Early Permian zircon uranium-lead ages for plagiogranites in the Yakuno ophiolite, Asago district, Southwest Japan

Abstract Prior work has defined a two-stage history for Yakuno ophiolite petrogenesis consisting of older oceanic basement, and younger island-arc rocks and subordinate elastic rocks. First-stage Yakuno rocks have transitional-type M orb or oceanic plateau affinities, and second-stage Yakuno rocks are more similar to immature island-arc settings. Zircon U-Pb isotopic dates of plagiogranite from the Asago district of Southwest Japan yield crystallization ages of 285 ± 2 Ma for the first-stage ophiolite component, and 282 ± 2 Ma for the second-stage component of the ophiolite. These results indicate that the two petrologi-cally distinct components of the Yakuno ophiolite in this area formed in a short time interval in the Early Permian. The zircon U-Pb crystallization ages provide a maximum age for the base of the stratigraphically overlying Maizuru Group.     

Geochemistry of the ophiolite and oceanic island basalt in the Kangxian-Pipasi-Nanping tectonic melange zone, South Qinling and their tectonic significance

Detailed studies indicate that Kangxian-Pipasi-Nanping tectonic zone is a complicated melange zone which includes many tectonic slabs of different origins. Ophiolite (MORB-type basalt), oceanic island tholeiite and alkaline basalt have been identified. Moreover, this tectonic melange zone is eastward connected with the Mianlue suture zone. The deformation characteristics, consisting components and volcanic rock geochemical features for the Kangxian-Pipasi-Nanping tectonic melange zone are much similar to those of the Mianlue suture zone and Deerni ophiolite. Therefore, the Kangxian-Pipasi-Nanping tectonic melange zone should be the westward extension part of the Mianlue suture zone. It indicates that the Mianlue suture zone had extended to the Nanping area.     

Fault tectonic analysis of Kii peninsula,Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.     

Modern tectonic stress field in Southwest Yunnan, China

IntroductionSouthwest Yunnan region is located in the southeastern margin of Qinghai-Tibet Plateau, bordering on the Sichuan-Yunnan rhomboidal block in the east and on the northeast corner of Indian Plate in the northwest. It is one of the regions of intense tectonic activities and frequent strong earthquakes in China continent. To study the features of modern tectonic stress field of this region is of significance for revealing the evolution mechanisms of Tibetan Plateau as well as for un…     

青岛马山-王哥庄断裂带构造活动性分析

本文从构造地质学、地震学和地球动力学的角度出发,论述了青岛及邻区的地质构造背景,阐述了青岛马山—王哥庄断裂带的构造活动特征,分析了青岛地区的地震活动性.通过分析2003年6月和2004年11月的崂山震群活动特征,进一步确定了马山—王哥庄断裂带的构造活动性.受来自南黄海的区域构造应力作用,该断裂带尤其是其东南段表现出较强的构造活动性,具有引发有感乃至中等地震的可能性.     

Crustal shortening of Southwest Japan in the Late Miocene

Abstract Tectonic deformation of an island arc is interpreted on the basis of geophysical data. Extensive reflection seismic, gravity, geomagnetic data around the back-arc region of Southwest Japan delineate east-west to northeast-southwest folding, and imply conspicuous compression on the southern margin of the Sea of Japan. Because geological data of exploration boreholes indicate that the coinpressive regime was dominant in the late Miocene, the tectonic event seems to be linked with coeval resumption of subduction of the Philippine Sea Plate. Strong coupling of the young buoyant oceanic plate brought about north-south shortening of the overriding continental lithosphere, and left wrench deformation at the southwestern corner of the Sea of Japan. Amount of shortening for the back-arc shelf and mountainous ranges of Southwest Japan is estimated to be ca 10 km, adopting a uniform ratio of shortening (0.944) since the Miocene determined on the shelf from depth-converted seismic profiles. Along the western side of a bend of boundary between the Eurasian Plate and Philippine Sea Plate, the middle Miocene and younger sediments upon the back-arc shelf are much less deformed than the northern equivalents, and the fore-arc Miocene strata are deformed by left wrenching, facts which are indicative of northerly initial convergence of the Philippine Sea Plate at the end of Miocene and crustal decoupling on the west of Kyushu Island.     

Isotopic equilibration ages for the Miyanohara tonalite from the Higo metamorphic belt in central Kyushu, Southwest Japan: Implications for the tectonic setting during the Triassic

Abstract Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early Permian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceous Shiraishino granodiorite in the north. The Miyanohara tonalite yielded three mineral ages: (i) 110–100 Ma for Sm–Nd and Rb–Sr internal isochrons and for K–Ar hornblende; (ii) 183 Ma for Sm–Nd internal isochron; and (iii) 211 Ma for Sm–Nd internal isochron. The ages of 110–100 Ma may indicate cooling age due to the thermal effect of the Shiraishino granodiorite intrusion. The ages of 183 Ma and 211 Ma are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in the sample which gave 183 Ma shows discontinuous zoning under microscope, whereas the one which gave 211 Ma does not show zonal structure. These mineralogical features suggest that the 183 Ma sample has suffered severely from later tectonothermal effect compared with the 211 Ma sample. Therefore, the age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a marginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active continental margin during the late Triassic.     

Paleoseismology of the Itoigawa-Shizuoka tectonic line in central Japan

The 150-km-long Itoigawa-Shizuoka tectonic line active fault system (ISTL)in central Japan is one of the most active Quaternary fault systems inJapan. Estimated slip-rates on the fault system are as large as 10 m/ka, butthe historic seismicity has been low since 841 A.D. with no largeearthquakes recorded. The high slip rates contrast with the long time sincethe last major earthquake on the ITSL and indicates the high potential of a largeearthquake from the ISTL. Based on slip-rate estimates, more than 10 mof potential slip may have accumulated on the fault system since the 841A.D. earthquake. Recent paleoseismolgical studies on the middle andnorthern parts of the ISTL have determined that the average recurrenceinterval of surface-faulting earthquakes on the middle ISTL is 680 to 825years (Gofukuji fault) and 1258 to 1510 years in the northern ISTL. These data suggest the most recent event on both northern and middleISTL occurred in 841 A.D. The results highlight the high seismic potentialof the ISTL. Additional studies of the entire ISTL are needed to define theextent of the next rupture.     

Paleoearthquakes and long-term seismic regime in the Longmenshan fault zone,Southwest China

The collected paleoseismological data about the ancient earthquakes are analyzed for the zone of the Wenchuan earthquake. Four earthquakes with magnitude М = 8.0 occurred over a period of 10000 years and two earthquakes with magnitude 7.5 occurred over 1200 years. The obtained data allowed us to reconstruct the long-term seismic regime within the studied territory by constructing the frequency–magnitude relationship based on the instrumental, historical, and paleoseismological data.     

Topography of the Moho and Conrad discontinuities in the Kyushu district,Southwest Japan

The first P-arrival time data from local earthquakes are inverted for two-dimensional variation of the depths to the Conrad and Moho discontinuities in the Kyushu district, southwest Japan. At the same time, earthquake hypocenters and station corrections are determined from the data. The depths to the discontinuities are estimated by minimizing the travel time residuals of first P-arrival phases for 608 earthquakes observed at 57 seismic stations. In the land area of Kyushu, the Conrad and Moho discontinuities are located within the depth ranges of 16–18 and 34–40 km, respectively. The Conrad discontinuity is not as largely undulated as the Moho discontinuity. The depth to the Moho is deep along the east coast of Kyushu, and the deepest Moho is closely related to markedly low velocity of P wave. We regard the deepest Moho as reflecting the Kyushu–Palau ridge subducting beneath the Kyushu district, together with the Philippine Sea slab. In western Kyushu, the shallow Moho is spreading in the north–northeast–south–southwest direction in the Okinawa trough region. Based on the presence of low-velocity anomaly in three-dimensional velocity structure and seismogenic stress field of shallow crustal earthquakes, the shallow Moho is interpreted as being due to lower crustal erosion associated with a small-scale mantle upwelling in the Okinawa trough region. The velocity discontinuity undulation basically has insignificant effect on hypocenter determination of the local earthquakes, but the Moho topography makes changes in focal depths of some upper mantle earthquakes. The depth variation of the Moho discontinuity has a good correlation with the Bouguer gravity anomaly map; i.e., the shallow Moho of western Kyushu and the deep Moho of eastern Kyushu closely correlate with the positive and negative Bouguer gravity anomalies, respectively.     

Volcanic rocks and the depth of the Benioff zone in Sud Lipez,Southwest Bolivia

The depth of the Benioff zone has been determined from the new analysis of volcanic rocks and from new seismic data from S.W. Bolivia. Both data give consistently 260 km as a depth of the Benioff zone and for the origin of magma of calc-alkali magma series in SW-Bolivia. These inferences are consistent with the plate tectonics model.     

Thermal structure and paleo-heat flow in the Shimanto accretionary prism, Southwest Japan

Abstract Thermal structural analysis and paleo-heat flow estimation provide clues to understanding the thermal evolution of the accretionary complex. The thermal structure and heat flow in the Jurassic Chichibu and Cretaceous to Tertiary Shimanto accretionary complex, Southwest Japan, have been investigated by vitrinite reflectance measurement and fluid inclusion analysis. As a result, the local and multistage metamorphisms were recognized as follows. First, the Tertiary complex around the Miocene Ashizuri granite underwent exposure to extra-high temperatures. Second, the Okitsu Melange underwent exposure to higher temperatures than the surrounding strata and was formed concurrently with the Kula-Pacific ridge subduction beneath the Japanese Islands in the Eocene. Finally, the thermal structure of most of the Cretaceous and southern Jurassic complexes is independent of the geologic structure, indicating that these areas suffered thermal overprint. Regional radiometric dating studies show that most of the Cretaceous Shimanto complex was heated in the Eocene; the thermal overprint might have occurred as a result of ridge subduction. The heat flow during peak heating was estimated to be 95–120 mW/m² except for the Cretaceous Okitsu melange and the Cretaceous Nonokawa formation, north of the Okitsu Melange; a much higher value of heat flow of ~200 mW/m² was estimated in the Okitsu Melange. An estimation of heat flow failed for the non-okawa formation because thermal equilibrium between the fluid and rocks has not yet been reached. It is probable that the southern strata underwent a higher heat flow. Such a trenchward increase in heat flow resembles the present situation of the Nankai Trough, although the heat flow in the Eocene was much higher.     

Magnetotelluric survey across the active spreading zone in Southwest Iceland

The results of 10 magnetotelluric soundings, performed along a 110-km-long profile crossing the constructive plate boundary in southwest Iceland, are presented. Apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross-section along the profile. The crust-mantle interface contains a well conductive layer. The depth to the good conductor increases with age of the crust and the distance from the axial zone. This layer is interpreted as partially molten basalt, at a temperature about 1100°C and a volume fraction of the melt phase in the range 10–20%.The high-conductivity layer probably disappears west of the Borgarnes anticlinal axis, which separates the older (to the west) and younger (to the east) flood basalts in western Iceland, indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt. Recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber, which agrees with the results of the magnetotelluric soundings.     

Pre-seismic crustal deformation caused by an underthrusting oceanic plate,in eastern Hokkaido,Japan

Eastern Hokkaido is being considered as one of the prime candidates for a future great earthquake in Japan. Geodetic work and seismological data suggest that the continental plate is being compressed and dragged down into the asthenosphere by the underthrusting Pacific plate.A quantitative examination of this idea was carried out by an application of the finite-element method to a static-field problem of crustal deformation. The effect of heterogeneous medium with Poisson's ratio differing from 0.25 is significant. An inverse problem was solved by applying a quadratic programming method to the observed vertical displacement.A model, which assumes a nearly uniform tangential displacement along the interface of the oceanic and continental lithospheres down to a depth of 100 km, explains a gravity-change as well as the general features of crustal deformation observed in eastern Hokkaido. The displacement rate is estimated to be 2.7 cm/year. It indicates that a rebound of the continental lithosphere now brings about a thrust-type earthquake whose reverse slip is comparable to that estimated for great shallow earthquakes along the Kuril trench. A possibility of aseismic rebound is suggested at the lower part of the interface.     

Segmentations of foreland belts and their tectonic mechanism in the Southwest Tarim Basin

Many geologists focus on the foreland structures, co-relationship between shallow and deep structures and their dynamics between intra-continent orogenies and foreland basin in recent years[1―17]. The intra-plate collision and deformed area of West Kunlun-Pamirand Southern Tianshan become the natural lab of this studies and there are many new developments con-cerned with the geometry and kinemics of foreland thrusting, back-thrusting and triangle zones[3―14]. Many types of foreland thrusti…     

Acoustic properties of deformed rocks in the Nobeoka thrust,in the Shimanto Belt,Kyushu, Southwest Japan

Laboratory measurements for compressional and shear wave velocities (Vp and Vs, respectively) and porosity were conducted with core samples from the Nobeoka Thrust Drilling Project (NOBELL) under controlled effective pressure (5–65 MPa at 5 MPa intervals) and wet conditions. Samples were classified according to deformation texture as phyllite, foliated cataclasite, or non‐foliated cataclasite. Measured values of Vp, Vs, and porosity are within a range of 5.17–5.57 km/s, 2.60–2.71 km/s, and 2.75–3.10 %, respectively, for phyllite; 4.89–5.23 km/s, 2.46–2.57 km/s, and 3.58–4.53 %, respectively, for foliated cataclasite; and 4.90–5.32 km/s, 2.51–2.63 km/s, and 3.79–4.60 %, respectively, for non‐foliated cataclasite, which are all consistent with the previous laboratory experiments conducted with outcrop samples under dry conditions. However, our results also indicate higher Vp and Vs and lower porosity than those measured by the previous studies that adopted the wire‐line logging methods. The variations in Vp, Vs, and porosity are controlled by deformation structure and are greater for phyllite and foliated cataclasite than for non‐foliated cataclasite.     

岩浆与构造作用对洋壳厚度的影响——以西北印度洋为例

洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度...     

Large amount of fluid migration around shallow seismogenic depth preserved in tectonic mélange: Yokonami mélange,the Cretaceous Shimanto Belt,Kochi, Southwest Japan

Studying subduction zone fluid at shallow seismogenic depths is important to understand the nature of fault rocks at the updip limit of the seismogenic zone because fluid–rock interactions affect heat and mass transfer, and fault strength. In this study, we conducted detailed analyses of distribution of shear veins, and estimation of pressure–temperature conditions for shear vein formation for the Yokonami mélange, Shikoku, Southwest Japan, which is tectonic mélange zone in an on‐land accretionary complex. We found a seismogenic fault at the upper boundary of the Yokonami mélange, indicating that the Yokonami mélange was active at seismogenic depth. The field‐transect distribution of shear veins was examined. The frequency, the total and mean thicknesses of the shear veins were about 3.7 per meter, about 10 mm per meter, and about 3 mm per shear vein, respectively. Quartz within the shear veins shows elongate‐blocky textures, suggesting precipitation from advective flow. The pressure and temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging 175–225°C and 143–215 MPa, respectively. The temperature is almost consistent with the paleotemperature determined from vitrinite reflectance, suggesting that the shear veins were formed at up to the maximum depth. The depth might be consistent with that where the seismogenic fault was formed. On the basis of the pressure and temperature conditions and the distribution of shear veins, we estimated the minimum volumetric ratio of fluid to host rocks, assuming that the shear veins had precipitated from advective flow. The estimated amount of fluid is about 106 m³ per cubic meter of host rocks. The results suggest that a large amount of fluid migrates through mélange zones at shallow seismogenic depths. This fluid possibly originates from the dehydration of clay minerals from underthrusted sediments and an altered subducting slab.