Does the Ulleung eddy owe its existence to β and nonlinearities?

A nonlinear theory for the generation of the Ulleung Warm Eddy (UWE) is proposed. Using the nonlinear reduced gravity (shallow water) equations, it is shown analytically that the eddy is established in order to balance the northward momentum flux (i.e., the flow force) exerted by the separating western boundary current (WBC). In this scenario, the presence of β produces a southward (eddy) force balancing the northward momentum flux imparted by the separating East Korean Warm Current (EKWC).It is found that, for a high Rossby number EKWC (i.e., highly nonlinear current), the eddy radius is roughly 2R_d/ε^1/6 (here ε≡βR_d/f₀, where R_d is the Rossby radius), implying that the UWE has a scale larger than that of most eddies (R_d). This solution suggests that, in contrast to the familiar idea attributing the formation of eddies to instabilities (i.e., the breakdown of a known steady solution), the UWE is an integral part of the steady stable solution. The solution also suggests that a weak WBC does not produce an eddy (due to the absence of nonlinearity).A reduced gravity numerical model is used to further analyze the relationship between β, nonlinearity and the eddy formation. First, we show that a high Rossby number WBC which is forced to separate from the wall on an f plane does not produce an eddy near the separation. To balance the northward momentum force imparted by the nonlinear boundary current, the f plane system moves constantly offshore, producing a southward Coriolis force. We then show that, as β is introduced to the problem, an anticyclonic eddy is formed. The numerical balance of forces shows that, as suggested by the analytical reasoning, the southward force produced by the eddy balances the northward flow force imparted by the boundary current. We also found that the observed eddy scale in the Japan/East Sea agrees with the analytical estimate for a nonlinear current.     

浙江景宁花岗岩基的解体划分及其演化

根据同源岩浆演化的观点,运用岩石谱系单位的划分方法,将景宁花岗岩基解体,划分为９个单元,归并成２个超单元。岩体定位方式为断裂扩张型。     

华北地块南缘太古宙灰色片麻岩及其成因

华北地块南缘太古宙登封花岗-绿岩地体和太华高级片麻岩地体中的英云闪长-奥氏花岗质和花岗闪长质灰色片麻岩多占据背斜核部或穹窿部位,它们是多期侵入的复合岩体。灰色片麻岩与表壳岩中的拉斑玄武岩构成双峰式岩套,属高Al_2O_3型,随岩浆演化,表现了Na_2O/K_2O,K/Rb比值逐渐降低,Rb/Sr比值升高的地球化学趋势,其LREE富集,HREE亏损U(Ce/Yb)_N=10—25]和Eu呈正异常。经模拟计算,登封灰色片麻岩是由石英榴辉岩约10—35％部分熔融形成,而石英榴辉岩系绿岩带中拉斑玄武岩的转变产物。太华灰色片麻岩可能由富含REE的母岩的部分熔融形成。     

A model for radial dike emplacement in composite cones based on observations from Summer Coon volcano,Colorado, USA

We mapped the geometry of 13 silicic dikes at Summer Coon, an eroded Oligocene stratovolcano in southern Colorado, to investigate various characteristics of radial dike emplacement in composite volcanoes. Exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries. Surprisingly, most dikes at Summer Coon increase in thickness with distance from the center of the volcano. Magma pressure in a dike is expected to lessen away from the pressurized source region, which would encourage a blade-like dike to decrease in thickness with distance from the center of the volcano. We attribute the observed thickness pattern as evidence of a driving pressure gradient, which is caused by decreasing host rock shear modulus and horizontal stress, both due to decreasing emplacement depths beneath the sloping flanks of the volcano. Based on data from Summer Coon, we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface. Near the interface between volcanic cone and basement, which may represent a neutral buoyancy surface or stress barrier, magma is transported subhorizontally and radially away from the center of the volcano in blade-like dikes. The dikes thicken with increasing radial distance, and offset segments and fingers form along the upper peripheries of the intrusions. Eruptions may occur anywhere along the length of the dikes, but the erupted volume will generally be greater for dike-fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source. Observed eruptive volumes, vent locations, and vent-area intrusions from inferred post-glacial dike-fed eruptions at Mount Adams, Washington, USA, support the proposed model. Hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano.     

The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe

The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo₆₀). The troctolite is composed of about 50% olivine (Fo_50-54), 40% plagioclase (An_53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product.     

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.     

The Effects of Protruding Rock Boulders in Regulating Sediment Intrusion within the Hyporheic Zone of Mountain Streams

Excessive sedimentation in mountain stream ecosystems is a critical environmental problem due to the clogging of streambeds by sediment particles within the hyporheic zone,with detrimental effects on fish spawning habitat.In this research,the effects of an array of boulders in regulating the intrusion of incoming sand within a gravel substrate were evaluated by performing detailed experiments in a laboratory flume.A unique experimental setup and two different sampling techniques were utilized for measuring the infiltrated sand within the gravel bed under two bed shear stress conditions（moderate vs.high）.For comparison purposes,experiments were performed without and with the presence of partially submerged to the flow（protruding） boulders,which is typical for the average flow conditions found in mountain streams.Results indicated that sand infiltrated primarily in the upper part of the gravel bed creating a surface seal which hindered the penetration of sand particles deeper into the bed.An exponential decrease of the amount of the infiltrated sand within the hyporheic zone was observed in all experiments regardless of the presence of boulders.However,the presence of boulders promoted sediment intrusion of sand particles especially for the moderate applied bed shear stress condition,since the total amount of the infiltrated sand was found to be on average 44% greater whenboulders were present.The findings from this study can provide additional insight regarding the role of boulders on promoting downwelling of flow and sediment within the gravel substrate with potential effects on fish habitat.     

Influence de l'hydrodynamique sur l'intrusion saline en aquifère karstique côtier

A new method of interpretation of the $Q\text{–}C$ hysteresis (discharge–salinity) during high water is proposed using the data recorded on coastal brackish karstic springs with salinity inversely proportional to the discharge. The method, based on the $Q_{\mathrm{mer}}\text{–}{C}_{\mathrm{T}}$ (seawater discharge–salinity) hysteresis, identifies the effect of the hydrodynamic changes on the flow and transport. Three phenomena are characterized: the emptying of the karstic conduit, the dilution by freshwater and the control of the seawater intrusion in the conduit by the hydraulic head variations. To cite this article: B. Arfib et al., C. R. Geoscience 338 (2006).     

云南梁河丝光坪锡矿成因及控矿因素

锡矿处于滇西腾中—梁河锡矿集区南部,产于来利山复式花岗岩体中东外接触带,形成于岩浆期后气化—高温热液成矿阶段。晚期低温现出木锡石—锡石一蛋白石型矿床,严格受花岗岩及构造控制。