Tectonic implications of a paleomagnetic study of the Sarmiento Ophiolitic Complex, southern Chile

A paleomagnetic study was carried out on the Late Jurassic Sarmiento Ophiolitic Complex (SOC) exposed in the Magallanes fold and thrust belt in the southern Patagonian Andes (southern Chile). This complex, mainly consisting of a thick succession of pillow-lavas, sheeted dikes and gabbros, is a seafloor remnant of the Late Jurassic to Early Cretaceous Rocas Verdes basin that developed along the south-western margin of South America. Stepwise thermal and alternating field demagnetization permitted the isolation of a post-folding characteristic remanence, apparently carried by fine grain (SD?) magnetite, both in the pillow-lavas and dikes. The mean “in situ” direction for the SOC is Dec: 286.9°, Inc: − 58.5°, α95: 6.9°, N: 11 (sites).Rock magnetic properties, petrography and whole-rock K–Ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the Late Cretaceous.The mean remanence direction is anomalous relative to the expected Late Cretaceous direction from stable South America. Particularly, a declination anomaly over 50° is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the Jurassic El Quemado Fm. located over 100 km north of the study area in Argentina. Nevertheless, a significant ccw rotation of the whole SOC is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back-arc basin sub-parallel to the continental margin. A rigid-body 30° westward tilting of the SOC block around a horizontal axis trending NNW, is considered a much simpler explanation, being consistent with geologic evidence. This may have occurred as a consequence of inverse reactivation of old normal faults, which limit both the SOC exposures and the Cordillera Sarmiento to the East. The age of tilting is unknown but it must postdate remanence acquisition in the Late Cretaceous. Two major orogenic events of the southern Patagonian Andes, in the Eocene (ca. 42 Ma) and Middle Miocene (ca. 12 Ma), respectively, could have caused the proposed tilting.     

Further paleomagnetic results from the ~ 155 Ma Tiaojishan Formation,Yanshan Belt,North China,and their implications for the tectonic evolution of the Mongol–Okhotsk suture

A new paleomagnetic study on well-dated (~ 155 Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A₉₅ = 5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~ 155 Ma. Our calculation shows a ~ 1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~ 155 Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~ 155 Ma.     

Oroclinal bending in the Alborz Mountains (Northern Iran): New constraints on the age of South Caspian subduction and extrusion tectonics

In this study, we report an extensive paleomagnetic study (76 sites) carried out in the Alborz Mts. (northern Iran), with the aim of reconstructing the rotation history and the origin of curvature of this orogenic chain. The analyzed deposits are the sedimentary successions of the Upper Red Formation (Miocene), Lower Red Formation (Oligocene) and Eocene clastic units. Paleomagnetic results indicate that the Alborz Mts. can be considered a secondary arc that originated as a linear mountain belt that progressively acquired its present day curvature through opposite vertical axis rotations along its strike. The curvature of the arc was entirely acquired after the middle-late Miocene, which is the age of the youngest investigated sediments (Upper Red Formation). Overall, our paleomagnetic data indicate that the Alborz Mts. can be considered an orocline.Our results define, for the first time, the rotational history of the entire Alborz curved mountain belt, and enable us to reconstruct the paleogeographic and tectonic evolution of northern Iran in the framework of Arabia-Eurasia continental deformation. The kinematics inferred by the pattern of paleomagnetic rotations is at odds with the present day kinematics of northern Iran, characterized by the westward extrusion of the South Caspian block, and by a left lateral shear between Central Iran and the central and western sectors of the Alborz Mts. By integrating paleomagnetic data with stratigraphic, thermochronological, structural and GPS information, we propose that the initiation of South Caspian subduction and the activation of westward extrusion of South Caspian block occurred diachronously and that the initiation of the present-day kinematics of northern Iran was quite recent (Lower Pleistocene, < 2 Ma).     

Tectonic rotations in the Deseado Massif, southern Patagonia, during the breakup of Western Gondwana

Paleomagnetic investigation in the Deseado Massif, southern Patagonia, suggests that Triassic sedimentary rocks carry a latest Triassic to Jurassic remagnetization and that earliest Jurassic plutonic complexes carry a reversed polarity magnetization of thermoremanent origin. Despite uncertainties in the timing of the observed remanence in the Triassic rocks and the lack of paleohorizontal control on the plutonic complexes, comparison of the derived pole positions with the most reliable Late Triassic–Jurassic apparent polar wander paths indicates that the study areas underwent significant clockwise vertical-axis rotation. In contrast, paleomagnetic results from mid-Cretaceous rocks in the region indicate no rotation. The observed crustal rotations in the Deseado Massif are thus bracketed to have occurred between Jurassic and Early Cretaceous times, documenting southern Patagonian deformation during the breakup of Western Gondwana and then enlarging the regional record of clockwise rotations associated with this event. These results suggest a more complex than previously supposed tectonic evolution of this part of South America.     

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 of the Central Iberian curve's putative hinge: Too many oroclines in the Iberian Variscides

The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~ 70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve.     

Geologic and geometric constraints on a kinematic model of the Bolivian orocline

The “Bolivian orocline” is the change in trend of the Andes from NW to N near 18°S. Paleomagnetic data have been used to infer that this bend was in part produced by wholesale opposed rotations (15–20° counterclockwise north of the bend and 15–20° clockwise south of it) of the two limbs of the orocline. Besides paleomagnetic data, shortening estimates from balanced cross-sections and variations in crustal cross-sectional area provide quantitative information on rotations and translations. The three sets of data do not agree closely, and therefore only loosely constrain the kinematics of the orocline. A map view kinematic model incorporating the major faults gives a more detailed picture. The resulting displacement field suggests that movement perpendicular to the orogen increases toward the bend, whereas the component of orogen-parallel movement increases away from it. Only weak rotations are indicated for the bend region. It is speculated that large-scale regional rotations of the limbs of the orocline are markedly lower than previously suggested, probably of the order of 5–10°. Stronger rotations are associated with strong lateral shortening gradients or may result from superimposed local phenomena.     

Paleomagnetism of the Late Paleozoic granites of the Angara-Vitim batholith and the host rocks of the Baikal-Patom folded area: Tectonic implications

The paper presents the results of paleomagnetic and geochronological studies of the Late Paleozoic granites of the Angara-Vitim batholith as well as Vendian-Early Cambrian sedimentary rocks and Late Devonian subvolcanic rocks of the Patom margin of the Siberian Platform. Primary and metachronous magnetization in the rocks of the study region was used to calculate an Early Permian (~ 290 Ma) paleomagnetic pole, which is proposed as a reference pole for the Siberian Platform in paleomagnetic reconstructions, plotting of the apparent polar-wander path curve, and other magnetotectonic studies. The published and obtained paleomagnetic data and analysis of the geological data confirm the Late Paleozoic age of the final folding in the Baikal-Patom area. Possible causes of Late Paleozoic deformations and large-scale granite formation in the Baikal-Patom area and Transbaikalia in the Late Paleozoic are discussed.     

Paleomagnetism of the late Cryogenian Nantuo Formation and paleogeographic implications for the South China Block

A new paleomagnetic pole position is obtained from the well-dated (636.3 ± 4.9 Ma) Nantuo Formation in the Guzhang section, western Hunan Province, and the correlative Long’e section in eastern Guizhou Province, South China. Remagnetization of the recent geomagnetic field was identified and removed for both sections. The hard dual-polarity, interpreted as primary, component of the Nantuo Formation, directs east–westward with medium inclinations, yielding an average pole of 9.3°N, 165°E, A₉₅ = 4.3° that, for the first time, passed a strata-bound reversals test. The new data are consistent with previously published paleomagnetic data of the Nantuo Formation from Malong county, central Yunnan Province, which passed a positive syn-sedimentary fold test. Together, these sites represent shallow- to deep-water sections across a shelf-to-basin transect centered at ∼33° paleolatitude. The sedimentary basin may have faced an expansive ocean toward the paleo-East. In the ∼750 Ma and ∼635 Ma global reconstructions, the South China Block (SCB) was best fitted in the northern hemisphere close to northwestern Australia. However, a direct SCB-northwestern Australia connection, inferred to have existed during the Early Cambrian–Early Devonian, had not formed by the time of ∼635 Ma.     

New thermochronometric constraints on the rapid Palaeogene exhumation of the Cordillera Darwin complex and related thrust sheets in the Fuegian Andes

Thermal modelling of new fission‐track and (U–Th–Sm)/He data from the Fuegian Andes reveals rapid cooling (～12–48 °C Ma^?1) during the middle and late Eocene followed by slow cooling ( ～ 1.5 °C Ma^?1) to the Recent. We interpret the rapid cooling as a result of exhumation from contractional uplift within the crystalline interior of the orogen. This interpretation is consistent with independent evidence of Eocene shortening, flexural subsidence and provenance changes in the study area, and is approximately coeval with marine geochemical evidence of the onset of Drake Passage opening. In light of the Palaeogene history of Nazca‐South American plate convergence and the differences in shortening magnitudes and exhumation histories between the Fuegian and Patagonian Andes, our data support Eocene development of the Patagonian orocline, which also provides a plausible explanation for early opening of Drake Passage.     

The longest voyage: Tectonic,magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia

The tectonic evolution of the Indian plate, which started in Late Jurassic about 167 million years ago (~ 167 Ma) with the breakup of Gondwana, presents an exceptional and intricate case history against which a variety of plate tectonic events such as: continental breakup, sea-floor spreading, birth of new oceans, flood basalt volcanism, hotspot tracks, transform faults, subduction, obduction, continental collision, accretion, and mountain building can be investigated. Plate tectonic maps are presented here illustrating the repeated rifting of the Indian plate from surrounding Gondwana continents, its northward migration, and its collision first with the Kohistan–Ladakh Arc at the Indus Suture Zone, and then with Tibet at the Shyok–Tsangpo Suture. The associations between flood basalts and the recurrent separation of the Indian plate from Gondwana are assessed. The breakup of India from Gondwana and the opening of the Indian Ocean is thought to have been caused by plate tectonic forces (i.e., slab pull emanating from the subduction of the Tethyan ocean floor beneath Eurasia) which were localized along zones of weakness caused by mantle plumes (Bouvet, Marion, Kerguelen, and Reunion plumes). The sequential spreading of the Southwest Indian Ridge/Davie Ridge, Southeast Indian Ridge, Central Indian Ridge, Palitana Ridge, and Carlsberg Ridge in the Indian Ocean were responsible for the fragmentation of the Indian plate during the Late Jurassic and Cretaceous times. The Réunion and the Kerguelen plumes left two spectacular hotspot tracks on either side of the Indian plate. With the breakup of Gondwana, India remained isolated as an island continent, but reestablished its biotic links with Africa during the Late Cretaceous during its collision with the Kohistan–Ladakh Arc (~ 85 Ma) along the Indus Suture. Soon after the Deccan eruption, India drifted northward as an island continent by rapid motion carrying Gondwana biota, about 20 cm/year, between 67 Ma to 50 Ma; it slowed down dramatically to 5 cm/year during its collision with Asia in Early Eocene (~ 50 Ma). A northern corridor was established between India and Asia soon after the collision allowing faunal interchange. This is reflected by mixed Gondwana and Eurasian elements in the fossil record preserved in several continental Eocene formations of India. A revised India–Asia collision model suggests that the Indus Suture represents the obduction zone between India and the Kohistan–Ladakh Arc, whereas the Shyok-Suture represents the collision between the Kohistan–Ladakh arc and Tibet. Eventually, the Indus–Tsangpo Zone became the locus of the final India–Asia collision, which probably began in Early Eocene (~ 50 Ma) with the closure of Neotethys Ocean. The post-collisional tectonics for the last 50 million years is best expressed in the evolution of the Himalaya–Tibetan orogen. The great thickness of crust beneath Tibet and Himalaya and a series of north vergent thrust zones in the Himalaya and the south-vergent subduction zones in Tibetan Plateau suggest the progressive convergence between India and Asia of about 2500 km since the time of collision. In the early Eohimalayan phase (~ 50 to 25 Ma) of Himalayan orogeny (Middle Eocene–Late Oligocene), thick sediments on the leading edge of the Indian plate were squeezed, folded, and faulted to form the Tethyan Himalaya. With continuing convergence of India, the architecture of the Himalayan–Tibetan orogen is dominated by deformational structures developed in the Neogene Period during the Neohimalayan phase (~ 21 Ma to present), creating a series of north-vergent thrust belt systems such as the Main Central Thrust, the Main Boundary Thrust, and the Main Frontal Thrust to accommodate crustal shortening. Neogene molassic sediment shed from the rise of the Himalaya was deposited in a nearly continuous foreland trough in the Siwalik Group containing rich vertebrate assemblages. Tomographic imaging of the India–Asia orogen reveals that Indian lithospheric slab has been subducted subhorizontally beneath the entire Tibetan Plateau that has played a key role in the uplift of the Tibetan Plateau. The low-viscosity channel flow in response to topographic loading of Tibet provides a mechanism to explain the Himalayan–Tibetan orogen. From the start of its voyage in Southern Hemisphere, to its final impact with the Asia, the Indian plate has experienced changes in climatic conditions both short-term and long-term. We present a series of paleoclimatic maps illustrating the temperature and precipitation conditions based on estimates of Fast Ocean Atmospheric Model (FOAM), a coupled global climate model. The uplift of the Himalaya–Tibetan Plateau above the snow line created two most important global climate phenomena—the birth of the Asian monsoon and the onset of Pleistocene glaciation. As the mountains rose, and the monsoon rains intensified, increasing erosional sediments from the Himalaya were carried down by the Ganga River in the east and the Indus River in the west, and were deposited in two great deep-sea fans, the Bengal and the Indus. Vertebrate fossils provide additional resolution for the timing of three crucial tectonic events: India–KL Arc collision during the Late Cretaceous, India–Asia collision during the Early Eocene, and the rise of the Himalaya during the Early Miocene.     

Palaeozoic to Early Jurassic history of the northwestern corner of Gondwana,and implications for the evolution of the Iapetus,Rheic and Pacific Oceans

The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic.     

Crustal segments in the North Patagonian Massif,Patagonia: An integrated perspective based on Sm–Nd isotope systematics

New insights on the Paleozoic evolution of the continental crust in the North Patagonian Massif are presented based on the analysis of Sm–Nd systematics. New evidence is presented to constrain tectonic models for the origin of Patagonia and its relations with the South American crustal blocks. Geologic, isotopic and tectonic characterization of the North Patagonian Massif and comparison of the Nd parameters lead us to conclude that: (1) The North Patagonian Massif is a crustal block with bulk crustal average ages between 2.1 and 1.6 Ga T_DM (Nd) and (2) At least three metamorphic episodes could be identified in the Paleozoic rocks of the North Patagonian Massif. In the northeastern corner, Famatinian metamorphism is widely identified. However field and petrographic evidence indicate a Middle to Late Cambrian metamorphism pre-dating the emplacement of the ca. 475 Ma granitoids. In the southwestern area, are apparent 425–420 Ma (?) and 380–360 Ma metamorphic peaks. The latter episode might have resulted from the collision of the Antonia terrane; and (3) Early Paleozoic magmatism in the northeastern area is coeval with the Famatinian arc. Nd isotopic compositions reveal that Ordovician magmatism was associated with attenuated crust. On the southwestern border, the first magmatic recycling record is Devonian. Nd data shows a step by step melting of different levels of the continental crust in the Late Palaeozoic. Between 330 and 295 Ma magmatism was likely the product of a crustal source with an average 1.5 Ga T_DM (Nd). Widespread magmatism represented by the 295–260 Ma granitoids involved a lower crustal mafic source, and continued with massive shallower-acid plutono volcanic complexes which might have recycled an upper crustal segment of the Proterozoic continental basement, resulting in a more felsic crust until the Triassic. (4) Sm–Nd parameters and detrital zircon age patterns of Early Paleozoic (meta)-sedimentary rocks from the North Patagonian Massif and those from the neighboring blocks, suggest crustal continuity between Eastern Sierras Pampeanas, southern Arequipa-Antofalla and the northeastern sector of the North Patagonian Massif by the Early Paleozoic. This evidence suggests that, at least, this corner of the North Patagonian Massif is not allochthonous to Gondwana. A Late Paleozoic frontal collision with the southwestern margin of Gondwana can be reconcilied in a para-autochthonous model including a rifting event from a similar or neighbouring position to its post-collision location. Possible Proterozoic or Early Paleozoic connections of the NPM with the Kalahari craton or the western Antartic blocks should be investigated.     

The stratoid granites of central Madagascar: paleomagnetism and further age constraints on neoproterozoic deformation

A paleomagnetic and ⁴⁰Ar/³⁹Ar study of a 630-Ma alkaline granite suite in Madagascar, the so-called ‘stratoid’ granites, reveals a complex history of remagnetization during the formation of the Antananarivo Zone de Virgation at ∼560 Ma (D₂) and the Angavo shear zone at ∼550 Ma (D₃). ⁴⁰Ar/³⁹Ar dating of hornblende, biotite and potassium feldspar from rocks affected by D₂/D₃ show initial cooling rates of 8 °C/Ma during the 550–520 Ma interval followed by slower cooling of 2.5 °C/Ma. The thermal effects of the D₂ and D₃ events appear to be restricted to regions surrounding the shear zones as evidenced by a ⁴⁰Ar/³⁹Ar biotite age of 611.9±1.7 Ma north of the virgation zone. The paleomagnetic data from the stratoid granites are complex and some sites, particularly in areas to the north of the virgation zone, may have been rotated about non-vertical axes following their emplacement and cooling. Because of these possible rotations, our best estimate for the paleomagnetic pole for Madagascar is derived from sites within the virgation zone. This pole falls at 6.7°S, 352.6°E (a₉₅=14.2°). A post-metamorphic cooling history for the virgation zone indicates a magnetization age of 521.4±11.9 Ma. Our work in central Madagascar, coupled with previous studies, suggests that emplacement of the 630 Ma stratoid granites followed a collisional (?) tectonic event beginning around 650 Ma, recently recognized in southern Madagascar and in Tanzania. Subsequently, the stratoid granites in the Antananarivo virgation zone were reheated (∼750–800 °C) at pressures between 3.5 and 3.6 kbars resulting in a pervasive remagnetization. We suggest that the younger shear events are genetically related to collisional tectonics elsewhere during the final stages of Gondwana assembly and are a consequence of the Kuunga Orogeny further south.     

Differential timing of vertical-axis block rotations in the northern Ryukyu Arc: Paleomagnetic evidence from the Koshikijima Islands,Japan

Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7^°, I = 51.8^°, α₉₅ = 9.6^°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1^°, I = 57.7^°, α₉₅ = 7.1^°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40^° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F₁ faults) without vertical-axis block rotation, (2) strike-slip reactivation of F₁ faults and oblique-normal displacement on NE–SW-trending faults (F₂ faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F₂ faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks.     

The 1420 Ma Indiavaí Mafic Intrusion (SW Amazonian Craton): Paleomagnetic results and implications for the Columbia supercontinent

The configuration and the timing of assembly and break-up of Columbia are still matter of debate. In order to improve our knowledge about the Mesoproterozoic evolution of Columbia, a paleomagnetic study was carried out on the 1420 Ma Indiavaí mafic intrusive rocks that crosscut the polycyclic Proterozoic basement of the SW Amazonian Craton, in southwestern Mato Grosso State (Brazil). Alternating field and thermal demagnetization revealed south/southwest ChRM directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests, including thermomagnetic data, hysteresis data and the progressive acquisition of isothermal remanent magnetization. Different stable magnetization components isolated in host rocks from the basement 10 km NW away to the Indiavaí intrusion, further support the primary origin of the ChRM. A mean of the site mean directions was calculated at Dm = 209.8°, Im = 50.7° (α₉₅ = 8.0°, K = 22.1), which yielded a paleomagnetic pole located at 249.7°E, 57.0°S (A₉₅ = 8.6°). The similarity of this pole with the recently published 1420 Ma pole from the Nova Guarita dykes in northern Mato Grosso State suggests a similar tectonic framework for these two sites located 600 km apart, implying the bulk rigidity of the Rondonian-San Ignacio crust at that time. Furthermore these data provide new insights on the tectonic significance of the 1100–1000 Ma Nova Brasilândia belt—a major EW feature that cuts across the basement rocks of this province, which can now be interpreted as intracratonic, in contrast to previous interpretation. From a global perspective, a new Mesoproterozoic paleogeography of Columbia has been proposed based on comparison of these 1420 Ma poles and a 1780 Ma pole from Amazonia with other paleomagnetic poles of similar age from Baltica and Laurentia, a reconstruction in agreement with geological correlations.     

Turmoil before the boring billion: Paleomagnetism of the 1880–1860 Ma Uatumã event in the Amazonian craton

The Uatumã silicic large igneous province (SLIP) has covered about 1,500,000 km² of the Amazonian craton at ca. 1880 Ma, when the Columbia/Nuna supercontinent has been assembled. Paleomagnetic and geochronological data for this unit were obtained for the Santa Rosa and Sobreiro Formations in the Carajás Province, southwestern Amazonian craton (Central-Brazil Shield). AF and thermal demagnetizations revealed northern (southern) directions with high upward (downward) inclinations (component SF1), which passes a ‘B’ reversal test, and is carried by magnetite and SD hematite with high-blocking temperature. This component is present on well-dated 1877.4 ± 4.3 Ma (U-Pb zrn - LA-ICPMS) rhyolitic lava flows, providing the SF1 key paleomagnetic pole (Q = 6) located at 319.7°E, 24.7°S (A₉₅ = 16.9°). A second southwestern (northeastern) direction with low inclination (Component SF2) was obtained for a well-dated 1853.7 ± 6.2 Ma (U-Pb zrn - LA-ICPMS) dike of the Velho Guilherme Suite. This component also appears as a secondary component in the host rhyolites of the Santa Rosa Fm and andesites of the Sobreiro Fm at the margins of the dike previously dated. Its primary origin is confirmed by a positive baked contact test, where a Velho Guilherme dike crosscuts the 1880 Ma andesite from the Sobreiro Formation. The corresponding SF2 key pole is located at 220.1°E, 31.1°S (A₉₅ = 5°) and is classified with a reliability criterion Q = 7. The large angular distance between the almost coeval (difference of ~ 25 Ma) SF1 and SF2 poles implies high plate velocities (~ 39.3 cm/yr) which are not consistent with modern plate tectonics. The similar significant discrepancy of paleomagnetic poles with ages between 1880 and 1860 Ma observed in several cratons could be explained by a true polar wander (TPW) event. This event is the consequence of the reorganization of the whole mantle convection, and is supported by paleomagnetic reconstructions at 1880 Ma and 1860 Ma and also by geological/geochronological evidence.     

Paleomagnetism of the Late Cretaceous intrusions from the Minusa trough (southern Siberia)

The paper summarizes paleomagnetic and rock-magnetic data on the Late Cretaceous diatremes and associated dikes from the Minusa trough located within the southwestern Siberian Platform. It is shown that the stable characteristic component of magnetization is superimposed magnetization (in physical sense). It is linked to Fe-rich titanomagnetite produced by the decay and oxidation of Ti-rich titanomagnetite derived from a primary magma. This process, however, coincides in time with the intrusion cooling, which is supported by paleomagnetic tests. Correlation of magnetic polarity with ³⁹Ar/⁴⁰Ar ages suggests that the acquired stable characteristic component of magnetization corresponds to magnetic Chrons C33-C32 and characterizes the Middle Campanian magnetic field (74–82 Ma). The mean paleomagnetic pole for this span is located at 82.8° N, 188.5° E, with α₉₅ = 6.1 and, within confidence intervals, coincides with the reference data from the European part of the Eurasian plate. The excellent agreement between virtual paleomagnetic poles testifies that the intraplate motions in the Mesozoic resulting in the crust deformation of Central Asia ceased in the late Cretaceous or were so small that elude detection by the paleomagnetic method.     

New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria

The central-western and the eastern Southern Alps are separated by the triangular shaped Adige embayment, which belongs to stable Adria and was the site of pelagic sedimentation from the Tithonian through Maastrichtian. The first part of this study presents paleomagnetic results from the Tithonian–Cenomanian Biancone and Turonian–Maastrichtian Scaglia Rossa formations sampled at 33 geographically distributed and biostratigraphically dated localities.The new and high quality paleomagnetic results from the Adige embayment are then combined with coeval paleomagnetic directions from autochthonous Istria (Márton et al., 2008), which also belongs to stable Adria. The combined data set (which for the Late Albian–Maastrichtian time period is constructed similarly to the synthetic African curve by Besse and Courtillot, 2002, 2003) reveals an important tectonic event (Late Aptian–Early Albian) characterized by 20° CCW rotation and sedimentary hiatus.Comparison between paleomagnetic declinations/inclinations expected in an African framework (i.e. with the assumption that Adria is still an African promontory) leads to the following conclusions. The time-distributed Tithonian and Berriasian (150–135 Ma) paleomagnetic directions exhibit the “African hairpin” with an inclination minimum and a sudden change from CW to CCW rotation at 145 Ma. Concerning the younger ages, the declinations for Adria continue to follow the African trend of CCW rotation till the end of Cretaceous. However, the Tithonian–Maastrichtian declination curve for stable Adria is displaced by 10° from the “African” curve as a result of two rotations. The first, an about 20° CW rotation of Adria with respect to Africa took place between the Maastrichtian and the mid-Eocene. During this time the orientation of Adria remained the same, while Africa continued its CCW rotation. The younger rotation (30°CCW) changed the orientation of Adria relative to Africa as well as to the present North.     

Ediacaran–Early Ordovician paleomagnetism of Baltica: A review

The Ediacaran–Early Ordovician interval is of great interest to paleogeographer's due to the vast evolutionary changes that occurred during this interval as well as other global changes in the marine, atmospheric and terrestrial systems. It is; however, precisely this time period where there are often wildly contradictory paleomagnetic results from similar-age rocks. These contradictions are often explained with a variety of innovative (and non-uniformitarian) scenarios such as intertial interchange true polar wander, true polar wander and/or non-dipolar magnetic fields. While these novel explanations may be the cause of the seemingly contradictory data, it is important to examine the paleomagnetic database for other potential issues.This review takes a careful and critical look at the paleomagnetic database from Baltica. Based on some new data and a re-evaluation of older data, the relationships between Baltica and Laurentia are examined for ~ 600–500 Ma interval. The new data from the Hedmark Group (Norway) confirms suspicions about possible remagnetization of the Fen Complex pole. For other Baltica results, data from sedimentary units were evaluated for the effects of inclination shallowing. In this review, a small correction was applied to sedimentary paleomagnetic data from Baltica. The filtered dataset does not demand extreme rates of latitudinal drift or apparent polar wander, but it does require complex gyrations of Baltica over the pole. In particular, average rates of APW range from 1.5° to 2.0°/Myr. This range of APW rates is consistent with ‘normal’ plate motion although the total path length (and its oscillatory nature) may indicate a component of true polar wander. In the TPW scenario, the motion of Baltica results in a back and forth path over the south pole between 600 and 550 Ma and again between 550 and 500 Ma. The rapid motion of Baltica over the pole is consistent with the extant database, but other explanations are possible given the relative paucity of high-quality paleomagnetic data during the Ediacaran–Cambrian interval from Baltica and other continental blocks.A sequence of three paleogeographic maps for Laurentia and Baltica is presented. Given the caveats involved in these reconstructions (polarity ambiguity, longitudinal uncertainty and errors), the data are consistent with geological models that posit the opening of the Iapetus Ocean around 600 Ma and subsequent evolution of the Baltica–Laurentia margin in the Late Ediacaran to Early Ordovician, but the complexity of the motion implied by the APWP remains enigmatic.