Fast static correction methods for high-frequency multichannel marine seismic reflection data: A high-resolution seismic study of channel-levee systems on the Bengal Fan

Very high-frequency marine multichannel seismic reflection data generated by small-volume air- or waterguns allow detailed, high-resolution studies of sedimentary structures of the order of one to few metres wavelength. The high-frequency content, however, requires (1) a very exact knowledge of the source and receiver positions, and (2) the development of data processing methods which take this exact geometry into account. Static corrections are crucial for the quality of very high-frequency stacked data because static shifts caused by variations of the source and streamer depths are of the order of half to one dominant wavelength, so that they can lead to destructive interference during stacking of CDP sorted traces. As common surface-consistent residual static correction methods developed for land seismic data require fixed shot and receiver locations two simple and fast techniques have been developed for marine seismic data with moving sources and receivers to correct such static shifts. The first method – called CDP static correction method – is based on a simultaneous recording of Parasound sediment echosounder and multichannel seismic reflection data. It compares the depth information derived from the first arrivals of both data sets to calculate static correction time shifts for each seismic channel relative to the Parasound water depths. The second method – called average static correction method – utilises the fact that the streamer depth is mainly controlled by bird units, which keep the streamer in a predefined depth at certain increments but do not prevent the streamer from being slightly buoyant in-between. In case of calm weather conditions these streamer bendings mainly contribute to the overall static time shifts, whereas depth variations of the source are negligible. Hence, mean static correction time shifts are calculated for each channel by averaging the depth values determined at each geophone group position for several subsequent shots. Application of both methods to data of a high-resolution seismic survey of channel-levee systems on the Bengal Fan shows that the quality of the stacked section can be improved significantly compared to stacking results achieved without preceding static corrections. The optimised records show sedimentary features in great detail, that are not visible without static corrections. Limitations only result from the sea floor topography. The CDP static correction method generally provides more coherent reflections than the average static correction method but can only be applied in areas with rather flat sea floor, where no diffraction hyperbolae occur. In contrast, the average static correction method can also be used in regions with rough morphology, but the coherency of reflections is slightly reduced compared to the results of the CDP static correction method.     

The Cauvery Shear Zone, Southern Granulite Terrain, India: A crustal-scale flower structure

The Cauvery Shear Zone (CSZ) is a crustal-scale shear system within the Southern Granulite Terrain along the southern margin of the Archaean Dharwar craton. Structural interpretation of satellite data and field observations reveal four major shear zones within the CSZ system. They show dextral shear kinematics synchronous with a major Neoproterozoic tectono-metamorphic event (D2) associated with intracrustal melting and migmatisation. The disposition, geometry and contemporaneity of shear fabrics of the CSZ system are modelled in terms of a crustal-scale flower structure akin to transpressional and collisional orogens. In the light of recent seismic evidence for a displaced Moho structure and a mid- to lower-crustal low velocity zone, the flower structure across the CSZ may extend to mantle depths.     

Neoproterozoic alkaline magmatism at Sivamalai, southern India

The Sivamalai alkaline complex lies at the southern margin of the Cauvery Shear System that separates the Archaean and Proterozoic domains of the Southern Granulite Terrain in India. U–Pb TIMS dating of zircon from a pegmatitic syenite sample in the complex yields a concordant age of 590.2 ± 1.3 (2σ) Ma which is interpreted to date the intrusion of the alkaline rocks. A lower concordia intercept at 168 ± 210 Ma defined by two grains with high common lead may indicate post-magmatic disturbances due to recrystallisation which is also evident in the CL images of the zircons. EPMA dating of monazite from a post-kinematic pegmatite which intrudes the crystalline basement hosting the alkaline rocks yields an age of 478 ± 29 (2σ) Ma and provides a lower bracket for the main phase of tectonism in this part of the Southern Granulite Terrain. The Pan-African high-grade metamorphism and ductile deformation has thus most likely affected the alkaline rocks. This is supported by the presence of a metamorphic foliation and extensive recrystallisation textures seen in the rocks. The major and trace element concentrations measured on selected samples reveals the presence of both enriched and depleted rock types. The enriched group includes ferrosyenite and nepheline syenite while the depleted group has only nepheline syenites. The trace element depletion of some nepheline syenites is interpreted to be a result of fractional crystallization involving the removal of accessory phases like zircon, titanite, apatite and allanite.     

Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France

Ion-microprobe U–Pb zircon dating of lower-crust metasedimentary granulite are reported on samples from two localities in Europe in order to determine (a) how this environment recorded the Variscan and eo-Alpine events, and (b) whether the transition between the two orogenic cycles was continuous or separated by a gap. The samples come from enclaves hosted by Miocene volcanoes at Bournac in the French Massif Central, and from the granulitic metasedimentary basement of the Alpine Santa Lucia nappe in Corsica, on the South European paleomargin of the Ligurian branch of the Tethys Sea. The zircon ages from Bournac range between 630 and 430 Ma and between 380 and 150 Ma with a major frequency peak at 285 Ma; the zircons older than 430 Ma are interpreted as detrital, whereas those younger than 380 Ma are considered to have formed by metamorphic processes after burial in the lower crust. Zircon ages from Santa Lucia range from to 356 to 157 Ma, with exception of one inherited Archean grain, and are interpreted like the younger Bournac zircons as having been formed by metamorphic processes.

In a granulite metamorphic environment, as opposed to an anatectic environment, new zircon growth can occur in the solid state. Once Zr has been incorporated into zircon, however, it is difficult to remobilize without dissolution; thus Zr available for new zircon growth must result from the breakdown of Zr-bearing minerals during prograde and/or retrograde events. In this light, the U–Pb zircon-age probability curves are interpreted as markers for major tectonometamorphic events, as suggested by the close correspondence between peaks in the curve and geological events recorded in the upper-crust, such as magma emplacement and basin subsidence.

Evidence of a tectonometamorphic gap between the Variscan and Alpine orogeneses is provided by the Santa Lucia zircon-age probability curve, which reveals a probable interlude during the Variscan–Alpine transition between 240 and 210 Ma. Here, the peak at 240 Ma is interpreted as the very beginning of crustal extension and the low at 210 Ma as a period of quiescence prior to the formation of an active margin and oceanization.     

Spinel–sapphirine–quartz bearing composite inclusion within garnet from an ultrahigh-temperature pelitic granulite: Implications for metamorphic history and P–T path

We report here a multiphase mineral inclusion composed of quartz, plagioclase, K-feldspar, sapphirine, spinel, orthopyroxene, and biotite, in porphyroblastic garnet within a pelitic granulite from Rajapalaiyam in the Madurai Granulite Block, southern India. In this unique textural association, hitherto unreported in previous studies, sapphirine shows four occurrences: (1) as anhedral mineral between spinel and quartz (Spr-1), (2) subhedral to euhedral needles mantled by quartz (Spr-2), (3) subhedral to anhedral mineral in orthopyroxene, and (4) isolated inclusion with quartz (Spr-4). Spr-1, Spr-2, and Spr-4 show direct grain contact with quartz, providing evidence for ultrahigh-temperature (UHT) metamorphism at temperatures exceeding 1000 °C. Associated orthopyroxene shows high Mg/(Fe + Mg) ratio ( 0.75) and Al₂O₃ content (up to 9.6 wt.%), also suggesting T > 1050 °C and P > 10 kbar during peak metamorphism.

Coarse spinel (Spl-1) with irregular grain morphology and adjacent quartz grains are separated by thin films of Spr-1 and K-feldspar, suggesting that Spl-1 and quartz were in equilibrium before the stability of Spr-1 + quartz. This texture implies that the P–T conditions of the rock shifted from the stability field of spinel + quartz to sapphirine + quartz. Petrogenetic grid considerations based on available data from the FMAS system favour exhumation along a counterclockwise P–T trajectory. The irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite-bearing silicate-melt pockets during the garnet growth at extreme UHT conditions.     

Impacts of Holocene climatic variations on alluvial fan activity below snowpatches in subarctic Québec

In the Lake Guillaume-Delisle area of subarctic Québec, storm-generated alluvial fans have been active sporadically throughout the Holocene. In this study, we propose that the persistence of late-lying snowpatches in fan catchments during Holocene cold episodes promoted alluvial fan activity by lowering the precipitation threshold required to trigger a torrential event. This hypothesis was tested by characterizing the depositional processes responsible for alluvial fan formation below snowpatches, and by reconstructing the Holocene alluvial fan activity. Stratigraphic and sedimentary analyses conducted on seven alluvial fans revealed that they were deposited by torrential activity leading to waterlaid, transient, or hyperconcentrated deposition. The chronology of the storm-generated alluvial fans — based on 22 radiocarbon dates — indicates that torrential activity was enhanced during the cooler Late Holocene (i.e., after ca. 3500 cal. yr BP). Snowier winters and cooler summers were beneficial to nival activity, allowing the persistence of larger snowpatches throughout the summer and fall seasons. Rainfall-induced thaw of such snowpatches during rainstorm events is inferred to have contributed to alluvial fan activity by increasing water availability. Three peaks of alluvial activity occurred during the Late Holocene (2950–2750, 1900–1400, and 800–300 cal. yr BP) and are indicative of increased storminess resulting in higher fan activity. Increased fan activity during cooler episodes was concurrent with increased runoff activity in the immediate pronival area. This stresses the importance of nivation below snowpatches and pinpoints the role of nivation in enhancing geomorphological activity during period of cooler and more humid climate in subarctic environments.     

Factors influencing the presence or absence of tributary-junction fans in the Iberian Range, Spain

This paper analyses the factors which influence the presence or absence of tributary-junction fans in the Iberian Range, northern Spain. Two valleys were selected, both characterised by wide variations in lithology, altitude, land use and plant cover. Two groups of factors were studied: those related to the internal characteristics of the drainage basins, which particularly control sediment generation; and those related to the characteristics of the depositional area which control accommodation space and main river power. Among the internal factors, the development of alluvial fans was related to: (i) the capacity of the basin to yield large volumes of sediment, (ii) the occurrence of intense human pressure until recent times, a good indicator of sediment yield, and (iii) the capacity of the basin to quickly increase discharge during rainstorms (discharge density and torrentiality). It is suggested that the areas that were intensively cultivated in the past, and have therefore been affected by intense erosion, have played a decisive role on the development of alluvial fans. This would imply that many of these alluvial fans have a relatively recent origin, perhaps related to the beginning of a widespread deforestation. The basins without alluvial fans are characterised by relatively steep hillslope gradients (that is, slopes that never were subjected to historical cultivation), low drainage densities and dense forest and shrub cover, mostly coinciding with high altitude basins composed of quartzite and shale bedrocks. Regarding the external factors, the shape, size and longitudinal gradient of the main river to which the fans are tributary are the most relevant conditioning factors determining the development of alluvial fans.     

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.     

Effective timescales of coupling within fluvial systems

This paper presents a review of the coupling concept in fluvial geomorphology, based mainly on previously published work. Coupling mechanisms link the components of the fluvial system, controlling sediment transport down the system and the propagation of the effects of base-level change up the system. They can be viewed at several scales: at the local scale involving within-hillslope coupling, hillslope-to-channel coupling, and within-channels, tributary junction and reach-to-reach coupling. At larger scales, coupling can be considered as zonal coupling, between major zones of the system or as regional coupling, relating to complete drainage basins. These trends are illustrated particularly by the examples of hillslope-to-channel coupling in the Howgill Fells, northwest England, badland systems in southeast Spain, alluvial fans in Spain, USA and UAE, and base-level-induced dissection of Neogene sedimentary basins in southeast Spain. As the spatial scales increase, so do the timescales involved. Effective temporal scales relate to magnitude and frequency characteristics, recovery time and propagation time, the relative importance changing with the spatial scale. For downsystem coupling at the local scale, the first two are important, with propagation time increasing in importance in larger systems, especially in those involving upsystem coupling related to base-level change. The effective timescales range from the individual event, with a return period of decades, through decadal to century timescales for downsystem coupling, to tens to hundreds of thousands of years for the basinwide response to base-level change. The effective timescales influence the relative importance of factors controlling landform development.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.