Petrologic consequences of rift propagation on oceanic spreading ridges

The production of anomalously differentiated lava compositions at several mid-ocean spreading centers can be attributed to magmatic processes associated with propagating rifts. The degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers. Low supply rates and moderate cooling rates allow advanced degrees of closed-system fractionation to occur. High supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma. Propagating rift tips are a special class of ridge-transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge. This balance is affected by the spreading rate, the propagation rate of the rift, the length of the bounding transform and proximity to hotspots. Maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance. Rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion. Geochemical anomalies may persist, though changing in degree and extent, for several million years on ridge segments that stop propagating. The concept of “magnetic telechemistry” is generally supported by our study, but in the vicinity of hotspots, magnetic anomaly amplitude may be controlled more by bathymetric and/or thickened magnetic layer effects than by geochemistry.     

Propagating rift explanation for the tectonic evolution of the northeast Pacific—the pseudomovie

We have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution. Using this technique, we have deciphered the evolutionary history of the Juan de Fuca area in the northeast Pacific, and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates.     

On the dynamic mechanism of formation of rift valleys

A dynamic mechanism that accounts for the sinking of a lithospheric plate near an accretion zone in the vicinity of a passive rift is revealed. It is shown that the influence of the underlying “cold” mantle can be described in terms of a concentrated vertical force applied to the rift axis. At a moderate spreading rate, the value of this force is an order of magnitude smaller than the characteristic values of forces acting in the plate tectonics. The average viscosity coefficient of the cold upper mantle is estimated at ～10²¹ P. The concentrated force at the rift axis produces a characteristic topography of the rift valley type of mid-ocean ridges.     

Some consequences of the subduction of young slabs

The negative buoyancy force exerted by a subducting oceanic slab depends on its descent velocity, and strongly on its age. For lithosphere close to thermal equilibrium, this force dominates by a large margin the resisting forces arising from friction on the plate boundary and compositional buoyancy. This may result in oceanward migration of the trench, with associated back-arc spreading. However, the strong age dependence of this force, and of the ridge push mean that a horizontal compressive stress is required to continue subduction if changing plate geometry should bring young lithosphere to the trench. Estimates can be made of the slab age, as a function of descent velocity, at which the driving forces are no longer sufficient to overcome a given resisting force. The transition corresponding to a resisting force of 8 × 10¹² N/m divides regions displaying back-arc extensional tectonics from those displaying compressional tectonics. This is in good agreement with other estimates of the forces resisting slab motion. It is suggested that an increase in the width of — or the shear stress on — the plate boundary, associated with the subduction of lithosphere to the buoyant side of this transition, can result in a compressional stress on the overriding plate which is great enough to account for cordilleran tectonics.The proposed reduction in the one of driving forces of plate motion is still consistent with observations, being compensated by the greaterrelative importance of the push from the ridges.     

The geometry of propagating rifts

The kinematics of two different processes are investigated, both of which have been described as rift propagation. Courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin. The term localisation is instead used here to describe this process, to distinguish it from Hey's type of propagation. Localisation generally leads to rotation of the direction of magnetisation.To Hey propagation means the extension of a rift into the undeformed plate beyond a transform fault. Detail surveys of the Galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault, but by a zone which is undergoing shear. The principal deformation is simple shear, and the kinematics of this deformation are investigated in some detail. The strike of most of the lineations observed in the area can be produced by such deformation. The mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others. Neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth.     

Constraints on Jalisco Block Motion and Tectonics of the Guadalajara Triple Junction from 1998�C2001 Campaign GPS Data

A GPS campaign network in the state of Jalisco was occupied for?~36?h per station most years between 1995 and 2005; we use data from 1998?C2001 to investigate tectonic motion and interseismic deformation in the Jalisco area with respect to the North America plate. The twelve stations used in this analysis provide coverage of the Jalisco Block and adjacent North America plate, and show a pattern of motion that implies some contribution to Jalisco Block boundary deformation from both tectonic motion and interseismic deformation due to the offshore 1995 earthquake. The consistent direction and magnitude of station motion on the Jalisco Block with respect to the North America reference frame,?~2?mm/year to the southwest (95% confidence level), perhaps can be attributed to tectonic motion. However, some station velocities within and across the boundaries of the Jalisco Block are also non-zero (95% confidence level), and the overall pattern of station velocities indicates both viscoelastic response to the 1995 earthquake and partial coupling of the subduction interface (together termed ??interseismic deformation??). Our results show motion across the northern Colima rift, the eastern boundary of the Jalisco Block, which is likely to be sinistral oblique extension rather than pure extension. We constrain extension across both the Colima rift and the northeastern boundary of the Jalisco Block, the Tepic-Zacoalco rift, to???8?mm/year (95% confidence level), slow compared to relative rates of motion at nearby plate boundaries.     

On the tectonics of Asia

This report describes an interpretation of the tectonics of central Asia made from seismic and geologic data. It is suggested that central Asia is not a tectonically passive unit, as previously proposed by others, responding solely to the convergence of the Indian plate with Asia. We postulate that the tectonics of central Asia can be represented by the motion of a few continental blocks which are influenced by spreading from the Baikal rift zone as well as compression due to the collision of the Indian plate. Here, a block is defined as a tectonic unit, within a continental plate, with boundaries delineated by broad zones of high seismicity with respect to the interior of the unit. Five tectonic units are postulated for central Asia. These are: the Siberian block, the East and West China blocks, the Southeast Asian block; and the Indian plate. An unusual phenomenon is noted along the boundary between the Siberian and West China blocks. There is general horizontal crustal compression along this boundary from the Hindu Kush north-eastward to the southern tip of Lake Baikal; however, there is general horizontal extension eastward from Lake Baikal through the Stanovoy range. Thus the West China block, to the south of this boundary, seems to be turning clockwise about a point near the southern tip of Lake Baikal. The major known faults within this block, which strike mainly northwest-southeast, may be interpreted as shear zones where interior stresses, due to the block rotation, are released. We cannot support this suggestion with an analytical model because of the uncertainties in various model parameters and geometries. The suggested model gives a possible explanation of why India, to the south of the Himalayas, is almost completely aseismic while the regions to the north and northeast have higher seismicity.     

Is there a Caroline plate?

We examine available marine geophysical and seismological data from the Caroline Sea region and conclude that a separate Caroline plate currently exists. The Caroline plate is moving relative to the Pacific plate on its northern and eastern boundaries, the Philippine plate on its western margin, and the Indian and smaller plates along its southern side in New Guinea and the Bismarck Sea area. The southern Yap Trench, the Palau Trench, and an accreting plate boundary within the Ayu Trough manifested as an axial rift valley comprise the Caroline-Philippine plate boundary. On the basis of sediment thickness and subsidence of basement away from the rift, we estimate that the Ayu Trough started to open during the Miocene. The northern section of Pacific-Caroline plate boundary coincides with the Sorol Trough which exhibits both strike-slip and extensional characteristics. The southeastern section of this boundary occurs along the Mussau Trench where Caroline plate underthrusts the Pacific plate. The section of plate boundary between the Sorol Trough and Mussau Trench is characterized by highly unusual deformational tectonics. Convergence between the Pacific and Caroline plates is apparently accommodated here by overthrusting of small slivers of sea floor towards the northeast. The intensity of deformation appears to increase southward towards the Mussau Trench. Our calculated instantaneous angular rotation vector for the Pacific-Caroline plates predicts that convergence rates increase uniformly south along the overthrust and underthrust sections of plate boundary. The transition in tectonic style from overthrusting to underthrusting occurs between 3° and 4°N.     

Subduction tectonics vs.plume tectonics——Discussion on driving forces for plate motion

Plate tectonics describes the horizontal motions of lithospheric plates,the Earths outer shell,and interactions among them across the Earths surface.Since the establishment of the theory of plate tectonics about half a century ago,considerable debates have remained regarding the driving forces for plate motion.The early"Bottom up"view,i.e.,the convecting mantledriven mechanism,states that mantle plumes originating from the core-mantle boundary act at the base of plates,accelerating continental breakup and driving plate motion.Toward the present,however,the"Top down"idea is more widely accepted,according to which the negative buoyancy of oceanic plates is the dominant driving force for plate motion,and the subducting slabs control surface tectonics and mantle convection.In this regard,plate tectonics is also known as subduction tectonics."Top down"tectonics has received wide supports from numerous geological and geophysical observations.On the other hand,recent studies indicate that the acceleration/deceleration of individual plates over the million-year timescale may reflect the effects of mantle plumes.It is also suggested that surface uplift and subsidence within stable cratonic areas are correlated with plumerelated magmatic activities over the hundred-million-year timescale.On the global scale,the cyclical supercontinent assembly and breakup seem to be coupled with superplume activities during the past two billion years.These correlations over various spatial and temporal scales indicate the close relationship and intensive interactions between plate tectonics and plume tectonics throughout the history of the Earth and the considerable influence of plumes on plate motion.Indeed,we can acquire a comprehensive understanding of the driving forces for plate motion and operation mechanism of the Earth's dynamic system only through joint analyses and integrated studies on plate tectonics and plume tectonics.     

Evolution of abyssal lavas along propagating segments of the Galapagos spreading center

The unusual petrological diversity of abyssal lavas erupted along some segments of the Galapagos spreading center is a direct consequence of the propagation (elongation) of these segments into older oceanic crust. With increasing distance behind propagating rift tips, relatively unfractionated MORB erupted close to the tips are joined first by FeTi basalts (bimodal assemblage) and then by a wide range of basaltic and siliceous lavas. Further behind propagating rift tips, this broad range diminishes again, approaching the narrow compositional range of adjacent normal ridge segments.These compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre-existing plate. We envisage this evolution as proceeding from small, isolated, ephemeral magma chambers through increasing numbers of larger, increasingly interconnected chambers to the steady-state buffered system of a normal ridge. Throughout this evolution, magma supply rates gradually increase and cooling rates of crustal magma bodies decrease. High degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved.At other propagating and non-propagating ridge-transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas. These effects may be evaluated provided a number of tectonic variables including transform length, spreading and propagation rates are taken into account.     

Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific)

The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets.During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift).The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates.The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction.The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant.The southern triple junction is a RRF junction with an overlapping ridge system.The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.     

Evolution of rifted ocean ridges

A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting along fault planes which dip away from the valley axis. The faulting occurs in a narrow zone just beyond the outer walls of the rift valley. This model allows for a steady-state evolution of the rift valley into the rift mountains in which the state of stress in the oceanic lithosphere continues to be in horizontal deviatoric tension throughout the entire process. Alternate mechanisms involving reverse faulting or regional tilt may be active but are found to be of less importance. Implications for various dynamic models of the rift valley are discussed.     

The generation of plate tectonics from mantle convection

In the last decade, significant progress has been made toward understanding how plate tectonics is generated from mantle dynamics. A primary goal of plate-generation studies has been the development of models that allow the top cold thermal boundary layer of mantle convection, i.e. the lithosphere, to develop broad and strong plate-like segments separated by narrow, weak and rapidly deforming boundaries; ideally, such models also permit significant strike-slip (toroidal) motion, passive ridges (i.e. pulled rather than pried apart), and self-consistent initiation of subduction. A major outcome of work so far is that nearly all aspects of plate generation require lithospheric rheologies and shear-localizing feedback mechanisms that are considerably more exotic than rheologies typically used in simple fluid-dynamical models of mantle flow. The search for plate-generating behavior has taken us through investigations of the effects of shear weakening (‘stick-slip’) and viscoplastic rheologies, of melting at ridges and low-viscosity asthenospheres, and of grain-size dependent rheologies and damage mechanics. Many such mechanisms, either by themselves or in combination, have led to self-consistent fluid-mechanical models of mantle flow that are remarkably plate-like, which is in itself a major accomplishment. However, many other important problems remain unsolved, such as subduction intiation and asymmetry, temporal evolution of plate geometry, rapid changes in plate motion, and the Archaean initiation of the plate-tectonic mode of convection. This paper presents a brief review of progress made in the plate-generation problem over the last decade, and discusses unresolved issues and future directions of research in this important area.     

弹性板块运动模型研究进展

传统板块构造理论认为板块是一个刚体，实际上板块是可变形的.板块内部几年到几十年时间尺度的变形主要是弹性变形,因此应当用弹性模型描述板块运动.推导了板块的弹性运动方程，由空间大地测量新的观测成果建立了菲律宾海、太平洋和澳大利亚板块的弹性运动模型.发现三个板块内部都存在明显的水平形变.板内应变场的空间变化有明显的规律：板块边界附近的应变率最大，从边界向内部逐渐减小；在板块扩散边界附近，主张应变率大于主压应变率，主张应变轴基本上与边界的扩张方向一致；在俯冲边界附近，主压应变率大于主张应变率，主压应变轴基本上与板块的俯冲方向一致；在走滑兼有俯冲性质的边界附近，最大剪应变的方向与边界断裂的走向基本一致.由GPS观测得到的主压应变轴与由震源机制解得到的主压应力轴方向具有很好的一致性.板内的应力－应变场基本上遵循广义胡克定律.     

America-Eurasia plate boundary in eastern Asia and the opening of marginal basins

The opening of the Arctic Ocean during the past 55 Ma resulted in relative rotation of America with respect to Eurasia about a pole located in eastern Siberia, near the plate boundary. The extensional plate boundary enters deep inside the Eurasian continent up to the rotation pole. On the opposite side of the pole, on the Pacific side of the plate boundary, compressive tectonics are recorded along Sakhalin and Hokkaido. From the Oligocene to Middle Miocene, the relative movement was accommodated by strike-slip motion along Sakhalin and Hokkaido although the rotation pole was not located at a significatively different position from now. In this paper we explain this by independent motion of the southernmost tip of the American plate towards the Pacific margin which behaves as a free boundary. This oceanward motion resulted in an extension of the American plate giving rise to the wedge structure of the Okhotsk Sea. The Japan Sea opened as a pull-apart basin along the strike-slip boundary; finally the increasing extension in the Okhotsk Sea led to the opening of the oceanic Kuril Basin.     

Break-up spots: Could the Pacific open as a consequence of plate kinematics?

The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. Noting that this volcanism is present from the Kermadec Tonga trench to the Easter microplate and forms a wide east–west channel, we propose to explain its occurrence in relation to the Pacific plate geometry and kinematics. We construct 2D numerical models of stress and strain within the Pacific plate using its velocity field and boundary conditions. The models indicate a shear band, associated to a change from compressional stresses to the south to tensional stresses to the north, which develop after 10 Myr between the Australian plate corner and the Easter microplate. We propose that the Central Pacific intraplate volcanism is related to this process, and may represent the first step of a future plate re-organization which will eventually break the Pacific plate in a southern and a northern plate due to intraplate stresses. Present-day intraplate volcanism would define break up spots of the future border.     

全球主要俯冲带处板块运动与地震各向异性及应力场的相关性讨论

在全球板块的很多地方,包括俯冲带、大洋中脊、甚至大陆板块的内部等,地震各向异性都与板块绝对运动图像存在一定的相关性,或者与板内应力场的优势取向一致。本文统计分析了全球9个包含主要俯冲带的板块边界内板块运动与地震各向异性及应力场的相关性,结果表明,板块的绝对或相对运动控制着板块边界的地震各向异性和应力场特征,尤其是板块的相对运动,在板块边界带处的影响十分明显;从计算结果还可以看出,板块的相对运动方向与地震各向异性及应力场的相关性要好于板块绝对运动。在包含俯冲带的板块边界处,由于俯冲机理的复杂性和控制因素的多样性,使得俯冲带处两者的相关性较为复杂,不同深度来源的各向异性表现出不同特征,且应力状态受多种因素的控制。     

Recent Tectonic Deformations and Stresses in the Frontal Part of the Rif Cordillera and the Saïss Basin (Fes and Rabat Regions,Morocco)

—Prerif Ridges are located at the frontal part of the Rif Cordillera, which develops at the Eurasian-African plate boundary. The ridges are formed by recent tectonic structures that also deform foreland basins (Saïss and Gharb basins) and the foreland (Moroccan Meseta). The position of the ridges is the consequence of inversion tectonics undergone in the area. The ENE-WSW trend of the northern edge of the Neogene Saïss basin is determined by the location of Mesozoic basins. Although Prerif ridges probably started to develop since the Early Miocene, the most active deformation phase affecting Pliocene rocks consisted of N-S to NW-SE oriented compression. Striated pebbles show that this compression has prolate stress ellipsoids. The deformation produces southwards vergent folds and NNW-SSE striae on reverse faults at the base of the ridges. The flexure of the Paleozoic basement by the emplacement of the Ridges produced extensional deformation and the development of the Saïss foreland basin. The extension in this basin is oblate and features a well determined NNE-SSW trend near the Ridges, whereas it becomes prolate and pluridirectional near the foreland edge represented by the Rabat region. This part of the Moroccan Meseta, commonly considered to be stable, is deformed by sets of orthogonal joints and faults with short slip that affect up to Quaternary sediments. Southwestward, the Meseta rocks are also deformed by transcurrent faults, which indicate NW-SE and N-S trends of compression. The NW-SE approximation of Eurasia and Africa determines a regional stress field with the same trend of compression. Regional stresses are notably disturbed by the development of the active structures in the Rif, which exhibit alternating trends of compression and extension. The clearest evidence of the relationship between the local deformation and the general plate motion is found at the deformation front of the Cordillera, that is, the Prerif Ridges.     

渐进式扩张海盆亚丁湾与南海西南次海盆扩张演化特征的对比

本文总结了渐进式扩张洋中脊和渐进式演化海盆的全球空间分布,并将西南次海盆与典型渐进式演化的亚丁湾加以比对,通过对海盆扩张中心的起源、扩张中心分段特征、火山活动、磁异常特征等的比较,为西南次海盆的演化提供新观点,为南海的演化观点寻找新证据.西南次海盆为渐进式扩张的海盆,与东部次海盆属于同一期扩张形成,海盆的渐进式扩张与渐进式扩张的方向很有可能受到地幔热柱(印支地幔柱、南海中部低速柱或海南地幔柱)的控制.南海的扩张演化模式并不是单一的,而是多种模式的综合,在考虑海底演化模式时应该同时考虑地幔柱的影响.     

Earthquake site effect modeling in the Granada basin using a 3-D indirect boundary element method

In this work, we develop the indirect boundary element method (IBEM) to simulate the seismic site response in a realistic, large-scale 3-D sedimentary basin. Most previous applications of boundary element method have used full-space Green’s functions for wave propagation between element points. We use half-space Green’s functions, which include the seismic wavefield interactions at the free surface and require only the boundary elements of the basin interface. In this way, the size of the matrix equation for solution in the IBEM can be reduced to approximately a quarter of that using full-space Green’s functions. The site response modeling of the Granada basin in southern Spain using the IBEM shows that the basin-induced scattering waves were identified as propagating back and forth inside the basin. The scattered waves also generate surface waves that are weakly propagated outside of the basin. The wave propagation inside and outside of the basin shows different patterns. We observe that the scattered wave is locally amplified, and its propagation direction deviates from that of the incident waves propagation direction. Therefore, the computed seismic response in the basin could provide us with good estimates of the seismic motion.