Secondary Magnetization Components Research Articles

Palaeomagnetic dating of the west Cumbrian hematite deposits and implications for their mode of formation

SUMMARY Palaeomagnetic analysis of six orientated hematite samples from the Florence Mine, Egremont, west Cumbria is described. A total of 21 specimens were subjected to incremental thermal demagnetization and the components of magnetization isolated by principal components analysis. All the specimens displayed a single component of magnetization with no evidence of either multicomponent magnetizations or significant low-temperature secondary magnetic components. The mean direction of the characteristic remanence based on the isolated components has a declination of 203°, and an inclination of −13° (α 95 = 10°, k = 12) with the corresponding pole position at 38.6°S, 333.5°E ( D p = 5.2, D m = 10.3). A single primary component is responsible for the characteristic remanent magnetism (ChRM) directions. The initial susceptibilities and magnetic intensities are compatible with petrographic observations that the samples comprise virtually pure hematite. The ChRM originated as chemical remanent magnetism when the hematite precipitated at temperatures over 100°C. Comparison of the calculated pole position for the west Cumbrian hematite deposits with the apparent polar wander path for Britain south of the Iapetus Suture reveals that the ore deposits formed during the Late Carboniferous and Early Permian. The west Cumbrian hematite deposits therefore are not precipitates of a brine expelled from the Triassic sediments of the East Irish Sea Basin. The hematite may have precipitated from a brine expelled from older sediments (Devonian?), a fluid associated with igneous activity resulting from Permian rifting in the Irish Sea area, or they may be associated with ingress of surface-derived fluids during the Permian.

Chemical remanent magnetization in oceanic sheeted dikes

Marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non‐vertical boundaries between normally and reversely magnetized crust. The acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness. A petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust, ODP (Ocean Drilling Program) Hole 504B, demonstrates that these rocks carry a secondary chemical remanent magnetization (CRM) and not a thermoremanent magnetization (TRM). Primary titanomagnetites have mostly been altered to non‐magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its Curie temperature (∼600°C at ambient pressure) mainly by the alteration of olivine. The natural remanent magnetization (NRM) has been compared with laboratory anhysteretic remanent magnetization (ARM) and TRM. The intensities relate NRM < ARM < TRM, each by a factor of ≈ two. Because previous studies have shown that CRMs are much lower in intensity than TRMs and because of the common presence of secondary magnetite we conclude that the NRMs of the lower Hole 504B dikes are CRMs. The dikes exhibit the same reversed magnetization as the overlying extrusive basalts, and, thus, the timing of CRM acquisition is most likely confined to the time span between the crustal age of Hole 504B and the next younger field reversal, i.e. 0.25 m.y. after emplacement.

Palaeomagnetism and magnetostratigraphy of the Middle Triassic in the Iberian Ranges (Central Spain)

Abstract The study of the structure and origin of remanence in five sections comprising the Rillo Mudstones and Sandstones and Torete Multicoloured Mudstone and sandstone Formations (Middle Triassic, Central Spain) reveals the presence of several components of magnetization of different ages. Detrital hematite and ilmenohaematite carry a primary magnetization of Triassic origin in the form of a DRM or PDRM. In addition, it is likely that an early CRM also contributes to the primary remanence. Goethite and several textural phases of authigenic hematite are responsible for a recent overprint in the form of a late CRM. Thermal demagnetization allowed the isolation of a characteristic Triassic remanence on which a reliable magnetostratigraphy for Anisian-Ladinian times could be based, and correlated with sections of similar age in North America and Asia. However, it did not succeed in entirely removing the CRM associated with the latest component of the overprint which resulted in the primary remanence being biased by residual components of Recent origin. Remanence acquisition was controlled by the sedimentary environment and the diagenetic processes that took place during burial and uplift. The sedimentary environment controlled the conditions of deposition and facies distribution which limited the occurrence of a particular diagenetic phase within the rock volume. Acquisition of secondary components of magnetization, and the consequent pervasive Holocene magnetization (in the form of a late CRM) is primarily related to coarser grained horizons. Sand body interconnectivity has also played an important role in the process. Primary remanence is better preserved in the finer-grained horizons. A PTRM associated with the uplift is easily removed during thermal demagnetization. Calculation of mean vectors by integrating data from adjacent magnetozones allows correction for the overprint bias. The corrected directions can then be closely compared with results from stable Europe.

Application of magnetostratigraphy to Brent Group reservoir zonation in the Visund Field

Abstract Magnetostratigraphy has been applied to an increasing number of oil fields in the North Sea region with promising results for detailed chronostratigraphic reservoir correlation. However, the magnetization processes in hydrocarbon reservoir successions, and the potential use of magnetostratigraphy for reservoir zonation purposes, are not very well known. Therefore, aspects of these subjects are discussed on the basis of a magnetostratigraphic study of three different wells covering the Middle Jurassic Brent Group at the Visund Field, northern North Sea. Results from scanning electron microscopy and remanent coercivity analysis suggest that the primary magnetization defining the magnetozones in the Brent Group is a combination of detrital remanent magnetization and a chemical remanent magnetization, probably acquired contemporaneously shortly after deposition. Detailed thermal demagnetization has uncovered an important secondary magnetization component in most of the core samples. This magnetization is probably due to cooling of the cores from a reservoir temperature of about 110°C to surface temperature, and it causes the main disturbances in the palaeomagnetic results. Based on analyses of individual core samples, characteristic remanent magnetization components have been defined. The magnetopolarity stratigraphy for the individual wells is based on these components and the results are comparable from well to well. The well correlation covers a rock sequence with a thickness of about 200m, and both magnetostratigraphic and biostratigraphic results have been used in combination with the lithostratigraphy. The acme of Corollina represents the most important palyno-event, and the top and bottom of this palynozone is used as the main tie-level for two of the wells. Sixteen additional correlation lines have been derived from the magnetostratigraphic results, six of these considered as tentative. The correlation reveals that the magnetostratigraphic results are facies independent and some of the lithostratigraphic formation lines appear to be diachronous within this deltaic mega-sequence. Although the magnetostratigraphy only provides a relative time framework for the Brent Group, the results are encouraging and suggest the possibility of achieving high resolution chronostratigraphic well correlations for Middle Jurassic reservoir sequences in the northern North Sea area.

Palaeomagnetic investigation of Middle Devonian limestones of Algeria and the Gondwana reconstruction

SUMMARY We have carried out a palaeomagnetic investigation of the upper middle Devonian marly limestone of the Hazzel Matti Formation outcropping at Meredoua in the northern Ahaggar in the Sahara Desert. the Givetian age of the formation is well constrained by palaeontological arguments. the formation has been deformed later than the Moscovian (upper middle Carboniferous). In the altered yellowish samples and in the heterogeneous samples made of mixed fresh and altered zones, several obviously secondary magnetization components are present. In the freshest bluegrey samples a SE down-dipping component can be isolated, particularly when a combined thermal and AF treatment is applied. This component, observed in 53 samples from three sites, passes the fold test of McFadden & Lowes (1981) and McFadden & Jones (1981) at the 95 per cent probability level. the mean direction for the three sites after tilt correction is: Ds= 118.9°, Is, = 34.2°, k = 378, α-95 = 6.3°, which corresponds to a south pole (MER) situated at Op=61.7°, λp= -16.2°, A95= 4.20 and places the site area under a palaeolatitude of -18.8°. However, the best grouping of the directions is achieved after 85 per cent of unfolding using a statistical method based on a bivariate extension of the Fisher's statistics (Legoff 1990; Legoff, Henry & Daly 1992). Such a tilt correction changes the pole position by 1.5°. As the α95 and K curves obtained from stepwise unfolding display a broad minimum and maximum value respectively, and because there was probably slight original syn-sedimentary dips, it is likely that 85 per cent of tilt correction is not significantly different from complete unfolding. The magnetic behaviour of the samples upon thermal and AF demagnetization, the evolution of the initial susceptibility upon thermal treatment, hysteresis cycles and investigation of the magnetic mineralogy through microscope examination and electron microprobe analyses lead to the following conclusion: greigite was probably a primary magnetic mineral formed during the diagenesis of the rock. However, its primary magnetization has been totally replaced by a VRM, except perhaps in rare samples in which the SE component can be found in the narrow range of 300-350°C. Magnetite is thought to carry the SE magnetization component and may have formed early during the diagenesis or soon after greigite. To consider the fold test negative would assign the MER pole a post-Moscovian age and place it in a situation inconsistent with the other poles of the West Gondwanian APWP, unless all the middle Carboniferous and probably lower Permian African poles are rejuvenated. We rather favour the interpretation of a positive fold test, which however does not constrain the magnetization to be of primary Givetian age; the SE component may as well have been acquired any time between the Givetian and at least the late Moscovian. Assuming that the MER pole has a Givetian age implies that the Rheic Ocean, which separated Laurussia from Gondwana in the early Palaeozoic, was almost closed in the middle Devonian, in agreement with palaeobiogeographic arguments; but it increases the discrepancy between the APWP of respectively West and East Gondwana. an intermediate lower Carboniferous age might be in better agreement with both African and Australian data. Clearly, however, more Gondwanian poles are needed to test the reliability of both West and East Gondwanian APWP, and check simultaneously the possibility of relative displacements between these two blocks during the Devono-Carboniferous time period.

Palaeomagnetism of the Ferrar dolerite in the northern Prince Albert Mountains (Victoria Land, Antarctica)

SUMMARY During the sixth Italian expedition to Victoria Land (Antarctica), Jurassic Ferrar dolerite sills were sampled at 23 sites in the northern Prince Albert Mountains. The rock magnetic properties were similar in all sites. Saturation remanence, remanent coercive force and Curie-temperature values pointed to titanomagnetite as the main carrier of magnetization. Thermal and AF demagnetizations were both effective in removing weak, secondary magnetization components below 250 C and 20 mT peak field. Further demagnetization resulted in single-component, stable ChRM. Palaeomagnetic directions were derived after AF demagnetization at 20 mT, and site-mean VGPs computed for all sites but one (Thern A), where systematic changes of the ChRM inclination have resulted in single-sample VGPs dispersed astride the equator. The site-mean VGPs distribution is clearly bimodal: 15 VGPs cluster around a palaeopole (lat. 48S, long. 226E) in good agreement with the literature data, whereas seven VGPs are displaced toward a palaeopole at higher latitude (lat. 79S, long. 181E). All VGPs can be regarded as Jurassic, because no evidence of substantial secondary components was found, and experimental data showed that the PTVRM (partial thermal viscous remanent magnetization) acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous VGPs. The Thern A and the southward displaced VGPs plot along a great circle through the East Antarctica Jurassic palaeopole. As a high reversal rate in the Jurassic is well documented by magnetostratigraphy, and the reversal VGP path is often confined to a narrow longitudinal band, we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous VGPs were intruded during transitional phases of the geomagnetic field. Similar anomalous VGPs previously reported from the Ferrar Supergroup rocks of Northern Victoria Land also plot along the same palaeomeridian, which is only 30 away from the palaeomeridian derived from the Jurassic intrusives of the Dufek Massif in the Transantarctic Mountains (Burmester & Sheriff 1980). The Jurassic palaeopole of East Antarctica has been recalculated from 103 VGPs available in the literature. The position is 57S, 221E when all VGPs are taken into account, and 51S, 223E when the high-latitude VGPs from Victoria Land are discarded as anomalous.

Paleomagnetism of the late Paleozoic Slide Mountain terrane, northern and central British Columbia

Paleomagnetic samples were collected from Mid-Pennsylvanian to Lower Permian red argillaceous cherts at two localities of the Slide Mountain terrane: 18 sites from the Sylvester allochthon in northern British Columbia and 11 sites from Sliding Mountain in central British Columbia. A secondary component of natural remanent magnetization in the Sylvester samples yields a paleomagnetic pole that can be brought into coincidence with the Jurassic portion of the North American apparent polar wander path by inferring vertical-axis rotation during obduction of the allochthon. Both localities yield a characteristic component (ChRM) with unblocking temperatures from 650 to 680 °C. After structural correction for bedding tilt, all inclinations of ChRM are negative, consistent with magnetization during a reversed-polarity interval in the northern hemisphere. Site-mean ChRM directions show consistent inclinations but distinct stratigraphic groupings of declinations. Inclination-only statistics indicate that the ChRM passes a tilt test within the Sylvester allochthon and regionally between the two localities. The ChRM was apparently acquired prior to structural imbrication within the Sylvester section and regional differential tilting. We interpret the ChRM to be a primary magnetization acquired at or soon after deposition during the Permo-Carboniferous reversed-polarity superchron. The mean ChRM inclination of −16.7° ± 6.0° from the Sylvester allochthon indicates a paleo-latitude of 8.8° ± 3.4°N, which is corroborated by a paleolatitude of 1.9° ± 1.5°N from the Sliding Mountain locality. When compared with expected Pennsylvanian–Permian paleolatitudes, a net poleward translation of 20.3 ± 3.7° is implied for at least the sampled lithotectonic component of the Sylvester allochthon.

Palaeomagnetic study and magnetostratigraphy of the Triassic Skagerrak Formation, Crawford Field, UK North Sea

Abstract A detailed palaeomagnetic study has been made of the Skagerrak Formation (Triassic) in two wells from the Crawford Field in the northern North Sea. In 9/28a-9 (unorientated) a well-defined magnetostratigraphy is present comprising up to four reversed and three normal zones. There are secondary magnetizations (Tertiary 8 Recent in age) particularly in areas which are oil stained. In 9/28a-13 (orientated), three normal and four reverse magnetozones can be isolated. Despite the existence of orientation data it is more difficult to establish a reliable Triassic magnetostratigraphy. This is partly due to the fact that the sandier sequence in this location is impregnated with oil and associated diagenetic effects have resulted in pervasive magnetic overprints which are difficult to separate from the original Triassic magnetization. The effects of oil emplacement included widespread dissolution and the precipitation of a suite of secondary iron minerals including pyrite and possible pyrrhotite or magnetite. These later phases carry a secondary component of magnetization which overprints the original Triassic characteristic remanence. Directions of secondary magnetization are consistent with oil emplacement in Tertiary times. In attempting to correlate the two wells a number of factors have to be taken into account, in particular variations in sedimentation rate and the lack of precision in the published Triassic polarity time scale. The results of this study strongly suggest that the cored sequences in the two wells are not time equivalent. However, the sequence in 9/28a-9 compares most closely with the Upper Triassic polarity time scale.

The magnetostratigraphy of the Chitarwata and lower Vihowa formations of the Dera Ghazi Khan area, Pakistan

Three sections of the Chitarwata and lower Vihowa formations were sampled along the Dalana River on the southeastern flank of the Zinda Pir Dome in the southern Sulaiman Range near Dera Ghazi Khan, Pakistan. Together they provide nearly 800 m of a continuous sedimentary record of the Miocene derived from the uplifted Himalayan highlands. Previous studies have examined the Middle and Upper Miocene sediments, the Siwalik Group, of the Potwar Plateau to the north. However, detailed investigations of earlier periods are impossible in that area due to the absence of Oligocene and Lower Miocene sediments caused by continued overthrusting associated with the Himalayan orogeny. Fortunately, the Sulaiman basin to the south, which was further removed from the tectonic activity, provides a record of the Early Miocene in the form of the Chitarwata Formation. The Dalana A, B, and C sections (DGA, DGB, and DGC) were examined and sampled for this study. A magnetostratigraphic analysis was carried out to correlate and date the Chitarwata and Vihowa formations exposed in this area. The samples were subjected to step-wise demagnetization to resolve the primary and secondary remanent magnetization components. Thermal demagnetization trajectories indicate that the majority of samples have sufficient internal consistency that their calculated polarities are reliable. Likewise, the majority of the 126 measured sites showed statistically significant agreement between the three measured samples per site. However, the bimodal data set does not pass the reversal test and so is deemed unsuitable for tectonic interpretations. The individual sections were initially correlated using lithologic and stratigraphic methods, and the relationship was reinforced by the magnetostratigraphy. The composite DG section was tentatively correlated with the standard magnetic polarity time scale placing the disconformable base of the Chitarwata in the DGA section at just older than 22 Ma, the Chitarwata/Vihowa contact at 18.6 Ma, and the top of the Vihowa in the DGC section at about 16 Ma. The assignment of these dates to the Chitarwata will aid in future biostratigraphic and lithostratigraphic correlation of Early Miocene sediments, effectively extending the well-established Siwalik faunal sequence back by four million years.

Paleomagnetic results in support of a model for the origin of the Asturian arc

Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.

A paleomagnetic investigation of the Minturn Formation, Colorado: A study in establishing the timing of remanence acquisition

One hundred sixty-five oriented hand samples were collected from 55 sites in the Minturn Formation located between Wellsville and Howard, Colorado, in an attempt to locate the base of the Permo-Carboniferous Reversed Superchron (PCRS) in western North America. After the samples were demagnetized using thermal and alternating field cleaning techniques, the normal polarity magnetizations (which marks the base of the Permo-Carboniferous Reversed Superchron (PCRS)) were not found in the Minturn Formation. Reversed polarities were consistent throughout the section studied. A conglomerate test was conducted on syndepositional rip-up clasts taken from the formation. The test indicates that a post-depositional, secondary component of magnetization has been acquired by the clasts and remains stable up to temperatures of 610–640°C. However, the high temperature components for three of the four test sites show random directions indicating that a primary remanent magnetization was acquired shortly after deposition. The conglomerate test demonstrates that a secondary Permian direction can be successfully isolated from a primary Pennsylvanian one revealed at blocking temperatures above 650°C. In this study, the two components were isolated using step-wise thermal demagnetization. The results of the study indicate that the base of the PCR superchron lies stratigraphically below the Minturn Formation, probably in Early Atokan or Morrowan age sediments.

Palaeomagnetic location of the Jaramillo Subchron and the Matuyama-Brunhes transition in the Castlecliffian stratotype section, Wanganui Basin, New Zealand

Magnetostratigraphy is described for the lower part of the Castlecliff section in Wanganui Basin, New Zealand, based on analysis of multiple samples from 37 horizons. The Castlecliff section is the stratotype of the middle Pleistocene Castlecliffian Stage and comprises eleven unconformity-bound sequences which accumulated at inner shelf depths, during interglacials. A range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition, in the stable single domain or pseudo single domain size range. However, at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral, chiefly within the superparamagnetic-single domain size range, resulting in hard secondary components of magnetization. Nevertheless, at most sites the primary component of remanence was clearly identified by stepwise demagnetization. Four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints. The section sampled spans the interval from below the Jaramillo Subchron to above the Matuyama-Brunhes boundary. The Matuyama-Brunhes transition extends over 10.5 m of section within the Upper Westmere Siltstone. Virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path. These represent the first published data on the occurrence of the Matuyama-Brunhes boundary on-land in New Zealand.

New paleomagnetic results from northern China: collision and suturing with Siberia and Kazakhstan

We report new results from our paleomagnetic studies of the Inner Mongolia autonomous region in northeastern China and the Xinjiang autonomous region in northwestern China. Primary magnetizations are isolated in Tertiary, Cretaceous, Late Jurassic and Late Permian rocks and secondary components of magnetization of Late Jurassic and Recent ages are seen in strata of Inner Mongolia. Western Junggar Basin and part of the Tarim block, suffered widespread complete tectono-thermal remagnetization at about 280 Ma (Permian). The paleomagnetic evidence suggests that the sampled areas in Inner Mongolia have been part of the North China Block at least since Late Permian time and the Junggar block collided with the Tarim block by Late Permian time. Our own paleomagnetic results together with available data from other parts of eastern Eurasia, suggest a tectonic model in which Kazakhstan, Europe and Siberia collided with each other and formed the Europe-Siberia-Kazakhstan plate (EUR-SIB-KAZ) in Late Permian time. To the east, Mongolia (= Outer Mongolia), Inner Mongolia and the North China Block were also wedged together forming the Mongolia-North China plate (MON-NC) by this time. These two composite continental plates were not a stable unit in the Late Permian. The geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the Late Permian near the western end (50°N, 75°E) of the Southern Siberian Fold Belt and progressed eastward as the MON-NC plate rotated and clockwise about 117° relative to the EUR-SIB-KAZ plate during the Early Mesozoic. Suturing and rotation were probably complete by the Late Jurassic. Significant relative motion between the Tarim and Siberia blocks continued from Early Triassic to Late Jurassic, as Tarim moved northeastward 1400 km relative to Siberia.

The Ras' Gharib dykes, Red Sea Hills (Egypt): an uplift origin of the secondary magnetic components?

Paleomagnetic results from post-⪢ Younger Granite⪢ mafic dykes west of Ras' Gharib (north Eastern Desert, Egypt) record complex secondary magnetizations. A sequence of poles calculated for the five different components isolated is as follows: C n = 4 sites λ' = 21.8° S φ' = 342.3° E A 95 = 18.1° D n = 11 samples λ' = 11.3° S φ' = 4.3° E a 95 = 11.6° E n = 3 sites λ' = 41.7°5 φ' = 69.8° E A 95 = 21.7° B n = 6 sites λ' = 55.5°5 φ' = 125.6° E A 95 = 10.3° A n = 6 sites λ' = 75.2° S φ' = 158.1° E A 95 = 6.7° Initially, it was expected that because of their stratigraphic age (latest Precambrian-early Paleozoic), the dykes should record magnetizations associated with the time of emplacement or the latest stages of Pan-African tectonothermal activity. This has not been the case, for when geomagnetic poles are calculated and compared with the Gondwana apparent polar wander curves (Morel and Irving, 1978 Thompson and Clark, 1982), acquisition of a magnetic moment during a period from the mid-Devonian to the Cenozoic is implied. Since there is no known major orogenic phase or igneous activity associated with either the Caledonian or Variscan in this part of northeastern Africa, we suggest uplift as the mechanism responsible for magnetic acquisition.

Magnetostratigraphy of the Cretaceous-Tertiary boundary at Agost (Spain)

A detailed magnetostratigraphic investigation of the Agost section (Spain) containing the Cretaceous-Tertiary (K/T) boundary is reported. Thermal demagnetization—contrary to alternating field demagnetization—succeeds in revealing the polarity of the characteristic remanent magnetization, although an overlap in blocking temperature spectrum exists with a normal polarity, secondary magnetization component. The K/T boundary occurs at two-thirds from the base of a reversed polarity zone and by comparison with earlier results [1] this polarity zone is correlated to chron C29r. Linear regression of sedimentation rates in other sections with an established magnetostratigraphy and containing the K/T boundary yields an age of 66.45 Ma for this boundary. On the basis of this age and the magnetic reversal ages of the Berggren et al. [13] polarity time scale, new first appearance (FAD) ages around the K/T boundary are suggested for the planktonic species Morozovella trinidadensis (Bolli) (66.04 Ma), Morozovella pseudobulloides (Plummer) (66.34 Ma), Eoglobigerina taurica (Bang) (66.41 Ma) and “Globigerina eugubina” Luterbacher & Premoli Silva (66.43 Ma).

Paleomagnetic results from the southeastern margin of the Ebro Basin (northeastern Spain): evidence for a Tertiary clockwise rotation

Abstract Triassic rocks underlying the Paleogene deposits of the Ebro Basin are exposed along its SE margin. About 200 samples of Middle Triassic and Paleogene rocks from this margin have been demagnetized and analysed. The Middle Triassic red beds contain one or two secondary components of magnetization. The high unblocking temperature component has a mean direction of D = 1° and I = 6°. The Eocene red beds display a very stable magnetization. Nevertheless some samples have a high-temperature component with the present-day field direction. The Eocene mean direction is D = 22° and I = 51°. The paleomagnetic results show a systematic 20° clockwise rotation relative to the data from the Pyrenean and Cantabrian Ranges and the western and SW part of Iberia. To explain these results two hypotheses are presented. One implies the rotation of the NE part of Iberia, whereas the other refers to a rigid block rotation mechanism along the Catalan Coastal Range.

On the secondary magnetization observed in the Nawakot Complex rocks, Malekhu area, central Nepal

Paleomagnetic investigation on some Lesser Himalayan low-grade metasediments in the Malekhu area detected no primary magnetization while two distinct secondary magnetization components of its own type have been revealed. These components area interpreted as a recent field component (post-tectonic) and a pre-tectonic component probably of Eocene time.
 A clockwise rotation of the Malekhu area by about 45° with respect to the Indian Shield since the early Tertiary time is inferred too.

Magnetostratigraphy of the Permo-Triassic boundary section at Shangsi (Guangyuan, Sichuan Province, China)

Continous marine sedimentation characterizes many Late Permian to Early Triassic sections on the Yangtze terrane in South China. The Permo-Triassic (P/Tr) boundary section at Shangsi (Sichuan Province) consists of limestones intercalated with clays and mudstones which belong to the Wuchiapingian and Changxingian (Upper Permian) and the Griesbachian and Dienerian (Lower Triassic) stages. The P/Tr boundary is formed by a clay horizon with an unusually high iridium concentration. The intensity of natural remanent magnetization is very low with a mean of 0.15 mA m −1. About 40% of the samples contain secondary or unstable magnetization components only, whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy. Normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones, the P/Tr boundary being situated very close to the transition from a reversed to a normal polarity zone. The Shangsi polarity sequence represents part of the Illawarra interval of mixed polarity, the exact beginning of which has still to be determined.

Paleomagnetic study of the Late Ordovician–Early Silurian platform sequence of Anticosti Island, Quebec

Eighteen sites (152 samples, 441 specimens) from the Late Ordovician — Early Silurian sequence of Anticosti Island were studied paleomagnetically. Six sites were collected in the Vauréal Formation, seven in the Ellis Bay Formation, three in the Becscie Formation, and one each in the Gun River and Jupiter formations. The lithologies sampled consisted of limestones, sandy limestones, marls, and sandy calcareous shales drawn from a sedimentary platform sequence of predominantly inter-bedded limestones and shales. The specimens were demagnetized in alternating fields (AF) and thermally and were found to be quite stable. Two components of magnetization were isolated. The memory carrier is fine- to medium-grained magnetite; AF and thermal cleanings are about equally efficient. The average directions of residual magnetization are D = 167°, I = 37°, α95 = 18°; D = 315°, I = −24°, α95 = 17 °for components B and C, respectively. The corresponding paleopoles for the B and C components are 129°E, 19°N (dp = 12°, dm = 21°) and 341°E, 16°S (dp = 10°, dm = 18°).Component A is most probably composed of component C and the present Earth's field (PEF); it was chiefly observed in the more altered Vauréal Formation and in the altered top part of the cored samples. The most common component in the Ellis Bay, Becscie, Gun River, and Jupiter formations is component C. Component B is mainly found in the Becscie and Ellis Bay formations. Both components B and C are isolated in the 300–500 °C and 10–40 mT ranges. Component C is either older than component B or synchronous and of reverse polarity to component B. If this last alternative is correct, then the Ordovician and Silurian formations are overprinted by a pre-Kiaman magnetization. The more plausible scenario is the following: component B is secondary, of pre-Middle Carboniferous age, and overprinted on an older secondary magnetization (component C), the acquisition age of which is confined to the Silurian–Carboniferous time interval.

Rotational overthrusting of the northwestern Himalaya: further palaeomagnetic evidence from the Riasi thrust sheet, Jammu foothills, India

Thermal demagnetization results (316 samples) are presented for the Tertiary succession of the Riasi thrust sheet (Jammu foothills, northwestern Himalaya). Primary and secondary magnetization directions of Murree Group red beds (Miocene to Upper Eocene) sampled northeast of Jammu indicate, for this part of the Riasi thrust sheet, a clockwise rotation over about 45° with respect to the Indian shield since Late Eocene/Early Miocene time. This accords with clockwise rotations of similar magnitude observed in the Panjal Nappe and the Krol Belt, and is interpreted as representative for the northwestern Himalaya. Results from the western part of the Kalakot inlier, sampled northwest of Jammu, i.e. basal Murree claystone (Middle Eocene) and carbonate from the Subathu Group (lower Middle to Lower Eocene), indicate an aberrant 20–25° counterclockwise rotation which is of local importance only. Available observations on rotation of Himalayan thrust sheets with respect to the Indian shield, indicate that the Himalayan Arc has formed through oroclinal bending. This supports Powell and Conaghan's and Veevers et al.'s model of Greater India with large-scale intracontinental underthrusting along the Main Central Thrust beneath the Tibetan Plateau. Minimal magnitudes of underthrusting of 550 km in the Krol Belt and 650 km in the Thakkhola region are concluded. Palaeolatitude observations (herein and in [1[) agree with absolute positioning of the Indian plate based on India-Africa relative movement data fixed to a hotspot frame in the Atlantic Ocean, and with palaeolatitude observations from DSDP cores on the Indian plate. Collision-related secondary magnetic components observed both to the north and to the south of the Indus-Tsangpo Suture zone show palaeolatitudes between the equator and 7°N. Comparison of both datasets indicates that initial contact between Greater India and south-central Asia had been established in the Hindu Kush—Karakorum region by about 60 Ma ago whereas eastwards progressive suturing had advanced to the Lhasa Block segment of the Indus-Tsangpo Suture zone before 50 Ma ago.