Structural control of the Gagua "Wedge" Zone east of Taiwan Island on the southern Okinawa Trough

Based on compositive analysis and interpretation of the observed and historical data, the geophysical field characters and structural properties of the Gagua "Wedge" Zone of the sea area east of Taiwan Island and the primary tectonic stress direction and its variabilities of backarc spreading in the southern Okinawa Trough are studied. It is concluded from the study results that the Gagua "Wedge" Zone is structurally consistent with the Gagua ridge and two fault basins on both sides of the Gagua ridge, and adjusts the moving direction and distance of the western Philippine Sea plate to make the northwestward motion of the plate on its east side change to the northward subduction of the plate on its west side so that the primary tectonic stress direction of the Okinawa Trough changed from NW-SE to nearly N-S, which provided the stress source for the Okinawa Trough to enter the second spreading stage.     

Gravity signals from the lithosphere in the Central European Basin System

We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian–Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic–Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS.Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north–east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco-Norwegian orogeny. The major part of the NGB is characterized by high-density lithosphere, which includes a high-velocity lower crust (relict of Baltica passive margin) overthrusted by the Avalonian terrane. The short wavelength pattern of the final residuals shows several north–west trending gravity highs between the Tornquist Zone and the Elbe Fault System. The NDB is separated by a gravity low at the Ringkøbing–Fyn high from a chain of positive anomalies in the NGB and the PT. In the NGB these anomalies correspond to the Prignitz (Rheinsberg anomaly), the Glueckstadt and Horn Graben, and they continue further west into the Central Graben, to join with the gravity high of the Central North Sea.     

Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution

We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region.     

Transport process of radionuclides in the East China Sea

Settling particles were collected from six stations at the Okinawa Trough and the East China Sea continental margin. Activities of U, Th, Pu isotopes and ^210Pb were determined for the particles to elucidate their transport processes. Surface sediment samples were also analyzed for their isotopes. There was a tendency for ^210Pb activities to increase almost linearly with depth from 72 m on the continental shelf edge to 1019 m in the Okinawa Trough. Increasing ^210Pb activities in settling particles with depth on the continental margin may be attributable to enhanced ^210Pb scavenging by particles and removal near the front. There also was a clear tendency for total mass fluxes and the radionuclide fluxes to increase with depth, with an especially large increase near the bottom. The ratio of the observed ^210Pb flux to the ^210Pb deficiency flux in the near-bottom traps ranged between 8.9 and 46. These high values show a strong ^210Pb excess which would be attributable to large advective import to the near-bottom and resuspended particles that have settled through the water column but have not been incorporated into the sediments. High variability of radionuclide fluxes occurred in very short time periods. Activities of ^238U, ^232Th, ^230Th, ^228Th, ^210Pb and ^239Pu＋^240Pu in settling particles were significantly higher than those in the underlying surface sediments.     

Modeling sideswipe in 2D oceanic seismic surveys from sonar data: Application to the Mariana arc

In two-dimensional (2D) marine seismic-reflection surveys, out-of-plane rough seafloor bathymetry can cause multiple ocean-bottom reflections that complicate the interpretation of shallow reflections. Although migration corrects for the in-plane position of reflectors, it cannot resolve the inherent ambiguity in their out-of-plane positions. We show how swath bathymetry, routinely collected in many such surveys, can be used to model out-of-plane seafloor reflections and prevent their misinterpretation as subsurface geology. We use both raw and gridded multi-beam bathymetry data to build images that represent seafloor reflections in migrated seismic data. Comparison of these images to the seismic sections reveals whether suspicious features are out-of-plane water bottom reflections or subsurface reflections. Multi-channel seismic surveys across the Marianas intra-oceanic arc system provide examples where rough seafloor topography produced reflections that were initially misinterpreted. We use our seafloor reflection modeling (SRM) approach to help distinguish a possible landslide from a volcanic cone, to help distinguish real from apparent fault-plane reflections bounding a sediment-filled basin, and to verify that a possible magma chamber reflection results from sub-surface structure, not seafloor sideswipe.     

Contribution de la gravimétrie et de la sismique réflexion pour une nouvelle interprétation géodynamique des fossés d'effondrement en Tunisie : exemple du fossé de Grombalia

Troughs in Tunisia are interpreted as Plio-Quaternary structures associated to normal faults (grabens) or to flexure faults. Gravity data and seismic sections are used in this study to clarify the structure and the geodynamic evolution of an example of trough: the Grombalia trough (northeastern Tunisia), since the Upper Miocene to the Quaternary. A high residual negative gravity anomaly, which reaches ?15 mGal, is interpreted as being related to the thickening of Mio-Plio-Quaternary deposits (and probably older), as illustrated by seismic data. This subsidence has been the result of a negative flower structure related to strike-slip faults that have been reactivated with normal component during the Upper Miocene and with reverse component during the Pliocene. Seismic and gravity data demonstrate that the fault system is rooted, and more than four kilometres deep. The Grombalia example outlines the association between troughs and strike-slip faults; such a system is recognized in Tunisia, in the Ionian Sea and in the Pelagian Sea. To cite this article: M. Hadj Sassi et al., C. R. Geoscience 338 (2006).     

The role of slump scars in slope channel initiation: A case study from the Miocene Jatiluhur Formation in the Bogor Trough,West Java

This paper investigates slope channel initiation by seabed irregularities that were initially formed by slump scars in the lower to middle Jatiluhur Formation, part of the middle- to late Miocene successions in the Bogor Trough, West Java. This Miocene succession is up to 1000 m thick in the study area, and is interpreted as a prograding slope–shelf system that formed during a period of falling- and lowstand stages in relative sea level. The lower part of the formation is a siltstone-dominated siliciclastic succession, containing slump deposits, slump-scar-fill deposits, and minor channel-fill deposits, which formed in slope and shelf-margin environments. In contrast, the middle part, which gradationally overlies the lower part, is characterized by shallow-marine carbonates.The slump-scars-fill deposits have an overall lenticular geometry, and are 140–480 m wide and 0.4–1.6 m thick. Some have distinct erosional bases, which cut into the underlying siltstones, in association with medium- to coarse-grained sandstones with lateral-accretion surfaces and tractional structures common in channel-fill deposits. The incident link of slump-scar-fill deposits and channel-fill deposits in the prograding slope–shelf succession of the lower to middle Jatiluhur Formation suggests that some slump scars formed incipient seabed irregularities that may have played an important role in the development of slope channels. The present study provides one example of the various potential mechanisms that can result in channel formation in a slope setting.     

Petrologic perspectives on tectonic evolution of a nascent basin (Okinawa Trough) behind Ryukyu Arc：A review

Okinawa Trough is a back-arc, initial marginal sea basin, located behind the Ryukyu Arc–Trench System. The formation and evolution of the Okinawa Trough is intimately related to the subduction process of the Philippine Sea Plate beneath the Eurasian Plate since the late Miocene. The tectonic evolution of the trough is similar to other active back-arcs, such as the Mariana Trough and southern Lau Basin, all of which are experiencing the initial rifting and subsequent spreading process. This study reviews all petrologic and geochemical data of mafic volcanic lavas from the Okinawa Trough, Ryukyu Arc, and Philippine Sea Plate, combined with geophysical data to indicate the relationship between the subduction sources(input) and arc or back-arc magmas(output) in the Philippine Sea Plate–Ryukyu Arc–Okinawa Trough system(PROS). The results obtained showed that several components were variably involved in the petrogenesis of the Okinawa Trough lavas: sub-continental lithospheric mantle underlying the Eurasian Plate, Indian mid-oceanic ridge basalt(MORB)-type mantle, and Pacific MORB-type mantle. The addition of shallow aqueous fluids and deep hydrous melts from subducted components with the characteristics of Indian MORB-type mantle into the mantle source of lavas variably modifies the primitive mantle wedge beneath the Ryukyu and subcontinental lithospheric mantle(SCLM) beneath the Okinawa Trough. In the northeastern end of the trough and arc, instead of Indian MORB-type mantle, Pacific MORB-type mantle dominates the magma source. Along the strike of the Ryukyu Arc and Okinawa Trough, the systematic variations in trace element ratios and isotopic compositions reflect the first-order effect of variable subduction input on the magma source. In general, petrologic data, combined with geophysical data, imply that the Okinawa Trough is experiencing the "seafloor spreading" process in the southwest segment, "rift propagation" process in the middle segment, and "crustal extension" process in the northeast segment, and a nascent ocean basin occurs in the southwest segment.     

冲绳海槽中段沉积物物源识别及其热液活动指示

冲绳海槽中段热液活动区表层沉积物的主、微量和稀土元素分析结果表明:区内沉积物主要由陆源物质与热液源物质组分组成,Hg、Au、Sb、Cu、Pb、Zn、Ba、As、Fe和Co等微量元素富集;沉积物的化学风化程度中等,A-CN-K图解表明其暂未受到钾交代影响,且其母岩成分接近花岗闪长岩;北美页岩标准化稀土配分模式曲线整体较为平坦,轻重稀土分馏较弱,部分样品具有与热液流体类似的明显正铕异常.受热液活动影响,部分沉积物中的Fe、Cu、As、Pb和Zn显著富集,结合样品所处站位,整体显示含金属沉积物-过渡沉积物-正常沉积物的空间分配模式.结合(La/Sm)N、(La/Yb)N比值指示区内沉积物的陆源物质主要来自于黄河与台湾岛河流.     

Effects of topography on the sub-tidal circulation in the southwestern Huanghai Sea (Yellow Sea) in summer

A nested circulation model system based on the Princeton ocean model(POM) is set up to simulate the currentmeter data from a bottom-mounted Acoustic Doppler Profiler(ADP) deployed at the 30 m depth in the Lunan(South Shandong Province,China) Trough south of the Shandong Peninsula in the summer of 2008,and to study the dynamics of the circulation in the southwestern Huanghai Sea(Yellow Sea).The model has reproduced well the observed subtidal current at the mooring site.The results of the model simulation suggest that the bottom topography has strong steering effects on the regional circulation in summer.The model simulation shows that the Subei(North Jiangsu Province,China)coastal current flows northward in summer,in contrast to the southeastward current in the center of the Lunan Trough measured by the moored currentmeter.The analyses of the model results suggest that the southeastward current at the mooring site in the Lunan Trough is forced by the westward wind-driven current along the Lunan coast,which meets the northward Subei coastal current at the head of the Haizhou Bay to flow along an offshore path in the southeastward direction in the Lunan Trough.Analysis suggests that the Subei coastal current,the Lunan coastal current,and the circulation in the Lunan Trough are independent current systems controlled by different dynamics.Therefore,the current measurements in the Lunan Trough cannot be used to represent the Subei coastal current in general.