Mantle plumes and plate motions

This paper elaborates the hypothesis that convection plumes may be rising from the lower mantle to spread out in the asthenosphere and drive lithospheric plates about and thus possibly provide the primary mechanism which governs the behaviour of the earth's surface. The paper notes some characteristics of plumes and identifies more than thirty by the hot spots which overlie them. Most lie close to mid-ocean ridges and have produced aseismic ridges trending away from them on either plate. A few have been overridden by plates to produce single, isolated chains of seamounts and islands. One plume may have uplifted the Colorado Plateau. Such distinctions serve to identify five types of hot spots.Most plates are in motion over the lower mantle. They are considered to be driven by the plumes, but their paths are influenced by interactions with other plates. Some temporarily become more or less stationary relative to the lower mantle. It is held that stationary plates, of which Africa and Southeast Asia may be present examples, develop special characteristics among which much volcanism, epeirogenic uplift, rifting and the development of basins and swells are diagnostic.It is well-known that if two plates approach one another at a subduction zone that a continental plate generally overrides an oceanic one. It is here suggested that the question of which plate is more nearly stationary over the mantle is important and determines the character of the continental margin. It is held that, if a continental plate advances over an oceanic one which is fixed over the mantle, a migrating marginal trench and mountains of Andean type with huge batholiths will form on the leading edge of the continent. On the other hand, if a continental plate is fixed and one or more oceanic plates are advancing and sliding under it, island arcs (and, when a collision with another continent occurs, mountains of Appalachian type) will form along each coast towards which a plate is advancing.     

New visualizations of global tectonic plate motions and plate boundary interactions

Clear understanding of detailed lithospheric plate motions has been impeded by lack of a suitable means of graphical representation. A series of coloured global maps are presented that reveal more detail in the patterns of both absolute and relative global plate motions. The use of continuous colour to represent velocities overcomes the limitations of earlier maps that used isolated vectors at selected points to indicate plate velocities. Velocity magnitudes and directions for entire surfaces of plates were computed at a resolution of 0.5°, and are shown on two separate maps. Relative motions between plates were decomposed into their shear and normal components, and are plotted on separate maps. Continuous colour is again used to indicate both the directions and magnitudes of sinistral/dextral and convergent/divergent motions for all plate boundaries. A final map of normalized velocity magnitudes for all plates reveals a global, fast 'belt' of plate motion that parallels a great circle aligned with the fastest portion of the Pacific Plate and orthogonal to the East Pacific Rise.     

Relations between fracture patterns, seismicity and plate motions in the Lesser Antilles

Measurements of orientations of joints that have been made on the Leeward and Windward Islands (Lesser Antilles) from St. Martin to St. Vincent are reported. The preferred orientations of the joints have been determined by a statistical analysis. Assuming the joints to be shearing phenomena, the principal directions of the stress field that caused them, can be calculated. The latter are in very good agreement with the principal stress directions obtained by a statistical analysis of fault plane solutions of earthquakes in the area; the inferred maximum compression direction being NW-SE (ca. N 130°E). This corresponds to a model which assumes that the American plate is subducted in that direction beneath the Caribbean plate.     

Possible effects of pre-existing basement topography on thrust fault ramping

Finite-element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments. In the simplest case, shear coupling between a planar basement—sediment contact causes the differential stresses in the sediments to die out with depth and distance from the applied load. For such cases, curved thrust faults may result if the strength of the rock is exceeded. Basement topography may also affect the location and shape of ramps by acting as a stress concentrator, by producing a stress shadow and by changing principal stress orientations. Modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it.Under the assumption of linear elasticity and for given boundary conditions, the Poisson's ratio plays an important role in determining the orientation and magnitude of the principal stresses. Calculations using experimentally measured Poisson's ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts. However, the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal. This suggests that non-elastic or ductile processes have an effect on the propagation of thrust faults.     

Absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the Indian Ocean

Paleopositions of the African, Indian and Antarctic plates are used as an independant set of data to test various hypotheses, derived from geophysical data, on the origin, emplacement mechanism and evolution of five aseismic structures of the Western and Southern Indian Ocean (Crozet and Kerguelen plateaus, Marion Dufresne, Léna and Ob seamount chain, Madagascar and Mascarene ridges).Except for a continental fragment of very limited extent bearing the Seychelles Islands, these structures, together with the Madagascar and Mascarene ridges, are probably due to anomalous volcanic episodes at —or near— active plate boundaries, rather than to intraplate emplacement. Their creation processes are therefore mainly related to the relative motions between the Antarctic, African and Indian plates. The volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the Western and Southern Indian Ocean, such as ridge jumps, asymmetric spreading and rapid variations in spreading velocity or direction. Finally, we think that thermal anomalies, located not far from active spreading boundaries, may have played a role in the physical processes creating the aseismic ridges.     

Lithospheric plate motions — One of the factors controlling distribution of ore deposits in some mineral belts

Determination of paleolatitudes of ore deposits, based on the reconstruction of lithospheric plate motions and the absolute ages of deposits, provides a basis for a new kind of space-time analysis of structural control of ore deposition. Such analysis shows that the formation of two ore deposits of different ages, each occurring at a different latitude along a north-south trend within a mineral belt, may be controlled by the same transversal fracture zone in the substratum underlying the lithospheric plate if rotation of the plate took place in the time-span between the formation of the two ore deposits (Fig. 3). This mechanism controlling ore deposition has been elucidated using a model which assumes horizontal movement of lithospheric plates on a mobile layer that originated within solid basement that is penetrated by a system of fracture zones. The distribution of porphyry copper deposits of the Andes mineral belt is used to study this process.     

Possible correlation between species extinction,evolution and plate adjustments to continental erosion

Data from continental erosion and from fluctuations in sea level are offered as a correlation with species extinction reported by Raup. Such data suggest a model for long term punctuate evolution.     

Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO₂ of 55–60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S^2?-rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ～80° (from SW to NW) at approximately 125–122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ?120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.     

非主题来搞选登

笔者建立了基于地震层析成像的热地幔对流模型,并分别计算了有、无地表板块运动约束下的中国大陆岩石圈底部的地幔对流速度场和水平剪切应力场。结果表明,欧亚板块运动速度对中国大陆地幔浅部的对流速度场影响较大,而对对流应力场的影响较小。在岩石圈底部(约100km深度),对流速度方向差异基本上达到了50%,幅度差异达到了80%以上,而地幔对流应力场的差异在方向上基本上小于10%、幅度上基本上小于20%。因此,如果仅研究岩石圈底部地幔对流应力场,则可以采用无板块速度约束的热对流模型。但如果要研究地幔浅部的对流状态和格局,则需要考虑板块运动速度。     

Possible effects of the 2004 and 2005 hurricanes on manatee survival rates and movement

Prior research on manatee (Trichechus manatus latirostris) survival in northwest Florida, based on mark-resighting photo-identification data from 1982–1998, showed that annual adult apparent survival rate was significantly lower during years with extreme storms. Mechanisms that we proposed could have led to lower estimates included stranding, injury from debris, being fatally swept out to sea, or displacement into poorly monitored areas due to storm-generated longshore currents or storm-related loss of habitat. In 2004 and 2005, seven major hurricanes impacted areas of Florida encompassing three regional manatee subpopulations, enabling us to further examine some of these mechanisms. Data from a group of manatees tracked in southwest Florida with satellite transmitters during Hurricanes Charley, Katrina, and Wilma showed that these animals made no significant movement before and during storm passage. Mark-resighting data are being collected to determine if survival rates were lower with the 2004 and 2005 storms.     

Notes on growth faults and sedimentary bed motions

The behavior of sediments in response to faults, and the corresponding influence of the sediments on the faults, are problems that control or modify substantially the evolution of sedimentary structures, fluid migration pathways, and the later deposition of post-fault sedimentary units. The self-consistent evolution of faults and sedimentary beds, including both footwall and hanging-wall motion of the beds, is investigated here using a novel inverse procedure which is guaranteed to honor present-day data on fault and bed positions. Paleo-evolution of both the faults and sediments is handled through constraint criteria such as conserved sediment area, bed-length, desired depositional and/or boundary conditions, or prescribed throw of sediments along a fault curve with time, etc. A procedure is given for using the constraint criteria either individually or collectively, with or without relative weighting conditions. In addition, for the cases of observed multiple faults, which either overlap laterally in a section or which are separated with no lateral overlap, a mapping procedure is given for transforming all such faults to a much simpler coordinate frame in which all the faults are (a) vertical, (b) of constant length, (c) do not overlap. The mapping procedure is shown to be universally valid for all times, so that the faults stay fixed and the sedimentary beds move through the new coordinate frame with time. Growth and arcuate faults are accommodated by the mapping procedure, as are flower and inverted flower arrangements of faults. The constraint criteria controlling paleo-evolution are also shown to be mappable to the new coordinate frame, so that desired paleo-evolutionary constraints can be easily accommodated through time-dependence of the mapping functions. The general mapping procedure, and the inverse method given for obtaining those parameters which describe paleo-evolution of faults and sediments in a self-consistent manner, are of use in addressing a wide variety of evolving fault/sediment interactions with a guarantee that the control criteria imposed will automatically honor present-day observations of both the faults and the sedimentary beds.     

On the effect of mid-air collisions on aeolian saltation

The effect of mid-air collisions on aeolian saltation is investigated using concentration profiles and grain velocities predicted by a numerical saltation model. The probability of a mid-air collision is found to be greater at high wind speeds. It is also found that mid-air collisions tend to reduce the number of grain/bed impacts and thus reduce the number of ejecta near the bed; this, in turn, reduces the intensity of mid-air collisions. It is suggested that this feed-back mechanism significantly influences transport rates at high wind speeds.     

Possible effects of iron oxide coating in the recovery of particulate gold from stream sediments

The influence of iron hydroxide/oxide coatings on the recovery of gold was studied using 13 stream sediment samples collected from an area of known gold mineralisation. Magnetic products collected from a Frantz isodynamic separator showed higher gold values than did non-magnetic fractions. An acid-treated magnetic product when run through the isodynamic separator yielded a significant amount of non-magnetic component which, when viewed by binocular microscope, showed the presence of gold grains in both bulk sample and in bromoform-separated heavy mineral concentrate. It is suggested that concentrates are treated with 10% HCl prior to magnetic separation where iron oxide coatings are conspicuous.     

Influence of seismic motions on behavior of piles in liquefied soils

In the present study an analytical procedure based on finite element technique is proposed to investigate the influence of vertical load on deflection and bending moment of a laterally loaded pile embedded in liquefiable soil, subjected to permanent ground displacement. The degradation of subgrade modulus due to soil liquefaction and effect of nonlinearity are also considered. A free headed vertical concrete elastic nonyielding pile with a floating tip subjected to vertical compressive loading, lateral load, and permanent ground displacement due to earthquake motions, in liquefiable soil underlain by nonliquefiable stratum, is considered. The input seismic motions, having varying range of ground motion parameters, considered here include 1989 Loma Gilroy, 1995 Kobe, 2001 Bhuj, and 2011 Sikkim motions. It is calculated that maximum bending moment occurred at the interface of liquefiable and nonliquefiable soil layers and when thickness of liquefiable soil layer is around 60% of total pile length. Maximum bending moment of 1210 kNm and pile head deflection of 110 cm is observed because of 1995 Kobe motion, while 2001 Bhuj and 2011 Sikkim motions amplify the pile head deflection by 14.2 and 14.4 times and bending moment approximately by 4 times, when compared to nonliquefiable soil. Further, the presence of inertial load at the pile head increases bending moment and deflection by approximately 52% when subjected to 1995 Kobe motion. Thus, it is necessary to have a proper assessment of both kinematic and inertial interactions due to free field seismic motions and vertical loads for evaluating pile response in liquefiable soil.     

Apparent motions of quasars due to microlensing

The effect of weak microlensing on the apparent velocities of extragalactic sources is considered; in particular, the apparent motions of sources from the ICRF list are discussed. We expect from two to seven cases of apparent motions of extragalactic sources due to weak microlensing by stars or dark bodies in our Galaxy to be detected over the next 30 years.     

Geometric evolution of a plate interface–branch fault system: Its effects on the tectonic development of the Himalayas

Crustal deformation due to fault slip depends strongly on fault geometry, and fault geometry is changed by the deformation of the crust. This feedback mechanism causes the geometrical evolution of the fault system. We have studied the progress of the geometrical evolution of a plate interface–branch fault system through numerical simulation, based on elastic–viscoelastic dislocation theory. If the plate interface is smooth, no significant change occurs in fault geometry. If the plate interface has a ramp, we observe the gradual horizontal motion of the ramp toward the hanging-wall side of the interface at half the plate convergence rate. The offset of the ramp decreases with time. The dip-angle of thrust faults branching from the plate interface increases more rapidly as the dip of the fault increases. We have applied these results to the plate interface–branch fault system at the India–Eurasia collision boundary and obtained a scenario for the tectonic development of the Himalayas for the last 30 Myr.     

Forecasting the polar motions of the deformable Earth

A mathematical model for the complicated phenomenon of the polar oscillations of the deformable Earth that adequately describes the astrometric data of the International Earth Rotation Service is constructed using celestial mechanics and asymptotic techniques. This model enables us to describe the observed phenomena (free nutation, annual oscillations, and trends) simply and with statistical reliability. The model contains a small number of parameters determined via a least-squares solution using well-known basis functions. Interpolations of the polar trajectory for intervals of 6 and 12 yrs and forecasts for 1–3 yrs are obtained using the theoretical curve. The calculated coordinates demonstrate a higher accuracy than those known earlier.     

Elliptical motions of stars in close binary systems

The motion of stars in close binary systems with conservative mass transfer is considered. It is shown that the Paczynski-Huang model that is currently used to determine the variations of the semimajor axis of the relative orbit of the stars is not correct, and leads to large errors in the derived semi-major axis. A new model is proposed, suitable for elliptical stellar orbits. The reaction forces and gravitational forces between the stars and the stream of overflowing matter are taken into account. The possibility of mass transfer in the presence of an accretion disk is considered.