Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite b_o, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.     

Cretaceous episodic growth of the Japanese Islands

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.     

Distribution of thermal areas on an active lava flow field: Landsat observations of Kilauea,Hawaii, July 1991

A Landsat Thematic Mapper (TM) image acquired on 23 July 1991 recorded widespread activity associated with the Episode 48 of the Pu'u 'O'o-Kupaianaha eruption of Kilauea Volcano, Hawaii. The scene contains a very large number (>3500) of thermally elevated near infrared (0.8–2.35 m) pixels (each 900 m²), which enable the spatial distribution of volcanic activity to be identified. This activity includes a lava lake within Pu'u 'O'o cone, an active lava tube system (7.9 km in length) with skylights between the Kupaianaha lava shield and several ocean entry points, and extensive active surface flows (total area of 1.3 km²) within a much larger area of cooling flows (total16 km²). The production of an average flux density map from the TM data of the flow field, wherein the average flux density is defined in units of Wm^-2, allows for the chronology of emplacement of active and cooling flows to be determined. The flux density map reveals that there were at least three breakouts (>5000 Wm^-2) feeding active flows, but on the day that the data were collected the TM recorded a waning phase of surface activity in this area, based on the relatively large amount of intermediate power-emitting (cooling) flows compared to high power-emitting (active) flows. The production of a comparable flux density map for future eruptions would aid in the assessment of volcanic hazards if the data were available in near-real time.     

西昆仑造山带复式增生楔的构造特征与演化

通过对西昆仑古生代主剪切带和早中生代增生楔杂岩剖面中的组成与构造等特征，西昆仑早古生代和晚古生代一早中生代两次向南的增生造山作用及其增生楔的组成与构造演化进行系统研究，探讨增生楔杂岩在大陆造山带及其对大陆侧向增生的意义。     

Preservation of aeolian genetic units by lava flows in the Lower Cretaceous of the Paraná Basin, southern Brazil

The Lower Cretaceous geological record of the intracratonic Paraná Basin in southern Brazil comprises a thick succession of aeolian sandstones and volcanic rocks. The intercalation between aeolian sandstone and volcanic floods allowed the preservation of distinct aeolian genetic units. Each genetic unit represents an accumulation episode, bounded by supersurfaces, that coincides with the base of lava flood events. The entire package can be subdivided into a Lower Genetic Unit, which corresponds to aeolian sandstones preserved below the initial lava flows (Botucatu Formation), and an upper set of genetic units, which comprises interlayered aeolian deposits and lava floods (Serra Geral Formation). The Lower Genetic Unit is up to 100 m thick. Its base is composed of ephemeral stream and aeolian sand sheet deposits that are overlain by cross‐bedded sandstones whose origin is ascribed to simple, locally composite, crescentic and complex linear aeolian dunes. Aeolian accumulation of the lower unit was possible as a result of the existence of a wide topographic basin, which caused wind deceleration, and a large sand availability that promoted a positive net sediment flux. The Upper Genetic Units comprise isolated sand bodies that occur in two different styles: (1) thin lenses (<3 m thick) formed by aeolian sand sheets; and (2) thick sand lenses (3–15 m) comprising cross‐bedded cosets generated by migration and climbing of simple to locally composite crescentic aeolian dunes. Accumulation of the aeolian strata was associated with wind deceleration within depressions on the irregular upper surface of the lava floods. The interruption of sedimentation in the Lower and Upper Genetic Units, and related development of supersurfaces, occurred as a result of widespread effusions of basaltic lava. Preservation of both wind‐rippled topset deposits of the aeolian dunes and pahoehoe lava imprints indicates that lava floods covered active aeolian dunes and, hence, protected the aeolian deposits from erosion, thus preserving the genetic units.     

Mid–Cretaceous Episodic Magmatism and Tin Mineralization in Khingan-Okhotsk Volcano–Plutonic Belt, Far East Russia

Abstract: Age of magmatism and tin mineralization in the Khingan‐Okhotsk volcano–plutonic belt, including the Khingan, Badzhal and Komsomolsk tin fields, were reviewed in terms of tectonic history of the continental margin of East Asia. This belt consists mainly of felsic volcanic rocks and granitoids of the reduced type, being free of remarkable geomagnetic anomaly, in contrast with the northern Sikhote‐Alin volcano–plutonic belt dominated by oxidized‐type rocks and gold mineralization. The northern end of the Khingan‐Okhotsk belt near the Sea of Okhotsk, accompanied by positive geomagnetic anomalies, may have been overprinted by magmatism of the Sikhote‐Alin belt. Tin–associated magmatism in the Khingan‐Okhotsk belt extending over 400 km occurred episodically in a short period (9510 Ma) in the middle Cretaceous time, which is coeval with the accretion of the Kiselevka‐Manoma complex, the youngest accretionary wedge in the eastern margin of the Khingan‐Okhotsk accretionary terranes. The episodic magmatism is in contrast with the Cretaceous‐Paleogene long–lasted magmatism in Sikhote–Alin, indicating the two belts are essentially different arcs, rather than juxtaposed arcs derived from a single arc. The tin‐associated magmatism may have been caused by the subduction of a young and hot back‐arc basin, which is inferred from oceanic plate stratigraphy of the coeval accre‐tionary complex and its heavy mineral assemblage of immature volcanic arc provenance. The subduction of the young basin may have resulted in dominance of the reduced‐type felsic magmas due to incorporation of carbonaceous sediments within the accretionary complex near the trench. Subsequently, the back‐arc basin may have been closed by the oblique collision of the accretionary terranes in Sikhote–Alin, which was subjected to the Late Cretaceous to Paleogene magmatism related to another younger subduction system. These processes could have proceeded under transpressional tectonic regime due to oblique subduction of the paleo‐Pacific plates under Eurasian continent.     

陕西宽坪岩群变基性熔岩锆石U-Pb年龄

宽坪岩群广东坪岩组绿片岩岩石学、岩石化学、稀土、微量元素、副矿物等研究表明其原岩为基性火山岩。对商州市板桥地区基性熔岩变质而成的绿片岩区域地质、地球化学特点、锆石特征进行了详细研究和单颗粒锆石U-Pb同位素年龄测定。放射性成因铅丢失最少的岩浆成因锆石的206Pb/238U表面年龄为1154Ma,变质成因锆石的206Pb/238U表面年龄为444Ma。5个锆石颗粒构成的不一致线上交点年龄为1827±11Ma,代表基性熔岩形成年龄;下交点年龄为418±8Ma,代表加里东期变质年龄。综合认为宽坪岩群形成时代为早元古代。     

Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene-Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.     

Audio frequency magneto-telluric survey of Norikura Volcano in central Japan

Norikura Volcano has not been active during the last 10,000 years in spite of the activity of the surrounding volcanic mountains. To study past volcanic activities, geological studies were carried out extensively. However, quite a few geophysical investigations were conducted to contribute to volcanology. Our objective is to detect the present subsurface structure of Norikura Volcano and to define volcanic stratifications. In the vicinity of Norikura Volcano, geothermal fields are still active. Subsurface volcanic rocks in this area have been exposed to geothermal activity and altered. To comprehend volcanic stratifications of Norikura and geothermal activity, we conducted audio frequency magneto-telluric (AMT) surveys around Norikura Volcano. AMT survey is useful in clearly defining the resistivity structure related to volcanic regions. The AMT data were acquired over a frequency range 10 Hz–10 kHz. Decomposition analysis was applied to the tensor impedance data. Subsequently, apparent resistivity and phase data were inverted using a two-dimensional magneto-telluric (MT) inversion and a model of Norikura was derived. The final model manifests that the surface resistors are in agreement with andesite lava or dacite lava. As for the deeper structure, a horizontal conductor is situated above resistive basements. The alteration of the conductor was weak, while basement rocks were strongly altered and/or heated through the thermal activity. The existence of these layers seems to indicate the degree of thermal activity of Norikura Volcano.