodies during the first 500 Myr was in part thermally demagnetized by general crustal warming after the dynamo field disappeared, from 500 to 1000 Myr. The Curie point isotherms around 1000 Myr established the maximum depth of TRM‐bearing crust. Shock and heating due to impacts demagnetized the uppermost ∼10 km of the crust around the same time, resulting in potential magnetic layer thicknesses of 15–20 km for pyrrhotite, 40–50 km for magnetite, and 50–60 km for hematite. Other magnetic phases, such as iron and finely exsolved low‐Ti titanohematite, are possible but less likely in a basaltic crust under oxidizing conditions. The prime candidates, in order of likelihood, are single‐domain magnetite (0.2–0.4 volume% or 0.4–0.8 weight% required), single‐domain pyrrhotite (1–2 volume% or 2–4 weight%), and either multidomain (&gt;15 μm) or 5–15 μm single‐domain hematite or a mixture of both (1.5–3 volume% or 3–6 weight%). A composite source with different combinations of these minerals at different depths is entirely possible. Viscous decay of TRM is difficult to assess without detailed knowledge of the distribution of minerals and blocking temperatures with depth but would increase the amounts of magnetic material required.

First mid-Neoproterozoic paleomagnetic results from the Tarim Basin (NW China) and their geodynamic implications

In order to improve the understanding of the configuration and breakup history of the Rodinia supercontinent, a paleomagnetic study has been carried out on the 807 ± 12 Ma Aksu dyke swarm (ADS) in Aksu area, northwestern Tarim Block (NW China). The magnetic mineralogical investigations show that the magnetic remanence is principally carried by automorphous titanium-poor magnetite. The measured samples from nine dykes present stable magnetic directions with both normal and reversed magnetic polarities. Because of the monoclinal bedding of overlying sedimentary rocks, no fold test can be provided. However, a positive reversal test is obtained. The magnetic site-mean direction in geographic coordinates is close to, but significantly different from, the present earth field (PEF). A tilt-corrected paleomagnetic pole, therefore, is computed: 19°N, 128°E, dp = 6°, dm = 7° with N = 9. This new paleomagnetic observation reveals that the Tarim Block was located at an intermediate latitude of 43 ± 6°N. Integrating the geochronological studies of dyke swarms from Australia and the Aksu area and referring to the configuration of the Rodinia supercontinent proposed by Moores [Geology 19 (1991) 425] and Li and Powell [Earth Sci. Rev. 53 (2001) 237], the Tarim Block was placed north of Australia with the Aksu dykes being a possible northward continuation of lamprophyre dykes and kimberlite pipes in the northeast part of the Kimberley Craton, Western Australia.

The Guiana and the West African Shield Palaeoproterozoic grouping: new palaeomagnetic data for French Guiana and the Ivory Coast

The aim of this study is to document the Palaeoproterozoic palaeomagnetic database for the Guiana and West African Shields in order to better understand the paleogeographic evolution of these two cratons. A total of 59 sites were sampled (33 in French Guiana and 26 in the Ivory Coast) in granites and metavolcanic rocks. Magnetic, petrographic and palaeomagnetic investigations were carried out on these rocks. Magnetic experiments and petrographic observations show that undeformed magnetite is the main magnetic remanent carrier in granites of French Guiana and both magnetite and haematite in rocks from the Ivory Coast. Both thermal and magnetic alternating-field demagnetizations were applied to the rocks. Four high-temperature magnetic remanent directions were isolated in French Guiana and the Ivory Coast. These directions are distinct from the present Earth's field and to the local Early Jurassic palaeomagnetic components. Reversal and contact tests were obtained for the collection from French Guiana. Based on these arguments and mineralogical investigations, we propose that the magnetic remanence represent a Palaeoproterozoic magnetization. Four virtual palaeomagnetic poles were calculated: GUI1, GUI2 for French Guiana; IC1 and IC2 for the Ivory Coast with their corresponding coordinates: GUI1: GUI1=-62°N, GUI1= 61°E, k= 18, A95= 10°, N= 15 ; GUI2: GUI2=-5°N, GUI2= 50°E, k= 26, A95= 18°, N= 5 ; IC1: IC1=-82°N, IC1= 292°N, k= 28, A95= 13°, N= 6 ; IC2: IC2=-25°N, IC2= 83°E, k= 11, A95= 16°, N= 9 . The magnetization age ranged from 2.04 to 1.97 Ga for the French Guiana poles (40Ar/39Ar) and between 2.10 to 2.00 Ga for the Ivory Coast poles (startigraphic ages). Combining these new palaeomagnetic poles and previously published data, two apparent polar wander paths were proposed for these two shields. The comparison of these two Palaeoproterozoic paths seems to indicate that the two cratons belonged to the same block at about 2.00 Ga but separated prior to 2.02 Ga. Although this hypothesis is supported by geological and tectonic observations in both shields, further palaeomagnetic, geochronological and petrographic constraints are needs between 2.04 and 2.10 Ga.

Tectonic implications of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China

A paleomagnetic study has been carried out in the east of the Tan‐Lu fault, in Liaoning Province, NE China, to understand the timing of Tan‐Lu fault activity. Samples aging from Early Paleozoic to Late Mesozoic from 51 sites have been analyzed. Paleozoic and Late Permian‐Early Triassic rocks are remagnetized by the recent geomagnetic field; however, Late Cretaceous (between 118 and 83 Ma) red tuffaceous sandstone passes a positive fold test, shows no Present Earth Field characteristic remanent magnetization carried by both magnetite and hematite, presents a solo normal polarity, and thus provides the only reliable paleomagnetic data in this study. The paleomagnetic pole calculated from these rocks (λp = 59.4°N, ϕp = 205.5°E, and A95 = 7.3°) is statistically undistinguishable from all available Cretaceous paleomagnetic data from Eastern Liaoning‐Korean Peninsula, indicating that these areas may belong to a single tectonic unit, here named the East Liaoning‐Korea (ELK) Block, at least since the Late Cretaceous. Conversely, a significant discrepancy between the ELK Block and Chinese Block (i.e., North and South China Blocks) characterized by a differential rotation (22.5° ± 10.2°) with a negligible latitudinal displacement (0.8° ± 6.1°) is demonstrated. This result indicates, first, that the left‐lateral displacement along the Tan‐Lu fault, if any, must have occurred before the Late Cretaceous and, second, that the Korean Block can not be considered as a rigid part of North China Block. Sedimentological and structural evidence show that the Meso‐Cenozoic triangle shaped plain, consisting of Songliao, Xialiaohe, Sanjiang, Zeya, and other smaller basins developing in northeast China and southeast Russia, south of the Mongol‐Okhotsk Belt, experienced a heterogeneous rifting from the Late Jurassic to Tertiary. The variable amount of extension, larger in the northeast (∼300 km) than in the southwest (∼80 km), is probably related to the clockwise rotation of the ELK Block centered at the south of the Bohai Bay Basin. This result shows that a significant segment of the East Eurasian margin (more than 1000 km long) experienced differential rotation in Cenozoic times.

Palaeomagnetic and rock magnetic studies on the dykes of Goa, west coast of Indian Precambrian Shield

Abstract Detailed alternating field (AF) and thermal demagnetization techniques on 120 samples belonging to 10 dykes of the Goa region along the west coast of the Indian Precambrian Shield, have yielded characteristic remanent magnetization (ChRM) directions from 9 dykes of which 6 dykes exhibited a reverse magnetic polarity direction (D=150°, I=44.30°, k=58, α95=5.8, N=6 dykes), while 3 dykes exhibited normal polarity D=351°, I=−50°, k=21, α95=14.9, N=3 dykes) direction. The remaining 1 dyke has shown stable direction, but with a strong overprint of the present earth field (PEF) direction, which could not be removed by either AF or thermal cleaning techniques. A Palaeomagnetic pole, computed by averaging the VGPs of 9 dykes is situated at 41.2°N 78.1°W (N=9 dykes; A95=9.81°), which is close to that of the Deccan super pole (36.9°N, 78.8°W), suggesting that the dykes in the Goa region are related to the Deccan basaltic eruptions. Recently published 40 Ar / 39 Ar dating on the normal polarity dykes of the studied area have assigned the ages as 62.8±0.2 Ma. These results demonstrate that the Deccan magnetism continued to affect western India for at least 1–2 Ma after the peak Deccan eruption during the K–T boundary. On the basis of published geological and geochemical information along with our Palaeomagnetic results, these intrusive bodies could be attributed as the feeders for the uppermost formations of Wai subgroup of the Western Ghat Deccan basalts, or to the major geodynamic events such as the opening of the Arabian Sea and the rifting of the Seychelles–Mascarene oceanic plateau, which occurred soon after the cessation of the Deccan volcanism. Rock magnetic experiments such as the Lowrie–Fuller test along with low-temperature magnetic susceptibility and isothermal remanent magnetization (IRM) measurements have identified single domain (SD) type titano-magnetite as the major characteristic remanent magnetization carrying magnetic mineral in the dyke samples.

New Early Cretaceous paleomagnetic results from Qilian orogenic belt and its tectonic implications

Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is l=62.2°N, f =193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous.