Paleomagnetic Survey Research Articles

Экспедиция «Итуруп 2022–2023»: основные направления работ и предварительные результаты

The paper provides information on the objectives, methods, targets and some preliminary results of the expeditionary work carried out within the framework of the RSF project No.21-17-00049 by the employees of the Institute of Volcanology and Seismology of the FEB RAS, Institute of Marine Geology and Geophysics of the FEB RAS, Pacific Geographical Institute of the FEB RAS and Trofimuk Institute of Petroleum-Gas Geology and Geophysics of the SB RAS in 2022 and 2023. The main objectives of the expedition were geological-volcanological, hydrogeological, geophysical, tephrochronological, and paleoseismological studies. In accordance with the set objectives, field teams were formed, which began work in February 2022. On the basis of the data obtained during the expedition, previously unknown hydrothermal manifestations and seismic events on Iturup Island were identified. A number of geophysical and paleomagnetic surveys were carried out.

Paleomagnetic survey of the Goulburn Supergroup, Kilohigok Basin, Nunavut, Canada: Toward an understanding of the Orosirian apparent polar wander path of the Slave craton

The Orosirian paleopoles from the three circum-Slave basins (i.e., the Great Slave, Coronation, and Kilohigok Basins) of the Slave craton show large (∼110°) and back-and-forth swings at 1.96–1.87 Ga, known as the Coronation loops. The Coronation loops, taken at face value, would imply rapid and substantial spin motions of the Slave craton, which is at odds with modern plate tectonics. Alternatively, the Coronation loops have been interpreted as a product of basin-scale rotations, local-scale vertical-axis rotations, or inertial interchange true polar wander (IITPW). One way to differentiate these models is to take advantage of the well-correlated stratigraphy in three circum-Slave basins and directly compare the time-equivalent paleomagnetic results. In this study, we collected ∼300 samples from nine formations from the Goulburn Supergroup of the Kilohigok Basin, in shallowly dipping autochthonous sections east of the Bathurst Fault. We provide seven new reconnaissance-level paleopoles of the Slave craton, namely from the Kenyon, Hackett, Rifle, Beechey, Link, Kuuvik, and Brown Sound Formations of the Goulburn Supergroup. Our results and the compiled Orosirian paleomagnetic data of the Slave craton suggest that although basin-scale rotation or local vertical-axis rotation in fault zones are able to explain some of the disagreements among time-correlative paleopoles, they could not account for the large declination variation within the homoclinal sections in individual basins. Notably, our results from the ∼1963 Ma Rifle Formation show progressive changes in declination through the stratigraphy, which cannot be explained by either basin-scale or local vertical-axis rotations. Selective remagnetization is also considered unlikely to be the cause. Instead, we suggest that IITPW could potentially be responsible for the Coronation loops, which could also provide an explanation for some discrepant paleomagnetic data observed globally during the Orosirian time.

An integrated magnetic survey on lava flows associated to the Paricutin volcano (Western Mexico)

We report a detailed aeromagnetic, field magnetometry and paleomagnetic survey from the most recent volcano on the American continent that emerged in February of 1943. Paleomagnetic measurements were performed to determine the characteristic remanent magnetization directions, considering that the Geomagnetic Field observed in nearby existing observatories at the time of extrusions is well known for the time of eruption of this volcano. The paleomagnetic records of the Paricutin flows showed lower values compared to the Teoloyucan Geomagnetic Observatory and those estimated with the International Geomagnetic Reference Field model (IGRF). Also, an aeromagnetic data modeling provided by the Mexican Geological Service together with field magnetometry along two profiles in perpendicular directions were carried out. The main objective was to model two levels of the internal structure, one that has to do with deep sources (not exclusively associated with the volcano itself), and another one of a more superficial area directly related to recent eruptions and the immediate geological basement.

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

AbstractThe Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (−9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.

Reassessment of the eruptive chronology of El Metate shield volcano (central-western Mexico) based on a comprehensive rock-magnetic, paleomagnetic and multi-approach paleointensity survey

Abstract We study seven out of the thirteen lava flows associated to El Metate shield volcano using paleomagnetic, absolute intensity and rock-magnetic experiments. Characteristic paleomagnetic directions were successfully determined from most of the flows. A combined mean direction of D = 346.4° and I = 32.1° (N = 34, R = 33.73, k = 121.74, α95 = 2.2) was obtained for the early flows (Flow 1 − Flow 3). Flow 6a (a flow from the late emplacement stage) yielded a statistically distinguishable mean paleomagnetic direction of D = 359.2° and I = 29.6° (N = 9, R = 8.97, k = 294.6, α95 = 3.0). Furthermore, the significant between-flow variation of absolute paleointensity values obtained using two different methodologies, which resembles the trend followed by that of the global model SHA. DIF 14k intensity curve for the period 1000 AD 1600 AD, disagrees with previous hypothesis supporting a short emplacement period between 50 and 100 years for the whole sequence. Additionally, paleodirectional and paleointensity dating of the studied flows allowed to refine the eruptive chronology indicating that early flows were produced over a short time period, whereas at least two of the later flows were erupted 200 years later; the activity ending just prior to the arrival of the Spaniards in ∼1520. Under these circumstances, and in light of the presented high quality paleomagnetic and paleointensity data, the assumed monogenetic nature of El Metate volcano seems to be questionable.

Rotations of Normal Fault Blocks Quantify Extension in the Central Tauride Intramontane Basins, SW Turkey

AbstractQuantifying the amount of stretching in extensional basin systems is often challenging in the absence of seismic profiles or boreholes. However, when fault spacing and orientation as well as vertical axis rotation patterns are known, map‐view restoration may provide a good estimate of total extension. This integrated structural and paleomagnetic approach provides a relatively straightforward tool in extensional basin restoration and fault zone kinematic analysis. Here we provide results of an extensive paleomagnetic survey of the Neogene Central Tauride intramontane basins (SW Turkey), where previous work revealed a complex array of basin‐bounding normal faults and relay ramps. In total, 437 oriented cores were sampled at 43 sites distributed within Miocene‐Pliocene continental sedimentary rocks from the Ilgın, Altınapa, Yalvaç, and Beyşehir basins. Despite the more or less coherent overall strike of the mountain belt and basins, rotations vary from ~42° clockwise (Yalvaç) to ~10° (Beyşehir), ~21° (Ilgın), and ~30° (Altınapa) counterclockwise. We show that the rotation pattern is related to normal faults and lateral variations in fault displacement superimposed on regional rotation patterns. We restore these to estimate a minimum NE‐SW horizontal extension of ~30–35 km across the basin system. As a consequence of our reconstruction, it appears that the Sultandağları range that exposes low‐grade metamorphic Paleozoic and Mesozoic rocks of the Geyikdağı and Bolkardağ nappes of the Taurides represents a Miocene extensional core complex.

Rock-magnetic and paleomagnetic survey on dated lava flows erupted during the Bruhnes and Matuyama chrons: the Mascota Volcanic Field revisited (Western Mexico)

A rock magnetic and paleomagnetic investigation was performed on some selected, radiometrically dated lava flows from the Mascota Volcanic Field (MVF), western Trans- Mexican Volcanic Belt. A set of rock-magnetic experiments and standard paleomagnetic analysis were carried out on 19 sites spanning the time interval from 2268 to 72 kyr. The paleomagnetic directions are anchored to absolute radiometric ages while no such information was available in previous studies. This makes possible to correctly evaluate the fluctuation of Earth’s magnetic field from Pliocene to Pleistocene and reveal the firm evidence of possible Levantine excursion. Both Ti-poor and Ti-rich titanomagnetites seem to carry the remanent magnetization with Curie temperatures ranging from 350°C to 537°C. Thirteen flows correspond to the Brunhes chron, one of them exhibits transitional directions, while the remaining six sites belong to the Matuyama chron. New and existing dataset for MVF were used to estimate the paleosecular variation parameters. The selected data include 35 Plio-Quaternary lava flows. After excluding the poor quality data, as well as the transitional directions, the mean paleodeclination is 356.1° and oaleoinclination 39.9°, which agree well with the geocentric axial dipole (GAD) and the expected paleodirections for the Plio-Pleistocene, as derived from the reference poles for the stable North America. The corresponding mean paleomagnetic poles are paleolongitude 226.7° and paleolatitude 86.0°. The virtual geomagnetic pole scatter for the MVF is 15.2°, which is consistent with the value expected from model G at latitude of 20° (this model provides an interpretation of the paleosecular variation at different latitudes for the time of interest). The combined paleomagnetic data, supported by positive reversal test, indicate no paleomagnetically detectable vertical-axis rotations in the study area. The evidence of one transitional directions was detected, which may correspond to the Levantine excursion (360-370 kyr) or unnamed event between 400-420 kyr.

A detailed paleomagnetic and rock-magnetic investigation around Cretaceous-Paleogene boundary: the Autlan (Western Mexico) volcanic sequence revisited

We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous.

Extension of the Middle Miocene Kleifakot geomagnetic instability event in Ísafjörður, Northwest Iceland

Detailed records of past geomagnetic polarity transitions and excursions are seldom found in extrusive volcanics. One type of such events seems to involve very irregular variations of the magnetic field direction. Several examples of that type have been discovered in paleomagnetic surveys on the Neogene lava pile of Iceland. Lava sequences spanning the most notable event are accessible in the two tributary fjords Mjóifjörður and Ísafjörður south of Ísafjarðardjúp, Northwest Iceland. Paleofield direction results from about 80 sampling sites in these sequences have been described in previous publications. The present paper adds 20 sampled sites in Ísafjörður, extending the area where parts of this „Kleifakot instability event“ of the geomagnetic field are recorded, to 5–6 kilometers along the fjord. Intermediate paleofield directions in the collection often agree closely with each other in correlated strata, even more than 4 kilometers apart. Events like this can provide valuable correlation tools for future stratigraphic mapping and various studies on volcanological features in the relatively uncharted region around Ísafjarðardjúp and beyond. Globally, recognition of the existence of such events will aid in the interpretation of results in other kinds of paleomagnetic studies. The role of geomagnetic paleo-intensity determinations is discussed briefly in this context.

Paleomagnetic and rock-magnetic survey of eocene dike swarms from the Tecalitlan area (Western Mexico): Tectonic implications

For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacan block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacan block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α95 = 8.9°. The corresponding mean paleomagnetic pole position is Plat = 75.7°, Plong = 166.6°, K = 31, A95 = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacan Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.

Report on the activities of geological and paleomagnetic surveys in the regions of Lutzow-Holm Bay, Prince Olav Coast and western Enderby Land during JARE-46

Report on the activities of geological and paleomagnetic surveys in the regions of Lutzow-Holm Bay, Prince Olav Coast and western Enderby Land during JARE-46

Magma flow directions of post Jotnian dolerite sills in central-east Sweden: a magnetic fabric and paleomagnetic survey

An anisotropy of magnetic susceptibility (AMS) and paleomagnetic study has been performed on post-Jotnian basic sills of the Ulvö complex, of the Central Scandinavian Dolerite Group (CSDG). The CSDG intruded some 1.27 Ga ago in the central part of the Fennoscandian Shield and extends over an area of c. 150 000 km2. Previously presented AMS data of the CSDG sills from south-central and northern Sweden indicated a NW or SE directed magma flow. In this study new measurements of the AMS reveal a fairly uniform pattern of NW-SE lineation directions also for the dolerites of the Ulvö complex. While the magma is flowing in a dyke or a sill, elongated particles become imbricated against the chilled margins, and the AMS can be used to determine not only the lineation but also the true direction of the magma flow. An imbrication pattern defined for one of the sills suggests that the magma propagated from the present NW towards SE. The established paleomagnetic pole position (Plat=−3° and Plon=157°) is in very good agreement with other paleomagnetic data from the Ulvö complex and from other parts of the CSDG. The new AMS data from the Ulvö complex presented in this study fit well in to the magnetic fabric pattern previously presented for the CSDG sills and support a plate tectonic model suggesting that the CSDG is a related to the break up of Baltica from Laurentia at c. 1.27 Ga ago.

The Sudbury Structure: A circular impact crater?

Petrological, geochemical, and structural evidence supports genesis of the Sudbury Structure by a meteorite impact. The Sudbury Igneous Complex (SIC) represents the ponded melt sheet. The primary morphology and size of the Sudbury Crater remains the subject of extensive discussion. A model is presented explaining the transformation of an originally circular crater to the current elliptical form. Processes involved in the geometrical transformation include; a) volume reduction through melt sheet crystallization, and repeated expulsion of melt into the surrounding footwall, b) translation and segmentation on multiple southeast dipping thrust surfaces, and c) relative displacement on steep north‐northwest trending faults. Paleomagnetic, geochemical and structural evidence supports the concept of periodic melt expulsions into the footwall. Seismic, potential field and paleomagnetic surveys provide evidence in support of multiple thrust surfaces. After over 9 km of subsequent erosion only a small volumetric portion of the original SIC remains, although complete vertical sections through the melt sheet have been preserved. None of the crater wall has been preserved. Thus the contacts between the SIC and the footwall on the North and South Ranges are interpreted as uplifted crater floor margins.

Tightening the Belt: Paleomagnetic-stratigraphic constraints on deposition, correlation, and deformation of the Middle Proterozoic (ca. 1.4 Ga) Belt-Purcell Supergroup, United States and Canada

The Belt-Purcell Supergroup crops out in Montana, Idaho, Washington, British Columbia, and Alberta. It is the largest and most intensively studied Middle Proterozoic succession in North America. A major paleomagnetic survey of the Belt-Purcell basin in Montana was carried out by the U.S. Geological Survey between 1975 and 1985, but only partial results have been published. We reanalyze these data and add new data from the Canadian part of the basin. Results come from >2700 samples (mostly red beds) from 93 localities, spanning 13 formations or members of formations. They typically exhibit stable magnetizations carried by hematite in detrital specularite grains and pigment. Coherent paleomagnetic directions have either southwest declinations and moderate positive inclinations (normal polarity) or northeast declinations and moderate negative inclinations (reverse polarity). These directions clearly pass fold and reversal tests. A stratigraphically coherent directional swing and a reproducible polarity zonation indicate that the stable magnetization is primary and was acquired during the course of deposition. U-Pb dates suggest an ∼50 m.y. duration of deposition of the upper part of the Belt-Purcell Supergroup. The limited amount of apparent polar wander and the few polarity reversals recorded in the Belt- Purcell deposits are similar to the rate of polar wander and a long interval of stable polarity observed for the middle Cretaceous. An apparent incompatibility between early Middle Proterozoic (Elsonian) paleomagnetic poles from different regions of North America is now resolved; the Belt- Purcell poles are middle Elsonian in age and form the southern extreme point on a hairpin loop of the 1500–1400 Ma part of a refined Elsonian apparent polar wander path.

Introduction to the special issue of Canadian Journal of Earth Sciences: The Nechako NATMAP Project of the central Canadian Cordillera

The Canadian Cordillera in central British Columbia has seen the Mesozoic subduction of an oceanic terrane; the amalgamation of volcanic-arc terranes; continued intermittent Mesozoic compression and magmatism; and Tertiary wrenching, extension and magmatism. Except in its northernmost mountain ranges, the area is extensively covered in glacial drift and thin veneers of Tertiary volcanic rocks. In 1994, a group of scientists and technologists believed they could understand that cover, see through it, and discover the components of that collision and extensional orogen. They would apply modern techniques of isotopic and paleontological geochronology; lake-sediment, till, and plant geochemistry; detailed gravity, magnetic, radiometric, paleomagnetic, and electromagnetic surveys; and isotopic and trace element lithochemistry, as they conducted extensive bedrock and surficial mapping. This special issue summarizes a cross-section of the scientific contributions derived from that mapping conducted under the auspices of the Nechako NATMAP Project. It demonstrates the absolute necessity of applying modern isotopic and paleontologic geochronology to understand the Phanerozoic geology of the Cordillera. It emphasizes the necessity of detailed aeromagnetic surveys (500 m or less line spacing) in looking through covered terranes at anything more than 1 : 250 000 scale. And, it shows the immense utility of applying various geochemical techniques to solve geological problems and establish baselines for future research and economic development. Bedrock and surficial mapping in the central Cordillera, using these and other techniques, have established the nature and timing of Mesozoic crustal growth, Tertiary crustal thinning, and the associated formation of mineral deposits.

Age and magnetism of lavas in Jökuldalur area, Eastern Iceland: Gilsá event revisited

We present results of paleomagnetic measurements and K–Ar age determinations of 38 lava flows collected in five separate sections in the Jökuldalur area of Eastern Iceland, including one section previously studied by Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444]. These sites are close to the type locality of the normal “Gilsá event” in the Matuyama chron first identified by McDougall and Wensink [McDougall, I., Wensink, H., 1966. Paleomagnetism and geochronology of the Pliocene–Pleistocene lavas in Iceland. Earth Planet. Sci. Lett. 1, 232–236]. Using new experimental results as well as field observations, we could correlate the sequence of lava flows in the Jökuldalur area spanning the time interval between 1.8 and 0.5 Ma. The magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for Brunhes–Matuyama ages given by Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095] based on marine magnetic anomalies, except that the Gilsá event needs to be added. Existence of Olduvai age lavas under Gilsá was inferred but not certain. Our results are fairly in good agreement with the former studies by Wensink [Wensink, H., 1964a. Secular variation of Earth magnetism in Plio–Pleistocene basalts of eastern Iceland. Geol. Mijnbouw 43, 403–413; Wensink, H., 1964b. Paleomagnetic stratigraphy of younger basalts and intercalated Plio–Pleistocene tillites in Iceland. Geol. Rund. 54, 364–384] and Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444], but the assignment to the polarity zones is different because of the newly obtained K–Ar ages. Based on the present study, we propose that the Gilsá event is a short normal subchron in the Matuyama chron distinct from and above the more well-established Oldvai subchron.

Paleomagnetism and tectonics of Malaita, Solomon Islands

Malaita, in the Solomon Islands, is the emergent expression of the Pacific Province, a geologically distinct suspect terrane which is commonly held to be the margin of the Ontong Java Plateau, a thickened portion of Pacific plate overthrust onto the rest of the Solomon Islands Arc during a late Miocene reversal of the polarity of the arc. Subduction of the Australian plate replaced Pacific plate subduction during this event, and the Solomon Islands Arc was transferred from the Australian plate to the Pacific plate. A paleomagnetic survey of the eastern Solomon Islands included sampling in southern Malaita. Magnetostratigraphic dating gives an age of 5.8 Ma for the Haruta Calcisiltite‐Hada Calcisiltite facies change on Malaita. This facies change resulted from renewed volcanic activity following the initiation of Australian plate subduction. Paleomagnetic poles from two pre‐Pliocene units do not lie near the Pacific apparent polar wander path (APWP), as the conventional model would suggest, but rather fall on the Australian APWP, suggesting that the Pacific Province developed as a part of the Australian plate. Correction for 6.5 Ma of Pacific‐Australia relative motion produces an improved, very close fit to the Australian APWP, consistent with the transfer of Malaita, along with the rest of the arc, to the Pacific plate at about that time. Magnetic paleolatitudes from a DSDP hole on the Ontong Java Plateau, which disagree with usually accepted Pacific Tertiary motion, also fail to match the Malaitan data; the possibility that Malaita could be a part of an Ontong Java sub‐plate, somehow detached from the rest of the Pacific plate, can be discounted. Malaita and the rest of the Pacific Province arose in the fore‐arc of the originally north‐east facing arc. Collision of the Ontong Java Plateau with the trench slowed subduction of the Pacific plate and began subduction of the Australian plate below the arc. Both subduction zones may have been active for a period, beginning at about 10 Ma and ending at about 3 Ma, until Pacific plate subduction broke down, and the Pacific Province was subsequently uplifted and deformed.

RECONSTRUCTION OF THE JAPANESE ISLANDS BEFORE JAPAN SEA OPENING

Recent paleomagnetic surveys revealed that Northeast Japan was rotated counter-clockwise about 20° during the middle Miocene opening of the Japan Sea, whereas Southwest Japan clockwise about 45°. A reconstruction of the pre-Miocene Japanese Islands is attempted on the basis of the following main premises. 1) The bending of zonal geologic units in the Central Japan was caused by the collision of the Izu-Ogasawara Ridge after the opening of the Japan Sea. 2) The Yamato Bank and several plateaus in the Japan Sea are regarded as continental fragments and there were no marginal basins before the opening. 3) Northeast Japan and Central Hokkaido are geologically continuous to West Hokkaido and Sakhalin, respectively. Each pair is treated as a single crustal block. The reconstruction gives consistent explanations for such geologic features as the change of volcanic front, distributions of Paleogene coal-bearing strata and Cretaceous to Paleogene subduction complex and felsic volcanic rocks, continuity of geotectonic units mainly consisting of the Jurassic subduction complex, and tectonic lines. A most significant corollary in this reconstruction is that Central Hokkaido was adjoined to Northeast Japan.

L’Occident équatorien : un terrain océanique pacifique accolé au continent sud-américain

Abstract We present here a review of the major geological features of western Ecuador including the first results of a paleomagnetic survey performed in 1985. The data are interpreted in the frame of global tectonics. We suggest that western Ecuador consists of exotic terranes that formed in the Pacific Ocean and were subsequently accreted to nuclear South America. This evolution is similar to that of other segments of the mountain belts surrounding the Pacific.

Spreading structure of the Troodos ophiolite, Cyprus: Some paleomagnetic constraints

It has been suggested that grabens on the northern flank of the Troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge-jumping. This paleomagnetic survey of the Solea graben was carried out to constrain the structural interpretation. An analysis was devised which restores both the dikes sampled to vertical and the primary remanent magnetizations to a previously derived, structurally corrected, remanent magnetization direction. Typically, two solutions for the original dike orientation and the pole and angle of net rotation are obtained for each dike sampled. These solutions can be separated if a locally consistent original dike strike is assumed. Reasons are discussed for the selection of the solution that gives an original northwesterly dike strike. Results are consistent with a graben: rotations are up to 78° at the sites studied. Rotation vectors are subhorizontal and subparallel to the original dike strike, suggestive of an axial process for the development of the structure. However, on the basis of field observations, an alternative mode of origin is proposed, invoking ephemeral extreme upper crustal extension by listric faulting on the west flank of an intermediate- to fast-spreading axis, the grabens being formed by antithetic faulting.