宇野 康司

Abstract To test the hypothesis that a Cretaceous hairpin turn is absent in the apparent polar wander path (APWP) of the inner arc of southwestern Japanese island (southwest Japan), we refined a mid-Cretaceous (100 Ma) paleomagnetic pole from southwest Japan. Red mudstone samples from the 100 Ma Hayama Formation were collected for paleomagnetic analysis from eight sites in the Hayama area in the central part of southwest Japan. A high-temperature remanent magnetization component carried by hematite was isolated from these sites and was found to be of primary mid-Cretaceous origin. The primary nature of the magnetization is supported by the detrital character of the magnetic carrier. The primary directions provided a paleomagnetic pole (35.0°N, 209.6°E, A₉₅ = 6.1°, N = 8), which represented southwest Japan at 100 Ma. This pole falls into a cluster of Cretaceous poles in southwest Japan. An APWP for southwest Japan between 110 and 70 Ma was updated to ascertain the stationarity of the pole positions for this region. Therefore, it is unlikely that the APWP for southwest Japan experienced a hairpin turn during the Cretaceous.

This study presents a description of a rhyolite lava-forming eruption, including the conduit system, degassing history during the lava flow dynamics. We examined the Pleistocene Shiroyama rhyolite lava on Himeshima Island, Japan. The lava is mainly characterized by locally developed obsidian. Based on the structural variation, the obsidian lithofacies correspond to the shallow conduit. The geological investigation and FTIR analyses showed that gas removal from the conduit magma proceeded via vesiculation, fracturing, and brecciation, allowing formation of the dense obsidian. Since the lava originally maintained some extent of water, the lava effervesced just after the effusion. This vesiculation resulted in pervasive bubble coalescence and the formation of abundant permeable pathways. The volcanic gasses escaped via those pathways, allowing collapse of the bubbles and deflation of the lava. AMS (anisotropy of magnetic susceptibility) results indicate that the lava spread concentrically.

<title>SUMMARY</title> A palaeomagnetic study has been conducted to examine the deformation of thick crusts of rhyolite lava while its inner portions continue to flow. The Sanukayama rhyolite lava, which erupted in the Pleistocene in Kozushima Island, Japan, was chosen as the investigation site because of its well-exposed vertical lithofacies variations classified into three distinct zones (pumiceous, obsidian and crystalline). The targets of this study are the pumiceous and obsidian zones, which constitute the crust of the lava. Thermal demagnetization reveals three remanent magnetization components from the pumiceous and obsidian samples but only a single magnetization component from the inner crystalline rhyolite samples. Alternating field demagnetization is ineffective in isolating the magnetization components in the pumiceous and obsidian samples. The multiple components of remanent magnetization of the crust are interpreted to have been acquired during cooling as thermoremanent magnetizations. We suspect intermittent lava transport of the inner portions, the primary mode of rhyolite lava advancement, to be responsible for the presence of multiple components in pumice and obsidian of the lava crust. When the inner portions of the lava retain mobility to flow out of the crust, the solidified crust of the lava surface below the magnetite Curie temperature remains susceptible to deformation. Analysis of palaeomagnetic directions from the crust allows the deformation of the crust to be described in terms of rotation. Although the mode of rhyolite lava advancement is not well understood, because of its infrequent occurrence, our observations offer an important insight on how the mobile part of the lava is associated with the deformation of the crust during continued lava advance.

To test the hypothesis that southwest Japan was involved in large-scale tectonic movement with southward translation as far as 2000 km with respect to Eurasia during the Cretaceous, we examined Cretaceous paleomagnetic poles from southwest Japan to compare with those from Eurasia. Red and gray sandstone samples from the Upper Cretaceous Onogawa Group were collected from twelve sites in the Onogawa Basin in the western part of southwest Japan for paleomagnetic analysis. This group formed over the time span in which the proposed tectonic event is hypothesized to have occurred. A characteristic remanent magnetization component was isolated from red sandstone at ten sites; it is interpreted to be of primary Late Cretaceous origin. The primary directions combined with previously reported data provide a mean direction (D = 76.8 degrees, I= 44.6 degrees, alpha(95) = 11.1 degrees, N = 15) and a paleomagnetic pole (24.4 degrees N, 202.6 degrees E, A(95) = 11.0 degrees) for the Onogawa area. This pole is consistent with other Late Cretaceous poles from a wide area of southwest Japan, and a mean Late Cretaceous pole (28.4 degrees N, 202.5 degrees E, A(95) = 7.5 degrees, N = 6) is calculated and regarded as representative of this region. The Late Cretaceous pole, together with mid-and Early Cretaceous poles, constitutes an apparent polar wander path (APWP) for southwest Japan during the Cretaceous. After restoration of post-Cretaceous tectonic rotation, each Cretaceous pole for southwest Japan shows agreement with the coeval poles for Eurasia; therefore, it is unlikely that the previously proposed tectonic model that includes southward translation of southwest Japan occurred in the Late Cretaceous. Southwest Japan is considered to have behaved as a stable part of the Eurasian continental margin during the Cretaceous. (C) 2017 Elsevier B.V. All rights reserved.

A primary remanent magnetization is identified in the Jurassic-Cretaceous red bed sandstones of the Tembeling Group in Peninsular Malaysia. This high-temperature magnetic component is unblocked at 680-690 degrees C, revealing a clockwise deflected direction of Ds = 56.8 degrees, I-s = 31.6 degrees (where k(s) = 8.5, alpha(95) = 11.3 degrees and N = 22) in stratigraphic coordinates. The primary origin of this component is ascertained by a positive fold test and a geomagnetic polarity reversal in the Kuala Wau section. Secondary remanent magnetizations are identified in the rocks of the Tembeling and Bertangga basins, which indicate a counter-clockwise deflection in the geographic coordinates (D-g = 349.1 degrees, I-g = 15.3 degrees where k(g) = 11.8, alpha(95) = 5.1 degrees, N = 72). The comparison with the expected paleomagnetic directions from the 130 Ma and 40 Ma Eurasian poles indicates two-stages of tectonic movement in the southern Malay Peninsula: (1) a clockwise rotation of 61.1 degrees +/- 11.9 degrees accompanied by a 13.3 degrees +/- 8.1 degrees southward displacement after the Cretaceous; and (2) a subsequent counter-clockwise rotation of 18.5 degrees +/- 5.0 degrees to the present day position. The first stage of rotation is ascribed to tectonic deformation caused by the indentation of India into Asia after 55 Ma, while the second stage is attributed to the collision of the Australian Plate with SE Asia after 30-20 Ma. The present paleomagnetic results from the Jurassic-Cretaceous Tembeling Group thus reveal impacts of both of these collisions on SE Asia in general and on Peninsular Malaysia in particular. (C) 2016 Elsevier Ltd. All rights reserved.

Late Jurassic to Early Cretaceous red beds of the Phuquoc Formation were sampled at 33 sites from the Sihanoukville and Koah Kong areas of the Phuquoc-Kampot Som Basin, southwestern Cambodia. Two high-temperature remanent components with unblocking temperature ranging 650 degrees-670 degrees C and 670-690 degrees C were identified. The magnetization direction for the former component (D = 5.2 degrees, I = 18.5 degrees with alpha(95) = 3.1 degrees in situ) reveals a negative fold test that indicates a post-folding secondary nature. However, the latter component, carried by specular hematite, is recognized as a primary remanent magnetization. A tilt-corrected mean direction of D = 43.4 degrees, I = 31.9 degrees (alpha(95) = 3.6 degrees) was calculated for the primary component at 11 sites, corresponding to a paleopole of 47.7 degrees N, 178.9 degrees E (A(95) = 3.6 degrees). When compared with the 130 Ma East Asian pole, a southward displacement of 6.0 degrees +/- 3.5 degrees and a clockwise rotation of 33.1 degrees +/- 4.0 degrees of the Phuquoc-Kampot Som Basin (as a part of the Indochina Block) with respect to East Asia were estimated. This estimate of the clockwise rotation is similar to 15 larger than that of the Khorat Basin, which we attribute to dextral motion along the Wang Chao Fault since the mid Oligocene. The comparison of the herein estimated clockwise rotation with the counter-clockwise rotation reported from the Da Lat area in Vietnam suggests the occurrence of a differential tectonic rotation in the southern tip of the Indochina Block. During the southward displacement of the Indochina Block, the non-rigid lithosphere under its southern tip moved heterogeneously, while the rigid lithosphere under the Khorat Basin moved homogeneously. (C) 2015 Elsevier B.V. All rights reserved.

Bedded chert samples from the Norian (Upper Triassic) Sakahogi section of the Mino Terrane in the Inuyama area, southwest Japan, have been collected for paleomagnetic study in order to determine the age of an impact ejecta layer interbedded within it and the paleolatitude of its deposition. Thermal demagnetization of the bedded chert samples revealed four distinct remanent magnetization components. The last demagnetized component with both polarities is interpreted to be the primary magnetization, which produced the first magnetostratigraphic data of the middle to upper Norian from Panthalassa, consistent with Tethyan magnetostratigraphic sections. Tie points were derived from paleomagnetic and paleontological data, from which the stratigraphic position of the ejecta layer was compared with the astronomically tuned geomagnetic polarity time scale (APTS). The age of ejecta layer is estimated to be about 212 Ma. This magnetostratigraphic age is consistent with the radiometric age of the Manicouagan impact that formed the second largest known crater in the Phanerozoic at Quebec, Canada. The results of this study, as well as previous studies, suggest that the chert section, which includes the ejecta layer, was deposited within a paleolatitude range of 0.3-14.4 degrees N. This indicates that the material ejected by the Manicouagan impact event reached near the equatorial region of the paleo-Pacific Ocean. (C) 2015 Elsevier B.V. All rights reserved.

We present new Early Cretaceous paleomagnetic results from the Zhuravlevka-Amur Terrane of the Eurasian Continent, Far East Russia. Out of 34 total sites, 14 were collected from Komsomolsk-on-Amur area (50.6 degrees N, 137.2 degrees E) and 20 from Vaninsky area (49.1 degrees N, 139.2 degrees E). Thermal demagnetization reveals the presence of two interpretable magnetization components in 19 sites, with laboratory unblocking temperatures of 350 degrees C and/or 500-580 degrees C. The remanent directions of the low-temperature component are either parallel or anti-parallel to those obtained from the high-temperature component. Results of fold tests show that both components are secondary. Rock magnetic and reflected light microscopic observations indicate a chemical origin for both of these components, as evident from the presence of secondary pyrrhotite and magnetite. The Komsomolsk-on-Amur area provides an in-situ formation mean direction of D = 127.5 degrees, I = 66.7 degrees (k = 28.2, alpha(95) = 9.3 degrees, N= 10 sites). When combined with the reported paleomagnetic data from Early to Middle Cretaceous accretionary wedge rocks of the Kiselevka-Manoma Terrane and the Early Cretaceous Western Sakhalin turbidite basin rocks (D = 94.2 degrees and D = 57.1 degrees, respectively), large magnitude of clockwise rotations of 66-118 degrees is demonstrated for the eastern part of the Sikhote-Alin Superterrane with respect to Eurasia. In addition, these three landmasses maintained their E-W elongated orientations before the start of rotation, implying southward directed subduction of the oceanic plates beneath northern margins. These reconstructions of the Sikhote-Alin Superterrane provide clues on the tectonic evolution of Panthalassa Ocean. (C) 2015 Elsevier Ltd. All rights reserved.

In rhyolite lavas, the high porosity bands are often developed. They potentially act as pathways for gas movement to the lava surface. Since explosive activities of lavas are generally considered to be controlled by degassing system, understanding the origin and deformation process of the high porosity bands is important to assessing volcanic hazards. The Takanoobane rhyolite lava in the middle of Kyushu Island in SW Japan is effused at 51±5ka. The volume, flow length, and thickness are 0.14km3, &gt 2km, and about 90m, respectively. The central crystalline part of the lava is characterized by the light-colored bands defined by the high porosity zone (HPZ). On the basis of geological and petrographical studies, we revealed that the HPZ was primary formed by ductile-brittle tearing of the lava (known as cavitation). According to the AMS results, the HPZs were subsequently stretched and flattened laterally during the concentric spreading of the lava. This deformation process could stretch the HPZ not only radially but also laterally. This effective stretching developed the HPZ into pervasive thin bands. Since the HPZs act as degassing pathways to the lava surface, the pervasive HPZ bands may play a role in providing volcanic gasses to void spaces created in surface fold hinges of rhyolite lavas. Thus, this degassing system may promote explosive activity of the lava surface.

A comparative study of the paleomagnetism of various lithofacies of pyroclastic flow deposits has been undertaken to examine the accuracy of their remanent magnetization direction at the time of deposition. The Aso-2 pyroclastic flow deposit, which erupted at 141 ka from Aso caldera, Japan, was chosen as the site of our investigation because the deposit formed over a short time period and the paleomagnetic directions of the deposit are expected to record substantial contemporaneous volcanism. Paleomagnetic samples of densely welded, lava-like rheomorphic, and non-welded pyroclastic deposits were collected at nine sites from the Aso-2 pyroclastic flow deposit. Remanence directions from densely welded pyroclastic deposits display good within- and between-site consistency and are considered to accurately record the ambient geomagnetic field direction at the time of emplacement. Rheomorphic pyroclastic deposits had directions that show good within-site consistency, and it is considered that this material obtains its remanence direction parallel to the ambient field corresponding to the time of its emplacement. Remanence directions from non-welded pyroclastic deposits show large confidence limits or deviations from their expected directions. Such deposits would likely be prone to modification of remanence direction introduced from random rotations of remanence-carrying material during syn- or post-depositional stages. In conclusion, we suggest that among various lithofacies of pyroclastic flow deposits, remanence directions observed from non-welded pyroclastic deposits may need to be interpreted cautiously. (C) 2014 Elsevier B.V. All rights reserved.

Takajosan breccia rocks are distributed around the southwestern caldera rim of the Aso Volcano in japan. They are characterized by coarse lithic breccias with a pumiceous matrix. The proximal coarse lithic breccias are divided into the lower massive unit and the upper stratified unit. The lower massive lithic breccias tend to transform laterally into tuff breccias and pumiceous lapilli tuffs. Paleomagnetic results showed that all of the deposits were deposited at high temperatures of 175-560 degrees C. This was also supported by geological characteristics such as spatter clasts, dasts with a bread-crust texture, and weakly welded parts. These features clearly show that the deposits originated from pyroclastic density currents (PDCs). The dense lithic-rich lithofacies, low vesicularity of pumice, lack of plinian fall deposits, and radial distribution indicate that the deposits were derived from boil-over PDCs rather than plinian column-collapse PDCs. The SiO2 contents of the matrix glasses of the proximal lower massive breccia showed a progressive decrease from the bottom toward the upper part. We interpret that this chemical variation corresponds to chemical zonation of the magma chamber. This indicates that the massive deposits aggraded progressively from the base upwards (progressive aggradation), rather than through en masse freezing. The vertical lithofacies changes of the proximal breccias from the lower massive to the upper stratified units indicate that a sustained current in a quasi-steady state switched to an unsteady current with the progression of the volcanic activity. (C) 2014 Elsevier B.V. All rights reserved.

We investigated the K-Ar ages and the petrological and geochemical features of lava units from lateral cones and lava distributed around the Aso caldera in central Kyushu, Japan, in order to constrain the spatial range of lateral magma intrusion during the caldera-forming stage. The results of the K-Ar age determination showed that most of the analyzed lava units erupted almost simultaneously with the Aso caldera-forming pyroclastic eruptions (266 to 89. ka). In addition, the petrography, major and trace element compositions, and Sr isotope ratios of these lava units are indistinguishable from the caldera-forming pyroclastic products. The contemporaneous activities of compositionally similar magmas inside and outside of the caldera presumably indicate the occurrence of a lateral intrusion of caldera-forming magma, which had accumulated in a huge magma chamber beneath the caldera system. Our results suggest that a total of 6.3. vol.% of caldera-forming magma in the Aso volcano migrated more than 20. km along the faults from the center of the caldera. © 2013 Elsevier B.V.

Rhyolite is a common volcanic rock; however, few studies have focused on the remanent magnetization of rhyolite lava, and few paleomagnetic studies have successfully investigated rhyolite lavas. We suspect that problems associated with paleomagnetic studies of rhyolite may be due to the nearly ubiquitous flow structure in rhyolite lava. In this study, we examined a thick rhyolite lava flow with clearly marked flow structure to assess its ability to record a consistent paleomagnetic direction, using material penetrated by two drill cores. Progressive thermal demagnetization isolated two magnetization components. A high-temperature component from each of the two cores yields inclinations that differ from each other. The low-temperature component had those that agreed with each other, and were also consistent with the direction expected from a geocentric axial dipole field. The modification of direction of the high-temperature component may be explained by post-magnetization acquisition tilting. The development of flow structure also leads to distortion of directions of the component, which is observed at stratigraphic positions where the volume fraction of light-colored parts of the flow structure &gt;30%. In the case of silicic lava, the low-temperature component may retain directions parallel to the ambient field direction at the time of lava emplacement.

We determined the K-Ar ages for post-caldera lavas from Aso volcano in central Kyushu, Japan using the unspiked sensitivity method. The following three peaks of highly-frequent volcanism are recognized in the post-caldera stage: 70-50 ka, 40-20 ka and later than 10 ka. The two peaks of 70-50 ka and 40-20 ka are characterized by activities of compositionally diverse magmas from basalt to rhyolite. The volumes of silicic magmas are significantly larger than those of basalt magmas in these peaks. On the other hand, the peak later than 10 ka is characterized by predominantly basaltic eruptions without silicic magma activities. The active center of the silicic magma production (=crustal assimilation of injected basalt) had presumably migrated northeastward from the southwestern part of the caldera between the peaks of 70-50 ka and 40-20 ka. The predominant basaltic activity in the peak later than 10 ka represents that the production rate of silicic magma decreased, and does not indicate the accumulation of voluminous silicic magma beneath the recent Aso caldera. (C) 2012 Elsevier B.V. All rights reserved.

Middle Triassic (Anisian) red bedded cherts were collected from the Ajiro area in Kyushu Island, southwest Japan, for a palaeomagnetic study. Thermal demagnetization showed five distinct remanent magnetization components from the cherts. The lowest temperature component below 200 degrees C (component A) is a present-day viscous overprint. The second- (up to 420 degrees C, component B) and third-demagnetized (up to 580 degrees C, component C) components are secondary magnetizations of thermoviscous and chemical origin, respectively, both of which reside in magnetite. The fourth-removed component (component D) was demagnetized at 630 degrees C and is a secondary magnetization carried by haematite. Components B and C are considered to be a result of widespread remagnetization in southwest Japan during the Cretaceous; the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area, are of the same origins with unblocking spectra similar to those observed in this study. The last-removed component during thermal demagnetization (up to 695 degrees C, component E) is a primary magnetization carried by haematite. The directions of this component, after correction for the inclination flattening, yielded a mean palaeolatitude of 2 degrees S at the time of deposition in the Middle Triassic. The Ajiro cherts accreted as part of an accretionary complex to the eastern margin of the South China Block during the Middle Jurassic and were then translated northwards from the continental margin to their present position through a margin-parallel translation process owing to the oblique convergence of the oceanic plate.

A new paleomagnetic study illustrates latitudinal translation of ribbon-shaped continental fragments along the western margin of the Pacific Ocean. Though tectonic translations have long been recognized as prominent features in the eastern Pacific margin (i.e., North American Cordillera), we present new. quantitative evidence for similar, margin-parallel motion through large-scale, strike-slip faulting in the western Pacific margin. The Kurosegawa Terrane in Southwest Japan is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of Southwest Japan. Late Triassic and Early Cretaceous sedimentary rocks from the terrane yielded paleolatitudes of 4 degrees N and 18 degrees N, respectively. Based on paleomagnetic data of the Kurosegawa Terrane and plate reconstructions of East Asia, we suggest that the terrane was the easternmost element of the South China Block at least from the Late Triassic to the Early Cretaceous. The terrane was then translated similar to 1500 km northward from the continental margin to its present position, associated with sinistral strike-slip movement along the East Asian continental margin in mid- to Late Cretaceous times. The average displacement rate is calculated to be 7 cm/yr and is comparable to rates reported for the eastern Pacific margin. (C) 2011 Elsevier B.V. All rights reserved.

Vertical variation of the oxidation state as recorded by Fe-Ti oxides of Takanoobane rhyolite lava (TR lava) is revealed on the basis of petrographic and rock magnetic studies. Petrographic observations of the interior of TR lava showed that Fe-Ti oxides are preferentially oxidized along the flow structure that is characterized by the light-colored band composed of aggregates of deformed minute cavities, whereas both the top and bottom parts of the lava are not oxidized. Rock magnetic studies measuring thermal demagnetization of a three-component isothermal remanent magnetization (IRM) and magnetic susceptibility revealed that hematite of a highly oxidized state is formed preferentially along the light-colored band, consistent with the petrographic observations. The observed preferential oxidation of the light-colored band is adequately explained by post-eruptive degassing of hydrogen from the interior of the lava, rather than migration of atmospheric oxygen into the hot interior of the cooling lava against the back-streaming volcanic gas. The magmatic volatiles emitted from the interior can be transported effectively through the light-colored band and microcracks which ubiquitously develop nearly vertical to the flow direction. The degassing mechanism should be limited to highly viscous silicic lavas such as rhyolite because the mechanism requires development of the flow structure and the microcracks as the pathways for gases. These structures are achieved only in highly viscous silicic lavas. (C) 2010 Elsevier B.V. All rights reserved.

P&gt;Intensity of the geomagnetic field during the Archaean can potentially be used to study an evolution of the dynamo activity in the Earth's core. In order to investigate this issue, we present new palaeomagnetic and geochronological results from the dolerite dyke, which have been intruded into the Archaean Gneisses of Nuuk area, southwest Greenland. &lt;SU40&lt;/SUAr/&lt;SU39&lt;/SUAr dating of the pyroxene grains from dolerite yield an age of 2585 +/- 21 Ma. The high temperature component, which has been recognized as a characteristic direction, is identified in 24 dolerite samples. Comparison of the palaeomagnetic directions from dolerite dykes with those from host gneisses suggests a primary origin for this component. Magnetic mineralogical and grain size investigations revealed a pseudo-single domain magnetite as a dominant career of magnetization. Thellier palaeointensity determinations of 14 dolerite specimens yield a mean field value of 13.9 +/- 2.5 mu T. Strength of the virtual dipole moment (2.30 +/- 0.42 x 10&lt;SU22&lt;/SU Am&lt;SU2&lt;/SU) obtained from this study is about one quarter of the present Earth's filed value. A simulation based numerical model of the Thellier experiments suggests that the presence of multidomain grains can play a role in enhancing the strength of palaeointensity. Even after taking in to account the effects of multidomain grains, an intensity value (13.9 mu T) from this study suggests that the strength of geomagnetic field at about 2.6 Ga was much lower than that of the present time.

Lower to Middle Cretaceous red sandstones were sampled at four localities in the Lanpin-Simao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 degrees C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5 degrees N, 100.8 degrees E) locality, which is located at a distance of 25 km from the Ailaoshan-Red River Fault, revealed northerly declination with steep inclination (Dec./Inc. = 8.3 degrees/48.8 degrees, alpha(95) = 7.7 degrees, N = 13). However, mean directions obtained from the Zhengyuan (24.0 degrees N, 101.1 degrees E), West Zhengyuan (24.0 degrees N, 101.1 degrees E) and South Mengla (21.4 degrees N, 101.6 degrees E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8 degrees/46.1 degrees, alpha(95) = 8.1 degrees (N = 7), Dec./Inc. = 324.2 degrees/-49.4 degrees, alpha(95) = 6.4 degrees (N = 4) and Dec./Inc. = 51.2 degrees/46.4 degrees, alpha(95) = 5.6 degrees (N = 13), respectively. The palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the Shan-Thai Block. When combined with geological, geochronological and GPS data, the processes of deformation in the Shan-Thai Block is described as follows: Subsequent to its rigid block clockwise rotation of about 20 degrees in the early stage of India-Asia collision, the Shan-Thai Block experienced a coherent but southward displacement along the Red River Fault prior to 32 Ma. This block was then subjected to a north-south compressive stresses during the 32-27 Ma period, which played a key role in shaping the structure of Chongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motion has occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as a result of internal block movements along the reactivated network of faults.

Neogene rift system configuration for the back-arc of southwest Japan, southern rim of the Japan Sea, is argued on the basis of reflection seismic interpretation. Divergent rifting and subsequent contraction provoked by an arc-arc collisional event are manifested by the formation of faulted grabens and their inverted deformation, respectively. We identified the following four Cenozoic tectonic epochs as a decomposition process of the eastern Eurasian margin based on reliable paleomagnetic data: (1) Plate margin rearrangement on a regional left-lateral fault through southwest Japan and Sikhote Alin, which constituted a continuous geologic province before the early Tertiary differential motion; (2) Early Tertiary clockwise rotation (&gt; 20 degrees) of the east Tan-Lu block relative to the North China block; (3) Oligocene to early Miocene divergent rifting and spreading of the Japan Sea, which divided southwest Japan from the east Tan-Lu block; (4) Middle Miocene bending and back-arc inversion of southwest Japan caused by collision with the Izu-Bonin arc. According to the estimation of relative motions during these events, a paleogeographic reconstruction is presented through Cenozoic time. (c) 2006 Elsevier Ltd. All rights reserved.

Red beds have been recognized as a useful source of ancient geomagnetic field. However, the timing of remanence acquisition in red beds is still incompletely understood. Fold tests may be used for an estimation of the timing of red bed remanence acquisition, but an essential ability of this kind of test is to conclude whether magnetizations are acquired before or after folding. Even a positive fold test does not necessarily represent a primary nature of magnetization. Because the relationship between the timing of fold growth and sedimentation is well understood for a small, syn-folding sedimentary basin, we select such a sedimentary basin as a focus of study. A case study example examining the timing of red bed remanence acquisition based on fold tests is described in this paper. It shows that the observed remanence directions carried by hematite are given a syn-folding nature. This syn-folding nature is more observable in the lower part of the sedimentary sequence that was under the more intense folding during its sedimentation than the upper part. A pure depositional remanent magnetization when beds were horizontal is not applicable to this case, but a post-depositional remanent magnetization, with possible existence of an initial slope of the deposition surface, is considered to have been imparted prior to complete lithification. Microscopic observations showing the presence of hematite as sedimentary particles can be used for an independent estimation supporting the detrital origin. Chemical compositions of the hematite grains agree well with the maximum laboratory unblocking temperatures of the samples, and we draw the conclusion that these grains are the carrier of the detrital magnetization. (c) 2005 Elsevier B.V. All rights reserved.

Early Jurassic sandstones and shales were collected at 14 sites from the Kimpo area in the western part of the Korean Peninsula for a palaeomagnetic study. A characteristic remanent magnetization component, which passes the fold test, was isolated from seven sites. This characteristic component is interpreted to be of primary Early Jurassic origin because the folding age is Middle-Late Jurassic. The primary directions, together with previously reported ones, provide a mean direction (D = 38.1degrees, I = 39.7degrees, alpha(95) = 3.8degrees, 11 sites) and a palaeomagnetic pole (54.1degreesN, 230.2degreesE, A(95) = 4.1degrees) for the Kimpo area. This palaeomagnetic pole shows good agreement with the coeval poles for the South China Block and is significantly removed from that for the North China Block, suggesting that the Kimpo area has been part of the South China Block since at least Early Jurassic times. The Kimpo area constitutes a part of the Imjingang Belt which is palaeontologically allocated to the South China Block in its origin, while all Precambrian tectonic blocks that compose the basement of the Korean Peninsula are interpreted to be of North China origin. We conclude that the Imjingang Belt, including the Kimpo area, is a tectonic extension of the South China Block on the Korean Peninsula. The South China elements were obducted onto the North China Block at the site of the present Korean Peninsula during the collision event of the North and South China Blocks, and hence the Korean Peninsula is composed of areas with both North and South China affinities.

In order to better constrain the Mesozoic apparent polar wander path for the North China Block, Late Triassic and Early Jurassic sandstones in the Ordos Basin and Early Jurassic sandstones in the Datong area were collected at 37 sites. Thermal demagnetization isolated a high-temperature magnetization component with unblocking temperatures of 500-580 degreesC from five sites of the Early Jurassic Yongdingzhuang Formation in the Datong area. Palaeomagnetic directions from the five sites yielded a positive fold test at the 95 per cent confidence level. These directions combined together with previously reported ones give a new Early Jurassic mean direction (D = 358.0degrees, I = 50.6degrees, alpha(95) = 4.4degrees) and corresponding palaeomagnetic pole (81.5degreesN, 302.5degreesE, A(95) = 5.5degrees). The palaeomagnetic pole is regarded as representative of the North China Block because this pole is derived from two localities in the undisputed part of the North China Block. Comparisons of the Early Jurassic pole for the North China Block with coeval ones for the South China Block identify significant differences between the poles from the two regions. At Early Jurassic times, the North China Block was at the peak of rotational motion associated with the collision of the North and South China Blocks. The North China Block underwent a counterclockwise rotation of 12degrees-32degrees with respect to the South China Block since the Early Jurassic, at least 17 per cent of the total rotational motion remaining at that time.