Chemical Diversity of the Ueno Basalts, Central Japan: Identification of Mantle and Crustal Contributions to Arc Basalts

The Ueno Basalts of central Japan comprise a monogenetic volcaniccone complex that was active between 2·76 and 1·34Ma. Basalts were erupted at more than 14 centers scattered overa region 40 km in diameter. Alkali basalt was erupted first,followed by sub-alkaline basalt. Quasi-concentric expansionof eruption centers coinciding with uplift and with decreasingalkalinity of the lavas suggests that Ueno magmatism originatedfrom a mantle diapir as it mushroomed at the base of the lithosphere.Depleted asthenospheric mantle (alkali basalt), enriched lithosphericmantle (sub-alkaline basalt), and crustal components are identifiedas chemical end-members in the petrogenesis of the Ueno Basalts.Incompatible trace element abundances indicate that the Uenoalkali basalts are typical within-plate basalts, whereas thesub-alkaline basalts show strong affinities with normal arclavas. Sr–Nd–Pb isotopic compositions indicate thatthe mantle source of the alkali basalts was more depleted thanthat of the sub-alkaline basalts. About 7% melting of asthenosphericmantle in the garnet-lherzolite stability field produced theprimitive alkali basalts and 12% melting of spinel lherzolitewithin the subcontinental lithosphere produced the primitivesub-alkaline basalts. Isotopic compositions and fluid mobile/immobileelement ratios broadly covary with SiO₂ contents in the sub-alkalinesuite, and increasing silica content is associated with strongerEMII (Enriched Mantle II) isotope affinities and fluid mobileelement abundances. A progressive AFC (assimilation–fractionalcrystallization) model assuming assimilation of a low-K silicicmelt reproduces the chemical variations observed in the sub-alkalinesuite. Melting of a flattening mantle diapir at the base ofthe lithosphere is the dominant cause of Ueno magmatism, accompaniedby the assimilation of older arc crust. KEY WORDS: arc basalt; crustal assimilation; mantle heterogeneity; Ueno Basalts     

Channel formation by flow stripping: large-scale scour features along the Monterey East Channel and their relation to sediment waves

The Monterey East system is formed by large‐scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour‐shaped depressions ranging from 3·5 to 4·5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude‐supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude‐supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream‐migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net‐erosional or net‐depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream‐migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.     

Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr^?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr^?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr^?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.     

Authigenic pyrite and gypsum in South West African continental slope sediments

An unusual association of authigenic pyrite and authigenic gypsum has been found in silty clays recovered from the South West African continental slope. Nannofossil content suggests that the sediments are Upper Miocene-Lower Pliocene in age. Pyrite occurs as (1) granular masses, (2) ‘worm’tubes, (3) foraminiferal infillings, and (4) framboids. Gypsum occurs as euhedral single or twinned crystals of selenite. ‘Worm’tubes and foraminiferal infillings of pyrite are partially or completely enclosed in some gypsum crystals. Electron microprobe analyses show a relatively high concentration of manganese in both the granular masses and tubes. Present-day waters off this coast are dominated by the upwelling of cold, nutrient-enriched waters (the Benguela Current). These rich waters support an enormous population of plankton. Death and decomposition of these plankton consume oxygen, thereby creating a belt of anaerobic sediments close to shore. An Upper Miocene-Lower Pliocene regression (Dingle & Scrutton, 1974) lowered sea level and shifted an older analogue of this upwelling zone seaward, eventually establishing an anaerobic environment in places on the present continental slope. Anaerobic bacteria thrived in these conditions. They reduced SO₄ dissolved in sea water, initiating the formation of H₂S. The H₂S reacted with iron minerals present in the sediment to form FeS. Addition of elemental sulphur produced pyrite. This strongly reducing, low pH, environment became saturated with calcium obtained by the dissolution of planktonic calcareous organisms. Gypsum was precipitated once the product of the concentrations of dissolved calcium and SO₄ exceeded the gypsum solubility product.     

Quantitative evaluation of the depth of the western Mediterranean before, during and after the Late Miocene salinity crisis

Quantitative geophysical calculations which take into consideration the isostatic loading of sediment overburden, the overlying water cover, and the thermal cooling history of the continental edge, and its adjacent oceanic lithosphere, demonstrate the foundering of the margins of the western Mediterranean had already commenced in the Aquitanian stage of the early Miocene. The calculations are based on magnitudes and rates of sediment accumulation observed along a profile of three commercial boreholes into the subsurface of the continental shelf of southern France. By the time of the late Miocene (Messinian) salinity crisis, the depth of the seafloor within the Balearic basin exceeded 2.5 km. Sea-level fluctuations induced by evaporitic draw-down permitted the exposure of large tracts of the former submerged continental margins to subaerial processes. The measured magnitude of sediment removal by erosion and channel incision near the outer shelf of the modern Gulf of Lion surpasses 1 km. The subsidence history of this shelf platform south of France provides new evidence that the continental lithosphere behaves as if it is rigidly coupled to its oceanic counterpart commencing with the initial phase of the pull-apart. No major vertical fault displacements have subsequently offset their overlying crustal layers. The sedimentary shaping and construction of the margin seaward of the Rhône delta resulted in a pronounced shelf edge migration and slope progradation during the pre-salinity crisis Miocene. It has taken 5 million years of predominant upbuilding to establish a new equilibrium profile similar in cross-section to the precrisis depositional surfaces created by outbuilding.     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

组合表面Bragg散射模型共极化SAR海表面风速反演

组合表面布拉格散射模型CSBS（Composite Surface Bragg Scattering）由布拉格（Bragg）散射模型和几何光学模型组成，是海洋微波散射的经典模型，可用于星载合成孔径雷达SAR（Synthetic Aperture Radar）海表面风场反演。研究指出，Bragg散射模型仅适用于中等入射角条件，几何光学模型则更适用于小入射角情形。然而，如何确定中等和小入射角的阈值，即CSBS模型最优入射角的选取目前尚无定论。基于142景成像于美国东西海岸和中国东海的RADARSAT-2精细四极化SAR影像数据和海洋浮标数据，本文提出了一种最优局地入射角查找算法，分别对VV和HH极化SAR数据进行最优入射角阈值的选取。结果显示，局地入射角14°和16°分别为VV和HH极化影像CSBS模型反演风速最优入射角。基于最优入射角的选取，本文在0—15 m/s海况区间内利用CSBS模型对VV和HH极化SAR影像开展风速反演实验，并将反演风速与浮标风速进行对比。结果显示，基于VV和HH极化数据的CSBS模型反演风速与浮标风速均方根误差分别为2.15 m/s和2.32 m/s，相关系数分别为0.79和0.75，两者具有良好的一致性。本文研究结论表明基于最优入射角设置后的CSBS模型在海面风速小于15 m/s条件下具有良好的应用性，后续研究将更加关注CSBS模型在高海况以及交叉极化SAR数据情况下的应用。