Paleomagnetism of gabbros of the Early Proterozoic Blachford Lake Intrusive Suite and the Easter Island dyke,Great Slave Lake,NWT: Possible evidence for the earliest continental drift

The Caribou Lake gabbro, part of the Blachford Lake Intrusive Suite accurately dated at –2186±10 mA, has a predominantNW–/SE+ magnetization with a mean, irrespective of sign, ofD=119°,I=50°, ₉₅=5° and a palaeopole 14°N, 064°W,A ₉₅=5°; it has not proved possible to determine if the magnetization is primary. The Easter Island dyke, less well-dated in the range –2200 to –2500 Ma, has a predominantWNW+ magnetization, whose mean, when corrected for an 8° tilt, isD=288°,I=46°, ₉₅=5 and palaeopole is 32°S, 2°W,A ₉₅=5°; the magnetization is probably primary. A vertical magnetization (D), not significantly different from the present field, occurs sporadically in both units and is considered to be Late Phanerozoic in age. Palaeopoles from the Caribou Lake gabbro and the Easter Island dyke, together with those already known from Early Proterozoic intrusives of the Archaean Slave Structural Province, roughly define a swath (the Slave Track) which maps the motion of the Slave Province relative to the geomagnetic axis during this interval. The corresponding array of palaeopoles (the Superior Track) from the Superior Structural Province does not fall in the same place. Hence it would appear that Slave and Superior were not in their present relative positions in the Early Proterozoic in disagreement with arguments that have been made for a fixed supercontinent during much of the Proterozoic. Mid-Proterozoic paleomagnetic signatures indicate that Slave and Superior had assumed their present relative position by about –1750 mA. These Early Proterozoic relative motions are the earliest for which there is palaeomagnetic evidence.Earth Physics Branch Contribution No. 1111.     

Palaeomagnetism of the Börzsöny Mountains (Hungary)

Summary Mean directions of magnetization (29 normal and 31 reversed) were recorded for 60 magmatic localities of middle Miocene age from the Börzsöny Mountains (Hungary). The overall mean direction of RM, irrespective of polarity, isD=0,9°;I=59,8°; withk=8,3 and ₉₅=6,8°. The coordinates of the corresponding geomagnetic north pole are =82,7°, A=193,8 with p=7,7° and m=10,2°.     

Paleomagnetism and Cyclostratigraphy of the Middle Ordovician Krivolutsky Suite,Krivaya Luka Section,Southern Siberian Platform: Record of Non-Synchronous NRM-Components or a Non-Axial Geomagnetic Field?

The Middle Ordovician Volginsky and Kirensky fossil zones were sampled in the Krivaya Luka section (Krivolutsky suite) that outcrops along the Lena river in Siberia. The Volginsky and Kirensky zones are coeval to the Llandeilo in the global geologic time scale. The Krivaya Luka section consists of siltstones, clays, sandstones, and limestones, and displays a remarkably distinct sedimentary cyclicity, especially in its reddish middle part.Stepwise thermal demagnetization yields three NRM components. Component A, isolated in the 100—250°C interval can be either normal or reversed. The normal A-component has a direction close to recent local magnetic field. The reversed A-component directions are scattered around a direction close to that of the lower Triassic Siberian traps. Component B has unblocking temperatures that range from 400 to 500°C and is represented mainly by normal polarity directions. The B-component, isolated from rocks of the middle part of the section is of a normal polarity with D = 176.5°, I = 30.0° and a North pole position at 16.2°S, 111.3°E. The other parts of the section are characterized by intermediate B-directions, which resulted possibly by partially overlapping A- and C-components. The highest temperature dual-polarity component C was isolated in the 550—670°C interval, resulting in the detection of two complete polarity zones and three magnetic reversals. The C-component is characterized by the following mean directions: for the reversed component D = 335.7°, I = 6.9°, and for the normal component D = 188.6°, I = 28.0°, which is very close to the normal polarity directions of the B-component. The corresponding paleomagnetic North pole for reversed polarity rocks is 32.6°S, 137°E, which is typical of Middle Ordovician rocks from Siberia – the mean pole for Llanvirn-Llandeilo is 30°S, 136°E (cf. Smethurst et al., 1998) – whereas for normal polarity rocks the pole position 17.2°S, 99.1°E is markedly different. Nevertheless, we assume that the C-component records the ancient geomagnetic field of Ordovician times, even though it does not pass the reversals test. This could be explained by overlapping NRM unblocking temperature spectra for the B and C components. In this case, the paleomagnetic pole positions should be interpreted with some caution.In addition, the section was logged and sampled in detail for cyclostratigraphic purposes. Spectral analysis in the depth domain using the high-field susceptibility as input parameter showed that the observed cyclicity is most likely orbitally forced. Detected spectral peaks (significant at the 95% confidence level) were close to the expected positions of the periodicities of precession, obliquity and eccentricity for the Ordovician. Consequently, the average sediment accumulation rate is estimated at 3.5 cm/kyr. Extrapolating this sedimentation rate yields a total duration of at least 1 Myr for the Volginsky fossil zone and 1.2 Myr for the entire Krivaya Luka section.     

Paleomagnetism of the Djebel Reouina Namurian Formation (Tindouf Basin,Algeria)

The paleomagnetic study of the Namurian of Reouina (28.9°N, 08.0°W) revealed the existence of two magnetization components, either juxtaposed or superimposed, besides a viscous component. The high blocking temperature component, carried by hematite, has a mean direction defined by D = 126.9° and I = 10.8°. It provides a Namurian paleomagnetic pole located at 28.4°S and 56.9°E (K = 642, A ₉₅=1.7°). The second component is carried at least in part, by grains with blocking temperatures lower than 550°C. Though well defined, it consists of two superimposed components, the high unblocking temperature component with a likely Permian overprint.     

A palaeomagnetic study of the Nigerian volcanic provinces

Summary Six Younger Granite localities showing normal and reverse magnetizations in equal proportion have given a Jurassic palaeomagnetic pole position =62.5°N, =241.6°E; (Fisher's precision parameter (k)=27.8 and ₉₅=13°). Individual palaeopole-positions have also been obtained for a Cretaceous pyroclastic rock and for two Pleistocene basalt flows.     

Paleomagnetic study of satyavedu sandstones of cretaceous age from Andhra Pradesh,India

Summary The paleomagnetism of twenty six oriented samples of red sandstones from three different sites located in Satyavedu hills (near 13°30N, 79°55E) and belonging to coastal upper Gondwana formations of India has been studied. These sandstones have been considered to be equivalent of Tirupati sandstones of Lower to Middle Cretaceous age from Godavari valley on stratigraphic considerations. Consistent directions of magnetization were obtained from two different sites after stability tests. Results from one site gave ancient pole position at 26°N, 67°W, very close to that obtained for Tirupati sandstones, thus confirming the geologic correlation. Results from the second site were rejected on account of instability and those from the third site gave pole position at 79°48N, 76°58W. This site appears to have been remagnetized during Pleistocene times when there was large deposition of laterite in the area. Study of magnetic properties of these sandstones revealed that the magnetization was of the nature of CRM and the NRM was carried almost entirely by red coating on silica grains.N.G.R.I. Contribution No. 70-169.     

Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements, Tshirege member (upper Bandelier Tuff), Valles caldera, New Mexico, USA

This study concludes that the elongation axis (K ₁) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K ₁ axes correlate well with postulated radial flow axes at 13 sites.K ₁ also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK ₁ axes, theK ₁ axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK ₁ axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK ₁ is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent mean flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K ₃) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK ₁ axes, not with the declination ofK ₃ axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, ₉₅=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A ₉₅=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.     

Palaeomagnetism of Mesozoic igneous rocks from the Maranha˜o Basin,Brazil, and the time of opening of the South Atlantic

From Middle-Upper Jurassic volcanics at the western margin of the Maranha?o Basin (6.4°S, 47.4°W) 15 sites (121 samples) have a mean magnetization directionD = 3.9°,I = ?17.9° withα₉₅ = 9.3°,k = 17.9 after AF cleaning (all sites have normal polarity). This yields a pole (named SAJ₂) at 85.3°N, 82.5°E (A₉₅ = 6.9°) which is near to the other known Middle Jurassic South American pole. For 21 sites (190 samples) from Lower Cretaceous basalt intrusions from the eastern part of the Maranha?o Basin (6.5°S, 42°W) the mean direction isD = 174.7°,I = +6.0° withα₉₅ = 2.8°,k = 122 (all sites have reversed polarity) yielding a pole (SAK₉) at 83.6°N, 261°E (A₉₅ = 1.9°) in agreement with other Lower Cretaceous pole positions for South America. Comparing Mesozoic pole positions for South America and Africa in the pre-drift configuration after Bullard et al. [13] one finds a significant difference (with more than 95% probability) for the Lower Cretaceous and Middle Jurassic poles and also a probable difference for the mean Triassic poles indicating a small but probably stationary separation of the two continents from the predrift position in the Mesozoic until Lower Cretaceous time which may be due to an early rifting event.     

Paleomagnetism of mid-Cretaceous red beds in west-central Kyushu Island, southwest Japan: paleoposition of Cretaceous sedimentary basins along the eastern margin of Asia

A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240-480°C), and high (480-680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α₉₅=5°, which yields a paleomagnetic pole at 39°N, 186°E and A₉₅=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.     

Paleomagnetism of the Lower Ordovician and Cambrian sedimentary rocks in the section of the Narva River right bank: For the construction of the Baltic Kinematic model in the Early Paleozoic

The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A₉₅ = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic.     

New Cambrian paleomagnetic pole for Yangtze Block

A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α₉₅ = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α₉₅ = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.     

Palaeomagnetism of middle Ordovician volcanic rocks from Quebec

This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (d_m = 14°, d_p = 7°) after thermal treatment and 164°E, 19°N (d_m = 11°, d_p = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.     

Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. ¹³ C analysis, total organic content measurements and C ¹⁴ dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.     

Aspects of Caledonian palaeomagnetism and their tectonic implications

Palaeomagnetic results from the Lower Palaeozoic inliers of northern England cover the upper part of the (Middle Ordovician) Borrowdale Volcanic Series (palaeomagnetic pole 208°E, 18°S, A₉₅ = 9.4°), minor extrusive units relating to the Caradoc and Ashgill stages of Ordovician times, intrusive episodes of Middle Ordovician and Middle Silurian to Late Devonian age, and the Shap Granite of Devonian (393 m.y.) age (palaeomagnetic pole 313°E, 33°S, A₉₅ = 5.6°).A complete assessment of Ordovician to Devonian palaeomagnetic data for the British region shows that the pole was nearly static relative to this region for long intervals which were separated by shifts occupying no more than a few millions of years. The mean palaeomagnetic poles are: Ordovician (6°E, 16°S), Lower Silurian (58°E, 16°N), Middle Silurian/Lower Devonian (318°E, 6°N) and Middle/Upper Devonian (338°E, 26°S); the first two shifts separating these mean poles can be explained predominantly in terms of rotational movements of the crustal plate but the last involved appreciable movement in palaeolatitude.Comparison of Lower Palaeozoic palaeomagnetic data from the British region with contemporaneous data from continental Europe/North America on the Pangaean reconstruction reveals a systematic discrepancy in palaeolatitude between the two regions prior to Middle Devonian times. This discrepancy was eliminated during a few millions of years of Lower/Middle Devonian times (ca. 395 m.y.) and can be explained in terms of ca. 3500 km of sinistral strike-slip movement close to the line of the orthotectonic Caledonides. This motion is linked both in time and place to the impingement of the Gondwanaland and Laurentian supercontinents during the Acadian orogeny; this appears to have displaced the British sub-plate until it became effectively locked between the Baltic and Laurentian regions. Although movement of the dipole field relative to the British region in Lower Palaeozoic times is now well defined, nearly one fifth of the total data show that the geomagnetic field was more complex than dipolar during this interval. Until the significance of these anomalies is fully resolved, the tectonic model derived from the palaeomagnetic data cannot be regarded as unambiguous.     

Notes on the variation of magnetization within basalt lava flows and dikes

Summary The magnetic properties of basaltic rocks are dominated by the contained primary Fe–Ti oxides. At solidus temperature (1000°C) the composition of these primary oxides is restricted to titanomagnetite (Fe_3-xTi_xO₄) and hemoilmenites (Fe_2-yTi_yO₃). The examination of 269 chemical analyses of the primary Fe–Ti oxides in basalts (in sensu lato) gives an average ofx=0.61 (T _c=168°C) for the titanomagnetites andy=0.89 (T _c=–121°C) for the hemoilmenites. If distinction is made between tholeiites, alkali basalts and andesites, a clear difference for thex-values is observed: the average for tholeiitesx=0.64 (T _c=144°C), for alkali basaltsx=0.52 (T _c=253°C), for andesitesx=0.38 (T _c=341°C).Environment of crystallization and cooling rate are major interrelated factors influencing subsequent changes in the mineralogy of the primary Fe–Ti oxides and resulting magnetic properties. This has been tested by studying the variation of magnetization and some of its parameters in three different basalt rock units: a dike, 180 cm, and two lava flows, 3 m and 33 m thick, respectively. Grain size and oxidation state of the titanomagnetites control the variation of magnetization in these basalt units.     

On farsidedness of palaeomagnetic poles: Magnetic refraction,sediment compaction and dipole off-set

Summary Three possible ways to explain the Caenozoically observed farsidedness of paleomagnetic poles (apart from lithospheric plate movements) are discussed: magnetic refraction, sediment compaction and dipole off-set. The dipole off-set, being a possible geomagnetic field property, will be of opposite sign on opposite hemispheres, and hence will not tend to smoothe out by sectorial averaging. Sediment compaction shallows the inclination on both hemispheres, and hence will tend to smoothe out by sectorial averaging, provided that sediment properties, site latitude coverage and number of investigations are equal (fairly unlikely).Magnetic refraction causes systematic directional distortions of the remanent magnetization in rocks of moderate to high magnetic intensity (or apparent susceptibility k_app=k(1+Q)) such as in many volcanics, some metamorphics, as well as in baked clays and slags, etc. A detailed discussion of this effect is given: If the k_app of the material is above 0.001 emu/cm ³ (×4 SI), this effect is likely to cause a significant palaeomagnetic refraction error of the NRM (typically a TRM or a CRM) of the rock. An apparent susceptibility of this order of magnitude is quite common in volcanic rocks; e.g. for oceanic floor basalts the average of k_app is about 0.02 emu/cm ³ corresponding to systematic errors (flattening) of some 3° to 6° in the inclination of a horizontal flow, depending on the latitude.To improve paleomagnetic results in general, a simple refraction correction is therefore suggested to be applied in the case of common two-dimensional (i.e. flat, elongated) geological bodies such as dykes, sills, lava flows and baked clays. Numerical solutions are given for the horizontal case, while a graphical solution is given for the general two-dimensional case.Being of systematic types, the refraction error together with the sediment compaction effect may be responsible for a major part of the observed farsidedness of the Caenozoic palaeomagnetic pole positions, the apparent farsidedness not yet beeing masked by the scatter of pole positions produced by older individual lithospheric plate movements.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

New paleomagnetic result from the Ethiopian flood basalts in the Abbay (Blue Nile) and Kessem gorges

New paleomagnetic investigations on the Ethiopian trap series have been undertaken at the Abbay and Kessem gorges in an attempt to better constrain the 30 Ma paleomagnetic pole of Africa. We sampled six thick massive basaltic lava flows, totaling 230 m, from Abbay Gorge and 10 lava flows, 180 m in thickness, from Kessem Gorge. Detailed paleomagnetic analyses disclosed that the carriers of the characteristic remanent magnetization (ChRM) are different in different lava flows. These are mostly titanomagnetites, titanomaghemites, and magnetite minerals with a broad range of coercive force and blocking temperatures. The heating and cooling susceptibility vs. temperature curves, many of which are irreversible, may indicate chemical remagnetization, notably low temperature maghemitization. Only one flow (KS04) with a clear 580°C Curie temperature was apparently unaffected by chemical remagnetization. The ChRM direction of this flow is identical to that in other flows, which suggests that if and when remagnetization occurred, this was shortly after emplacement of the lava flows. All of the flows sampled have normal polarity. However, a reversed component of low to medium coercive force and low to medium unblocking temperature occurs in flow KS01 at Kessem Gorge. The ChRM directions for the 16 sites are D=3.1°, I=5.8° (α₉₅=12.7°). The paleomagnetic pole obtained from these is at λ=83.0°N, φ=193.3°E (A₉₅=9.0°). Comparison with three previous studies of the traps shows remarkable consistency and a number of means are derived and discussed. Two final preferred poles for the traps are at λ=79.0°N, φ=196.9°E (A₉₅=2.8°) when all 112 published flows are used, and λ=78.7°N, φ=209.4°E (A₉₅=3.4°) when only the 76 flows from the four more recently analyzed sections are included. Both are compatible with the recent reference synthetic pole for Africa of Courtillot and Besse [J. Geophys. Res. (2002) in press]. In that sense, the Ethiopian trap pole is not anomalous and does not require more of a non-dipolar contribution than indicated by analyses of the global paleomagnetic data base covering the last few million years.     

Analysis of aeromagnetic anomalies over the central part of the Narmada-Son lineament

The Narmada-Son lineament (NSL) forms a major tectonic feature on the Indian subcontinent. The importance of this lineament lies in its evolution as well as its tectonic history. The lineament seems to have been active since Precambrian times. In order to understand the history of its evolution, it is necessary to know what igenous activity has been taking place along this lineament, and how the Deccan trap volcanics, which cover large areas along this lineament, have erupted.For the study of this problem an analysis of the aeromagnetic anomaly map lying between 76°15 to 77°30E and 21°45 to 22°50N has been carried out. Four different profiles (B ₁ B ₁,B ₂ B ₂,B ₃ B ₃ andB ₄ B ₄) have been drawn in N-S direction over this area and interpreted in terms of the intrusive bodies present within or below the surface of Deccan trap exposures. Inversion and forward modelling techniques have been adopted for interpretation purposes. An analysis of frequency spectra along the profiles has also been carried out to estimate the average depth of the different magnetic bodies. These results have been correlated with the available geological information. It has been found that most of the small wavelength anomalies are caused by dyke-like bodies within or below the Deccan trap at a depth of less than 0.5 km.     

Paleomagnetic and rock magnetic results from the Twin Creek Formation (Middle Jurassic), Wyoming

A magnetization which passes the fold test has been observed in 73 limestone samples (10 sites) from the Middle Jurassic Twin Creek Formation. The pole calculated from the site mean poles is located at 68.4°N, 145.0°E (K = 31.8,A₉₅ = 8.7°). This pole lies in a segment of the North American apparent polar wander (APW) path for which there are only a few reliable poles in the literature. The results corroborate earlier studies which conclude that the Jurassic segment of the APW path does not include the present north pole. However, the position of the Twin Creek pole suggests that significantly more APW took place prior to the late Jurassic than previous studies indicated.     

Paleomagnetism of Lower Devonian volcanics and Devonian dykes from northcentral New Brunswick,Canada

Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α₉₅ = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, d_p = 6.5°, d_m = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.