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.     

Reversal asymmetry in Mesoproterozoic overprinting of the 1.88-Ga Gunflint Formation, Ontario, Canada: non-dipole effects or apparent polar wander?

Eighty-two palaeomagnetic samples of calcareous and jaspilitic grainstones (iron-formation or ‘taconite’) and chert carbonate were collected from the 1.88-Ga Gunflint Formation at 22 sites in the Thunder Bay area, Ontario. Twenty clasts of Gunflint taconite also were sampled from the basal conglomerate of the overlying Mesoproterozoic Sibley Group. Anisotropy of magnetic susceptibility measurements indicate the Gunflint Formation in the sampling area has not experienced regional dynamic metamorphism. Analyses by variable-field translation balance and X-ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples. Altogether, 213 Gunflint specimens and 59 Sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 Gunflint specimens to stepwise alternating-frequency demagnetisation. The following components were isolated for the taconites:

• Gunflint magnetite: normal declination D=293.4°, inclination I=30.8°, α₉₅=7.2°, n=21; reverse D=86.7°, I=–54.6°, α₉₅=5.8°, n=29.

• Gunflint hematite: normal D=243.6°, I=23.6°, α₉₅=6.0°, n=11; reverse D=70.3°, I=–51.4°, α₉₅=3.2°, n=79.

• Sibley clasts magnetite: normal D=282.7°, I=33.4°, α₉₅=7.6°, n=20.

• Sibley clasts hematite: normal D=254.5°, I=56.2°, α₉₅=8.4°, n=13; reverse D=110.6°, I=–55.7°, α₉₅=8.3°, n=11.

None of these sets passed the reversal test, with the normal component generally being the shallower. Fold tests were negative or inconclusive and the conglomerate test also was negative. Chert carbonate at one other site appears to have acquired a remanence carried by magnetite (D=97.3°, I=−78.2°, α₉₅=6.3°, n=18) prior to folding related to Keweenawan (1.1 Ga) Logan diabase sill emplacement. Most of the components we identified match components for Keweenawan sills, volcanic rocks, intrusions and baked contact rocks in the Thunder Bay area, indicating that Keweenawan magmatism caused widespread chemical remagnetisation of the Proterozoic country rock in our sampling area. Although others have argued that asymmetry was a feature of the Keweenawan geomagnetic field, the declinations of our Gunflint and Sibley hematite and magnetite components are different, suggesting that the components were acquired at significantly different times. We conclude that the reversal asymmetry shown by our Gunflint and Sibley data may best be ascribed to apparent polar wander during Keweenawan times.     

Palaeomagnetic results from the Early Permian Copacabana Group, southern Peru: Implication for Pangaea palaeogeography

Samples collected from folded carbonate rocks of the Early Permian Copacabana Group exposed in the Peruvian Subandean Zone have been subjected to detailed palaeomagnetic analysis. Thermal demagnetisation of most samples yield stable high unblocking temperature directions dominantly carried by titanomagnetite minerals. This remanence, identified in 32 samples (43 specimens), is exclusively of reverse polarity consistent with the Permian–Carboniferous Reversal Superchron (PCRS). The overall directions pass the fold test at the 99% confidence level and are considered as being a pre-folding remanence acquired in Early Permian times. The Copacabana Group yields an overall mean direction of D = 166°, I = +49° (α₉₅ = 4.5°, k = 131.5, N = 9 sites) in stratigraphic coordinates and a corresponding palaeosouth pole position situated at λ = 68°S,  = 321°E (A₉₅ = 5.2°, K = 100). Combining this pole with the coeval high quality data from South America, Africa and Australia results in a mean pole for Gondwana situated at λ = 34.4°S,  = 065.6°E (A₉₅ = 4.9°, K = 73.6, N = 13 studies) in African coordinates. This pole position supports a Pangaea B palaeogeography in Early Permian times. In contrast, the combined pole for Gondwana diverges from the coeval Laurasian mean pole when assuming the Pangaea A-type configuration. Poor quality of the Gondwana dataset and inclination shallowing in sediments seem to play no role in the misfit between the Permian–Triassic poles from Gondwana and Laurasia in Pangaea A reconstruction.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

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.     

Palaeomagnetic study of Archaean Banded Hematite Jasper Rocks from the Singhbhum-Orissa Craton, India

The Banded Hematite Jasper Formation within the Iron Ore Supergroup of the Singhbhum Craton in eastern India comprises fine alternating layers of jasper and specularite. It was deposited at 3000 Ma and deformed by a mobile episode at 2700 Ma. Hematite pigment (<1 μm) mixed with cryptocrystalline silica and specularite (> 10 μm) is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts (1–2%) of magnetite are present. Palaeomagnetic study identifies a dual polarity remanence resident in hematite (D/I = 283/60°, α₉₅ = 12°) which predates deformation. Studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite. The palaeopole (32°E, 24°N, dp/dm = 14/18°) records the earliest post-metamorphic magnetisation event in the Orissa Craton. A minimum apparent polar wander motion of the Orissa-Singhbhum craton of through 80° is identified during Late Archaean times (2900-2600 Ma).     

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.     

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of D_m = 148.1, I_m = + 48.6 (K = 22.2, α₉₅ = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.     

Palaeomagnetism of the Loch Doon Granite Complex, Southern Uplands of Scotland: The Late Caledonian palaeomagnetic record and an Early Devonian episode of True Polar Wander

The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km³ intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α₉₅ = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.     

Geochronology and paleomagnetism of mafic igneous rocks in the Olenek Uplift, northern Siberia: Implications for Mesoproterozoic supercontinents and paleogeography

We present a new, reliably dated Mesoproterozoic paleopole for Siberia, based on a combined geochronological and paleomagnetic study of mafic rocks within the Mesoproterozoic Sololi Group of the Olenek Uplift in northern Siberia. Ion microprobe (SHRIMP) U–Pb analysis yields crystallisation ages of 2036 ± 11 Ma for zircon from a basement granite and 1473 ± 24 Ma for baddeleyite from a large dolerite sill within the Kyutingde Formation. The baddeleyite result indicates that the lower Sololi Group is significantly older than was suggested by previous K–Ar results. Paleomagnetic analysis of the dolerite sill and related mafic intrusive rocks yields a paleopole at 33.6°N, 253.1°E, A₉₅ = 10.4°. A positive baked-contact test between the Kyutingde sill and sedimentary country rocks shows that the magnetisation is primary. Comparison of this paleopole with coeval results for Laurentia provides a revised reconstruction between Siberia and Laurentia, and implies that these two continents were parts of a single Mesoproterozoic supercontinent since at least 1473 Ma. We argue that Siberia, Laurentia, and Baltica belonged to the same supercontinent between 1473 Ma and mid-Neoproterozoic time.     

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.     

Reconnaissance paleomagnetism of Late Triassic blocks, Kuyul region, Northern Kamchatka Peninsula, Russia

The northernmost Kamchatka Peninsula is located along the northwestern margin of the Bering Sea and consists of complexly deformed accreted terranes. Progressing inland from the northwestern Bering Sea, the Olyutorskiy, Ukelayat and Koryak superterranes (OLY, UKL and KOR) are crossed. These terranes were accreted to the backstop Okhotsk-Chukotsk volcanic-plutonic belt (OChVB) in northernmost Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlaps the terranes and units of the Koryak superterrane, and constrains their accretion time. A paleomagnetic study of blocks within the Kuyul (KUY) terrane of the Koryak superterrane was completed at two localities (Camp 2: λ=61.83°N, φ=165.83°E and Camp 3: λ=61.67°N, φ=164.75°E). At both localities, paleomagnetic samples were collected from Late Triassic (225–208 Ma) limestone blocks (2–10 m in outcrop height) within a melange zone. Although weak in remanent magnetization, two components of remanent magnetization were observed during stepwise thermal demagnetization at 32 sites. The A component of magnetization was observed between room temperature and approximately 250 °C. This magnetic component is always of downward directed inclination and shows the best grouping at relatively low degrees of unfolding. Using McFadden–Reid inclination-only statistics and averaging all site means, the resulting A component mean is I_opt=60.3°, I₉₅=5.0° and n=36 (sites). The B magnetic component is observed up to 565 °C, at which temperature, most samples have no measurable remanent magnetization, or growth of magnetic minerals has disrupted the thermal demagnetization process. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, where bedding orientation differs within a block, most of these sites show the best grouping of B component directions at 100% unfolding, and two of the blocks display remanent magnetizations of both upward and downward directed magnetic inclination. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, the resulting overall B component paleolatitude and associated uncertainty are λ_obs=30.4°N or S, λ₉₅=8.9° and n=19 (sites). When compared with the expected North America paleolatitude of λ_{APWP expected}=57.9°N, our data support a model in which blocks within the Koryak superterrane are allochthonous and far travelled.     

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.     

Palaeomagnetic results from the ca. 1130 Ma Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, Antarctica, and tectonic implications

Detailed palaeomagnetic and rock magnetic analyses provide improved palaeomagnetic results from 23 sites in the Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, East Antarctica. These intrusions are of similar age to their host, the ca. 1130 Ma Ritscherflya Supergroup (RSG). A mean direction of D=235.4°, I=−7.6° with k=45.9 and α₉₅=4.5° was obtained from this study. When combined with previously reported results from 11 sites in the same region, including sites from the Ritscherflya Supergroup, it gives an overall mean direction for 34 sites from the igneous suite with D=236.5°, I=−3.6°, k=27.9 and α₉₅=4.8°. Isothermal remanent magnetization (IRM) experiments on several specimens suggest magnetite or titanomagnetite as the primary remanence carrier, while high temperature magnetic susceptibility experiments indicate the presence of single domain particles. These observations, together with field evidence and the high coercivities and unblocking temperatures, support a primary origin for the observed characteristic remanence. The Borgmassivet palaeomagnetic pole lies at 54.5°E, 8.3°N with A₉₅=3.3°. If Antarctica is moved to its Gondwanan position adjacent to southeast Africa, the Borgmassivet pole (BM) coincides with that of the African well-established, well-dated (1100 Ma) Umkondo Large Igneous Province pole, supporting the hypothesis that the Grunehogna craton of Dronning Maud Land was part of the Kalahari craton of southern Africa at ca. 1100 Ma.     

Paleomagnetism of the middle Cretaceous Iritono granite in the Abukuma region, northeast Japan

We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with ⁴⁰Ar–³⁹Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma ⁴⁰Ar–³⁹Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity (B_c > 50–90 mT) or high blocking temperature (T_b > 350–560 °C) component and (2) L component, relatively low B_c or low T_b component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination (I = 29.9°, D = 311.0°, α₉₅ = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude (I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low-P/high-T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination (I = 42.8°, D = 311.5°, α₉₅ = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point ( 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.     

Paleomagnetic study of Cenozoic sediments from the Zaisan basin (SE Kazakhstan) and the Chuya depression (Siberian Altai): tectonic implications for central Asia

This paper presents new paleomagnetic results on Cenozoic rocks from northern central Asia. Eighteen sites were sampled in Pliocene to Miocene clays and sandy clays of the Zaisan basin (southeastern Kazakhstan) and 12 sites in the upper Oligocene to Pleistocene clays and sandstones of the Chuya depression (Siberian Altai).Thermal demagnetization of isothermal remanent magnetization (IRM) showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the Zaisan basin. Stepwise thermal demagnetization up to 640–660 °C isolated a characteristic (ChRM) component of either normal or reverse polarity at nine sites. At two other sites, the great circles convergence method yielded a definite direction. Measurements of the anisotropy of magnetic susceptibility showed that the hematite-bearing sediments preserved their depositional fabric. These results suggest a primary origin of the ChRM and were substantiated by positive fold and reversal tests. The mean paleomagnetic direction for the Zaisan basin (D=9°, I=59°, k=19, α₉₅=11°) is close to the expected direction derived from the APW path of Eurasia [J. Geophys. Res. 96 (1991) 4029] and shows that the basin did not rotated relative to stable Asia during the Tertiary.In the upper Pliocene–Pleistocene sandstones of the Chuya depression, a very stable ChRM carried by hematite was found. Its mean direction (D=9°, I=46°, k=25, α₉₅=7°) is characterized by declination close to the one excepted for early Quaternary, whereas inclination is lower. In the middle Miocene to lower Pliocene clays and sandstones, a stable ChRM of both normal and reverse polarities carried by magnetite was isolated. Its mean direction (D=332°, I=63°, k=31, α₉₅=4°) is deviated with respect to the reference direction and implies a Neogene, 39±8° counterclockwise rotation of the Chuya depression relative to stable Asia. These results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style, predominantly compressive in the Zaisan basin and transpressive in the Siberian Altai. At a larger scale, the pattern of vertical axis rotations deduced from paleomagnetic data in northern central Asia is consistent with the hypothesis of a large left-lateral shear zone running from the Pamirs to the Baikal. Heterogeneous rotations, however, indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations.     

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.     

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.     

Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and Ar/Ar study of the Jurassic Fourth of July Batholith, Canadian Cordillera

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. ⁴⁰Ar/³⁹Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of 165 Ma, which dates cooling through 280 °C.Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by 9°.Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N (d_p=5°, d_m=5°) and a tilt-corrected paleopole at 81°W, 55°N (d_p=5°, d_m=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated.