Autunian age constrained by fold tests for paleomagnetic data from the Mezarif and Abadla basins (Algeria)

A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM (D = 131.8°, I = 15.7°, k = 196, α₉₅ = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.     

Paleomagnetic Study of the Juiz de Fora Complex, SE Brazil: Implications for Gondwana

The Juiz de Fora Complex is mainly composed of granulites, and granodioritic-migmatite gneisses and is a cratonic basement of the Ribeira belt. Paleomagnetic analysis on samples from 64 sites widely distributed along the Além Paraíba dextral shear zone (SE Brazil, Rio de Janeiro State) yielded a northeastern, steep downward inclination direction (Dm=40.4°, Im=75.4, a₉₅=6.0°, K=20.1) for 30 sites. The corresponding paleomagnetic pole (RB) is situated at 335.2°E; 0.6°S (a₉₅=10.0°; K=7.9). Rock magnetism indicates that both (titano)magnetite and titanohematite are the main magnetic minerals responsible for this direction. Anisotropy of low-field magnetic susceptibility (AMS) measurements were used to correct the ChRM directions and consequently its corresponding paleomagnetic pole. This correction yielded a new mean ChRM (Dm = 2.9°, Im = 75.4°, a₉₅ = 6.4°, K = 17.9) whose paleomagnetic pole RBc is located at 320.1°E, 4.2° N (a₉₅=10.3°, K=7.5). Both mean ChRM and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95% confidence circle. Geological and geochronological data suggest that the age of the Juiz de Fora Complex pole is probably between 535–500 Ma, and paleomagnetic results permit further constraint on these ages to the interval 520–500 Ma by comparison with high quality paleomagnetic poles in the 560–500 Ma Gondwana APW path.     

Paleomagnetic dating of Alleghanian orogenesis and mineralisation in the Mascot–Jefferson City zinc district of East Tennessee, USA

The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (d_p=18°, d_m=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (d_p=5.3°, d_m=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (d_p=2.4°, d_m=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.     

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, α₉₅ = 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, α₉₅ = 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.     

Paleomagnetic evidence for post-Cretaceous internal deformation of the Chuan Dian Fragment in the Yangtze block: a consequence of indentation of India into Asia

Paleomagnetic samples of Paleocene–Eocene red sandstones were collected at 36 sites from the Jiangdihe-4 and Zhaojiadian formations around the Yongren (26.1°N, 101.7°E) and Dayao areas (25.7°N, 101.3°E). These areas are located in the Chuxiong basin of the Chuan Dian Fragment, southwestern part of the Yangtze block. After stepwise thermal demagnetization, a high-temperature component with unblocking temperature of about 680 °C is isolated from 26 sites. The primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. The tilt-corrected mean paleomagnetic directions for the Yongren and Dayao areas are D=17.2°, I=26.6° with α₉₅=5.8° and D=16.5°, I=31.1° with α₉₅=4.8, respectively. Easterly deflected declinations from this study are consistent with those reported from other areas of the Chuxiong basin, indicating its wide presence in the Cretaceous–Eocene formations of the said basin. Comparison with declination values expected from the Cretaceous–Eocene APWP of Eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the Chuxiong basin as well as in the Chuan Dian Fragment. This trend of the differential tectonic rotation in the Chuan Dian Fragment is consistent with curvature of the Xianshuihe–Xiojiang fault system. Deformation of the Chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the Chuan Dian fragment. During southward displacement, the Chuan Dian Fragment was probably subjected to tectonic stresses as a result interaction with the Yangtze and Indochina blocks, which resulted into east–west extension and north–south shortening.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

The motion of Adria during the Late Jurassic and Cretaceous: New paleomagnetic results from stable Istria

The motion of Adria, the largest lithospheric fragment in the Central Mediterranean region, has played an important role in the tectonic development of the surrounding mountain chains and even of distant areas, like the Eastern Alps or the Pannonian basin. The available paleomagnetic data were insufficient to constrain this motion, except in a general way. In this paper, new paleomagnetic results are presented from one of the stable parts of Adria which emerge from the Adriatic Sea. The results were obtained on weakly magnetic platform carbonates of the mud-supported type, collected from 21 geographically distributed localities.The results, combined with mean paleomagnetic directions from selected localities from a pioneer study in Istria that were chosen using statistical criteria, were divided into three age groups (Tithonian–Aptian, Albian–Cenomanian, Turonian–Coniacian). The paleomagnetic poles calculated for each of them (Tithonian–Aptian): λ(N) = 47°, (E) = 275°, k = 67, α₉₅ = 9.4°, N = 5; Albian-Cenomanian: λ(N) = 58°, (E) = 253°, k = 145, α₉₅ = 4.3°, N = 9; Turonian–Coniacian: λ(N) = 63°, (E) = 261°, k = 50, α₉₅ = 7.3°, N = 9) reveal a moderate shift during the Cretataceous, which is comparable with that calculated from the African reference poles. However, the Istrian apparent polar wander path is slightly displaced from the African curve, as a consequence of about 10° counterclockwise rotation of Istria, with respect to Africa. This rotation angle is more that 10° smaller than the difference measured for the Mid-Late Eocene between the paleomagnetic direction of platform carbonates from Istria and the African reference direction. This difference may be the consequence of a small clockwise rotation of Istria, with respect to Africa, most probably at the end of Cretaceous.     

Paleomagnetism of the Cretaceous Morelos and Mezcala Formations, southern Mexico

A paleomagnetic study of platform-facies carbonate rocks of the mid-Cretaceous Morelos Formation and deep-water carbonate rocks of the overlying Upper Cretaceous Mezcala Formation, sampled at Zopilote canyon, in Guerrero State, southern Mexico, indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the Late Cretaceous Laramide orogeny. The remanence carrier is interpreted to be magnetite, although other mineral phases of high coercivity carry recent secondary overprints. The overall mean is of Dec=323.1° and Inc=36.5° (k=162.7; α₉₅=2.7°; N=18 sites; 64% unfolding). Comparison with the North America reference direction indicates that this area has experienced a small, yet statistically significant, counterclockwise direction of 19.2±4.0°. Similar rotations are documented in other localities from southern Mexico; rotations are linked to mid-Tertiary deformation associated with the left-lateral strike-slip fault system that accommodated motion of the Chortis and Xolapa blocks.     

New African Lower Carboniferous paleomagnetic pole from intrusive rocks of the Tin Serririne basin (Southern border of the Hoggar, Algeria)

A paleomagnetic study has been conducted on intrusive doleritic rocks cropping out within Devonian horizontal tabular formations of the Saharan craton (Tin Serririne basin, South of Hoggar shield). The ⁴⁰K/⁴⁰Ar dating of the dolerites gave an age of 347.6 ± 8.1 Ma, i.e. Tournaisian. The paleomagnetic data present three different directions. The first has a paleomagnetic pole close to the previous African poles of Permian age. This direction is therefore interpreted as a Permian remagnetization. The second direction, which is defined by both linear regression and remagnetization circles analysis, is considered as the primary magnetization. It yields a new African Tournaisian paleomagnetic pole (λ = 18.8° S,  = 31.2° E, K = 29, A₉₅ = 7.5°) very close to the Ben Zireg Tounaisian pole [Aifa, T., Feinberg, H., Pozzi, J.P., 1990. Devonian/Carboniferous paleopoles for Africa. Consequences for Hercynian geodynamics. Tectonophysics, 179, 288–304]. The third direction has intermediate orientation between those of the first or second directions and that of the Upper Cenozoic field. It is interpreted as related to a composite magnetization. This new Tin Serririne pole improves the APWP of Gondwana, for this key period of the evolution of the Pangea. This APWP confirms the previous paleogeographic reconstruction which shows that the pre-Hercynian ocean between Gondwana and Laurussia is still not close during the beginning of the Carboniferous.     

Paleomagnetism of the earliest Cretaceous to early late Cretaceous sandstones, Khorat Group, Northeast Thailand: Implications for tectonic plate movement of the Indochina block

A total of 400 samples (33 sites) were collected from the earliest Cretaceous to early Late Cretaceous sandstones of the Khorat Group in the Indochina block for paleomagnetic study to unravel the tectonic evolution of the region. The sites were adopted from 3 traverses located in the northern edge of the Khorat Plateau, northeastern Thailand. Results indicate that almost all the sandstones exhibit similar magnetic values with an average declination (D) = 31.7°, inclination (I) = 30.3°, λ = 59.7°,  = 190.9°, K = 54.4, and A₉₅ = 3.7 at reference point 17°30′N and 103°30′E. The calculated paleolatitude points are inferred to deviate from the present latitude point by 1.2 ± 2.3°. Only the lowermost part of the Cretaceous sandstones can pass a positive fold test at 95% confidence level. The relationship between the virtual geomagnetic poles (VGPs) of Cretaceous rocks of the Indochina plate in Thailand and those of the South China plate advocate that there is a major displacement of Indochina along the northwest-trending Red River and associated faults by about 950 ± 150 km with a 16.0–17.0° clockwise rotation relative to the South China plate during earliest Cretaceous times. Paleomagnetic results of the early Late Cretaceous Indochina plate point to a 20–25° clockwise rotation relative to the present occurring since very Late Cretaceous (65 Myrs)–Early Neogene times which may be due to the collision between India and Asia.     

Conical folds and apparent rotations in paleomagnetism (a case study in the Southern Pyrenees)

The so-called apparent rotation was defined as the angular deviation between a local paleomagnetic direction (after the standard bedding correction) and their corresponding paleomagnetic reference [J. Geophys. Res. 85 (1980) 3659]. In this paper, we make a theoretical exploration on this concept and we conclude that (depending on the number, sequence, orientation and magnitude of the deformation axes that have affected to the rock volume) the apparent rotation may be the addition of a vertical-axis rotation plus a spurious rotation. The later is an error whose origin is the inappropriate application of the bedding correction during the restoration (which does not fit the reverse sequence of deformations). Then, Apparent rot. (s.l.) (δ)=Spurious rot. (θ)+Vertical-axis rot. (β).Conical folds are complex geometries that cannot be restored by using the bedding correction. However, appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts. In this paper, we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection. The modelling has to be based on a good characterization of the geometry (fold axis orientation) and understanding of the kinematics of the fold. General modelling has also allowed us to predict the apparent rotation in conical synclines. Its magnitude depends on the semiapical angle and on the degree of development of the fold; the sense of the rotation (clockwise or counter-clockwise) will depend on the sense of rotation the fold axis.The western External Sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall (Ebro foreland basin) of the South Pyrenean sole thrust. In addition, a vertical-axis clockwise rotation up to 47° (32° in average) has been detected in the hagingwall. An apparent rotation up to 28° (20° in average) is observed in the footwall of the structure when a simple bedding correction is used. This deviation does not fit with the expected Ebro basin direction (reference) and is caused by the effect of the Riglos conical syncline, developed by the flexure of the foot wall ramp of the South Pyrenean sole thrust. The forward modelling carried out considering the geometry and kinematics (non significant rotations in the autochthonous foot wall) of this structure predicts very well the paleomagnetic observations in the field (geographic coordinates) with angular departures of only 5° (in average). The only application of the bedding correction would introduce errors (spurious rotations up to 21°, 12 in average) related to the conical geometry that would not allow the differentiation of these distinct structural units.     

Fluid flow in the Duperow and Winnepegosis Formations, Williston Basin, Canada: Evidence from preliminary magnetic measurements

The Devonian Winnepegosis and Duperow Formations were examined in well 4-27-11-22W1, located in at the eastern edge of the Williston Basin in Manitoba. The variation in characteristic remanent magnetization (ChRM) direction and magnetic mineral carrier is obvious: the older Winnepegosis Formation has a primary or early post-depositional magnetization held in magnetite or pyrrhotite (n = 15; D = 324.1°, I = − 27.3°, α₉₅ = 10.4°, k = 15.7), whereas the younger Duperow Formation magnetizations are carried by hematite and could be as late as Early Jurassic. The variability may be attributable to the intervening Prairie Evaporite acting as an aquitard to fluid migration.     

Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism from Kutch basin, northwest India

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill.IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° (α₉₅=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (dp/dm=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.     

Paleomagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China

A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield ⁴⁰Ar / ³⁹Ar isochron ages of 58.5 ± 1.3 Ma (2σ, MSWD = 0.9) and 60.4 ± 1.3 Ma (2σ, MSWD = 1.7). These compare well with a previously published K–Ar dilution age of 61.7 ± 2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower ( 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya.     

Autunian age constrained by fold tests for paleomagnetic data from the Mezarif and Abadla basins (Algeria)

A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM (D = 131.8°, I = 15.7°, k = 196, α₉₅ = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.     

Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Rind River Range, Wyoming, USA

We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U–Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170±8 Ma (95% confidence). An ⁴⁰Ar/³⁹Ar amphibole date from the same sample yields a similar apparent age of about 2124±30 Ma (2σ), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355°, Inc.=65°, k=145, α₉₅=5°) with a paleomagnetic pole at 84°N, 215°E (δm=6°, δp=7°). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20°, N20°E), gives a tilt corrected pole of 75°N, 58°E (δm=4°, δp=6°), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic.     

Reappraisal of paleomagnetic data from Gargano (South Italy)

The platform limestones of Apulia (Italy) outcropping in the Gargano peninsula have been restudied. Paleomagnetic research has been carried out on Upper Cretaceous, Lower Cretaceous and Jurassic rocks. Despite the low intensities of the NRM (10–100 μA/m), all samples (268) could be cleaned by stepwise A.F. and/or thermal demagnetization treatments. NRM directions could be determined accurately and reproducibly for 85% of the samples, using a ScT cryogenic magnetometer and double precision measuring procedures. NRM of the Jurassic limestone is carried by secondary haematite and the results are therefore rejected from further consideration. The Upper and Lower Cretaceous limestones have an NRM carried by magnetite. Minor bedding tilt corrections improve the grouping of the site-mean results. The Upper Cretaceous “Scaglia” limestone (Turonian-Senonian) reveals a characteristic mean direction of decl. = 327.7°, incl. = 38.2°, α₉₅ = 4.3° (21 sites), while the Lower Cretaceous “Maiolica” limestone (Neocomian-Aptian/Albian) reveals a characteristic mean direction of decl. = 303.1°, incl. = 35.1°, α₉₅ = 8.7° (8 sites). The Cretaceous results show a post-Aptian/Albian counterclockwise rotation of about 25°, which is expressed by the smeared distribution of the Late Cretaceous site-mean results and a post-Senonian (i.e. Tertiary) counterclockwise rotation of the same amount with respect to the pole. These results are in excellent agreement with contemporaneous paleomagnetic results from other peri-Adriatic regions. A Tertiary counterclockwise rotation of all the stable Adriatic block is strongly supported by the new results.     

Palaeomagnetism of Upper Bhander Sandstones from Central India and implications for a tentative Cambrian Gondwanaland reconstruction

A virtual geomagnetic pole position for the uppermost Vindhyan sediments in Central India, probably correlative with the Cambrian of the Salt Range (W. Pakistan), was obtained from 43 oriented cores (7 sites) drilled from the Upper Bhander sandstones in the Great Vindhyan basin.Alternating field and thermal demagnetization resulted in a mean direction: D = 207.5°, I = +9.5° (k = 137.5, α₉₅ = 5.5°) and a virtual geomagnetic northpole position: 146.5° W48.5°N (dp = 3°, dm = 5.5°).This pole position disagrees with the Cambrian palaeomagnetic results from the Salt Range. Implications of this disagreement for the pre-drift configuration of Gondwanaland, based on palaeomagnetc results, are discussed.     

Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000 Ma Closure age for the Purana Basins?

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including ⁸⁷Sr/⁸⁶Sr and δ ¹³C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α₉₅ = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α₉₅ = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.     

Palaeomagnetic data from the precambrian Gwalior traps, Central India

From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction: D=78°, I=+34.5°, α₉₅=5°, k=369, N=4 (Pole): 155.5°E19°N, dp=3°, dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y.