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1.
Radial structures to the north-east of Irnini Mons are discernable as ridges at the full Magellan image resolution of 75 m/pixel using FMAP tiles. While a magma source modeled as a pressurized hole in an elastic plate usually allows for only extensional radial structures, the perturbation of regional stresses around the hole can explain the presence of radial ridges. Sixteen numerical models were run to see what effect changes in magma pressure and regional stresses had on the magnitude of the maximum principal stresses and the orientation of potential features around a hole in a plate. These models were arranged into four groups based on model input and setup: 1) uniaxial compressive stress; 2) uniaxial tensile stress; 3) biaxial stress; and 4) multiple holes. Nine of the sixteen models result in radial ridges in the orientation observed around Irnini Mons; all of these models incorporate a regional N-S compression. Flows from Irnini Mons are superimposed on an older, regional plains material deformed by a regional set of east-west trending wrinkle ridges, implying a regional north-south compression affected the area. The existence of radial ridges on the Irnini flows implies that the regional N-S compression that caused the E-W trending wrinkle ridges was still active during the formation of Irnini Mons. However, the magnitude of the regional compressive stress required for radial ridge formation could be 1–5 MPa less than the compressive strength of the material, indicating that radial ridges could form after wrinkle ridge formation has ceased. 相似文献
2.
The effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two-dimensional (2D) horizontal stresses arising from isostatically compensated vertical loads. The topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge-parallel tensile stresses. These may predominate over the ridge-perpendicular stresses and explain observations at midocean ridge offsets such as (1) oblique normal faulting at ridge-transform intersections trending up to 60° relative to the ridge axis, and (2) nontransform offsets consisting of structures oriented at 45° relative to the ridge trend. At midocean ridge overlaps, rotation of the ridge-parallel tensile stresses favours rift propagation at more than 45° relative to the ridge trend. It is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets, and as a result large overlaps may eventually start to rotate and evolve into a microplate. 相似文献
3.
月岭是月球表面上最常见的线性构造之一,大多分布于被玄武岩充填的月海之中。虽对于月岭成因的观点不一,但多数月岭具有明显的挤压变形特征,其展布形式有单列、组或群(常见)、网状或雁列状等。月岭的形成和展布既受到月海盆地构造的控制,又受到区域应力状态的影响。文中以雨海为研究区域,利用美国环月球巡逻者(LRO)的宽视角影像(WAC)数据和激光高度计(LOLA)数据对雨海地区的月岭构造进行解译和提取,对月岭进行分类和统计分析,结合雨海盆地构造特征以及月岭构造的样式解析,并运用沙箱构造物理模拟方法对月岭的形成和分布进行模拟实验,讨论了雨海地区影响月岭形成和分布的主控因素。结果表明,月岭是挤压体制下形成的;盆地构造是控制月岭类型Ⅰ分布的主要因素;月岭类型Ⅱ的分布主要受区域构造应力控制,其走向主要反映区域构造的最大主应力方向。综上推断,雨海地区月岭形成于近东西向最大主应力并受到雨海盆地构造的控制,表现为月岭类型Ⅰ和月岭类型Ⅱ的综合样式。 相似文献
4.
This paper studies the depth variability of uniaxial compressive laboratory test results on intact Toki granite (i.e., sound
rock without macroscopic fractures) from the Shobasama and Mizunami Construction Sites, Japan. Some of the depth variability
observed in the laboratory results can be indirectly attributed to the high fracture frequency of the “upper highly fractured
rock domain” from which some of the samples were taken. For samples taken from the “lower sparsely fractured rock domain,”
however, the uniaxial compressive strength of the granite seems to be very strongly correlated to the level of in situ rock
stress (i.e., maximum shear stress) determined by measurement results obtained from hydrofracturing tests. The correlation
between the laboratory results and the level of in situ stress is explained by the damage due to the complex stress path that
the cores undergo during drilling, besides the stress concentrations at the drill-bit/rock contact, which can also affect
the microcracking of the samples. An attempt to adjust laboratory test results to estimate the in situ intact rock strength
of Toki granite based on its correlation with in situ stresses was carried out. 相似文献
5.
Global Correlations of Ocean Ridge Basalt Chemistry with Axial Depth: a New Perspective 总被引：4，自引：0，他引：4
The petrological parameters Na8 and Fe8, which are Na2O andFeO contents in mid-ocean ridge basalt (MORB) melts correctedfor fractionation effects to MgO = 8 wt%, have been widely usedas indicators of the extent and pressure of mantle melting beneathocean ridges. We find that these parameters are unreliable.Fe8 is used to compute the mantle solidus depth (Po) and temperature(To), and it is the values and range of Fe8 that have led tothe notion that mantle potential temperature variation of TP= 250 K is required to explain the global ocean ridge systematics.This interpreted TP = 250 K range applies to ocean ridges awayfrom ‘hotspots’. We find no convincing evidencethat calculated values for Po, To, and TP using Fe8 have anysignificance. We correct for fractionation effect to Mg# = 0·72,which reveals mostly signals of mantle processes because meltswith Mg# = 0·72 are in equilibrium with mantle olivineof Fo89·6 (vs evolved olivine of Fo88·1–79·6in equilibrium with melts of Fe8). To reveal first-order MORBchemical systematics as a function of ridge axial depth, weaverage out possible effects of spreading rate variation, local-scalemantle source heterogeneity, melting region geometry variation,and dynamic topography on regional and segment scales by usingactual sample depths, regardless of geographical location, withineach of 22 ridge depth intervals of 250 m on a global scale.These depth-interval averages give Fe72 = 7·5–8·5,which would give TP = 41 K (vs 250 K based on Fe8) beneathglobal ocean ridges. The lack of Fe72–Si72 and Si72–ridgedepth correlations provides no evidence that MORB melts preservepressure signatures as a function of ridge axial depth. We thusfind no convincing evidence for TP > 50 K beneath globalocean ridges. The averages have also revealed significantcorrelations of MORB chemistry (e.g. Ti72, Al72, Fe72,Mg72, Ca72, Na72 and Ca72/Al72) with ridge axial depth. Thechemistry–depth correlation points to an intrinsic linkbetween the two. That is, the 5 km global ridge axial reliefand MORB chemistry both result from a common cause: subsolidusmantle compositional variation (vs TP), which determines themineralogy, lithology and density variations that (1) isostaticallycompensate the 5 km ocean ridge relief and (2) determine thefirst-order MORB compositional variation on a global scale.A progressively more enriched (or less depleted) fertileperidotite source (i.e. high Al2O3 and Na2O, and low CaO/Al2O3)beneath deep ridges ensures a greater amount of modal garnet(high Al2O3) and higher jadeite/diopside ratios in clinopyroxene(high Na2O and Al2O3, and lower CaO), making a denser mantle,and thus deeper ridges. The dense fertile mantle beneath deepridges retards the rate and restricts the amplitude of the upwelling,reduces the rate and extent of decompression melting, givesway to conductive cooling to a deep level, forces melting tostop at such a deep level, leads to a short melting column,and thus produces less melt and probably a thin magmatic crustrelative to the less dense (more refractory) fertile mantlebeneath shallow ridges. Compositions of primitive MORB meltsresult from the combination of two different, but geneticallyrelated processes: (1) mantle source inheritance and (2) meltingprocess enhancement. The subsolidus mantle compositional variationneeded to explain MORB chemistry and ridge axial depth variationrequires a deep isostatic compensation depth, probably in thetransition zone. Therefore, although ocean ridges are of shalloworigin, their working is largely controlled by deep processesas well as the effect of plate spreading rate variation at shallowlevels. KEY WORDS: mid-ocean ridges; mantle melting; magma differentiation; petrogenesis; MORB chemistry variation; ridge depth variation; global correlations; mantle compositional variation; mantle source density variation; mantle potential temperature variation; isostatic compensation 相似文献
6.
《Sedimentology》2018,65(3):721-744
Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges. 相似文献
7.
浅埋偏压连拱隧道施工的力学响应分析 总被引：1，自引：0，他引：1
以江西某高速公路一浅埋偏压连拱隧道为背景,用MARC有限元程序对其出口段进行了动态施工的三维数值模拟。系统研究了塑性区分布和发展、拱顶下沉、正应力与剪应力的集中和转移、中隔墙竖向应力随施工过程的变化规律。研究表明:1.非对称开挖是引起中墙偏压的最关键因素,初衬和二衬的施作对改善中墙偏压作用不大,对称开挖才是最有效途径;2.在浅埋条件下,拱顶下沉有随埋深增大而增大的趋势,位移释放在开挖完成、支护之前就已经大部分完成。3.左右洞上台阶开挖后拱顶出现拉应力区,是易坍方部位,应超前或及时支护;4.由于偏压作用,山脊一侧边墙和中墙墙踵处塑性区更发育,该侧更易失稳;5.施工完毕,隧道两侧边墙附近集中的压应力转移到二衬和仰拱上,使二者成为应力集中部位,从而改善了隧道围岩的受力状况。 相似文献
8.
MATTHEW R. BENNETT MICHAEL J. HAMBREY DAVID HUDDART JEAN FRANOIS GHIENNE 《第四纪科学杂志》1996,11(6):437-449
Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges. 相似文献
9.
《Tectonophysics》1987,135(4):329-345
Hydraulic fracturing stress measurements have been made in fifteen boreholes of various depths from 100 to 800 m in the Kanto-Tokai area since 1978. About 90 sets of in-situ stresses have been obtained successfully. The maximum and minimum horizontal compressive stresses increase linearly with depth in each borehole. The difference between the maximum and minimum principal stresses also increases with depth. The increase of stress difference with depth is interpreted in terms of large stress relaxation in shallow parts of the boreholes where low confining pressure and many pre-existing microcracks are dominant. The maximum shear stress at a depth of 400 m ranges from 1 to 8 MPa depending on the site. It is not always the case that the regions of small shear stresses are inactive in microseismicity and crustal movement. This phenomenon is attributed to the relaxation of shear stress at the measured depths and to the regional variation in increasing rate of shear stress. The maximum compressive stress direction is obtained from detection of the fracture azimuth after hydraulic fracturing. Stress direction measured at each borehole agrees well with that estimated from geologic and seismic methods near the measurement site. Several stress provinces where the stress directions appear almost uniform are defined in the Kanto-Tokai area. The regional distribution of the stress directions is understood in terms of the relative movement of three plates, the Philippine Sea, Pacific, and Eurasian plates, which are in contact with each other in this area. 相似文献
10.
Lawrence M. Cathles 《Mineralium Deposita》2011,46(5-6):639-657
Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ??350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000?years at current production rates. 相似文献
11.
Seabeam, seismic and submersible surveys took place during the Kaiko Project and revealed significant compressive deformation at the northeastern end of the Philippine Sea plate, related to the recent collision of the Izu-Ogasawara Arc against Central Japan. Intraoceanic thrusting at the base of the Zenisu Ridge, a linear topographic high running a few tens of kilometers south of the Nankai Trough, is supported by tectonic, magnetic and gravimetric data. We investigate the formation of the Zenisu Ridge in terms of compressive mechanical failure of a thin elastic-perfectly plastic plate, subducting at a trench and subject to a regional compressive axial force. The rheological envelope concept is used throughout the numerical calculations. Based on a detailed study of flexure of the present-day bending far from the deformation zone, we evaluate the bending forces involved: the bulge is 120 to 150 m high and the compressive stress all along the Nankai Trough is about −100 MPa. In the Zenisu Ridge area, an additional compressive stress is superimposed due to the nearby collision at Izu-Peninsula. We compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20–25 km in the trench area, and again at the surface 60 to 100 km seaward, in the vicinity of the bulge. The development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed. The model predicts that the formation of the Zenisu Ridge did not occur before 4 Ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the Nankai Trough. 相似文献
12.
Structural Petrology of Plagioclase Peridotites in the West Othris Mountains (Greece): Melt Impregnation in Mantle Lithosphere 总被引：6，自引：2，他引：4
We present the results of a structural and petrological studyof mantle rocks from the strongly dismembered Othris Ophiolite.Part of the mantle section was impregnated with melt, crystallizingplagioclase and clinopyroxene as cumulate phases and refertilizingpreviously depleted peridotites. Melt impregnation occurredlate in the deformation history of the host peridotites. Thedeformation took place at stresses of 13–26 MPa and attemperatures around 1000–1200°C, at the base of thethermal lithosphere. The melt therefore impregnated relativelycold mantle rocks, implying that the thermal lithosphere reachedinto the mantle during magmatic activity. We conclude that theOthris Ophiolite represents a spreading environment with a relativelythick lithosphere, such as that near an axial discontinuityor transform fault of a slow-spreading ridge. The proposed magmaticand deformation history of the peridotites is in agreement withepisodic magmatism at slow-spreading ridges. We thus concludethat the heterogeneous character of the mantle section of theOthris Ophiolite results from melt impregnation processes. Wesuggest that the presence of lherzolitic ophiolite types amongharzburgitic ophiolite types in the Hellenic–Dinaric chainreflects variable degrees of melt impregnation and refertilizationrather than partial melting and melt extraction. KEY WORDS: lithospheric mantle deformation; melt impregnation; microstructures; Othris Ophiolite; plagioclase peridotites 相似文献
13.
Particulate organic carbon (POC) was measured for 77 water samples collected over a 3000 m water column along 88° E in the
central Bay of Bengal. The POC values varied from 80 to 895 μg per litre at the surface and 171 to 261 μg per litre at 2000
m. The POC decreased with increasing water depth at all the stations. Deep water concentrations of POC were higher than those
reported from other oceanic waters. Distribution of POC was not influenced by water masses. The POC was not significantly
correlated with chlorophylla. 相似文献
14.
The near-solidus transition from garnet lherzolite to spinel lherzolite 总被引：20，自引：1，他引：19
The position of the transition from spinel lherzolite to garnet lherzolite in the system CaO-MgO-Al2O3-SiO2 (CMAS) has been determined experimentally at near-solidus temperatures. In reversed experiments, the transition occurs between
18 and 20 kbar at 1200 °C and between 26 and 27 kbar at 1500 °C, corresponding to higher pressures than previously envisaged.
A position for the transition deeper within the Earth further complicates the explanation of the so-called garnet signatures
in the trace element and isotope patterns of mid-ocean ridge basalts. If melting during adiabatic upwelling beneath a mid-ocean
ridge begins at the depth required for the stability of garnet in peridotitic compositions, simple melting models predict
that the amount of melt produced should be much greater than the observed thickness of the oceanic crust. A partial solution
to the apparent conflict might be that (1) the rather simplistic melting models are in error, (2) that melting begins in garnet
pyroxenite veins that are believed to be stable at lower pressures than garnet lherzolite or (3) that melting does not involve
garnet at all, but it is clinopyroxene causing the trace element patterns observed in basalts erupted at mid-ocean ridges.
A second set of reversal experiments were conducted to investigate the solubility of alumina in both orthopyroxenes and clinopyroxenes
at the high temperatures near the solidus in the system CMAS. The results are compatible with most previous studies, and may
be used as a starting point to calibrate thermodynamic models for pyroxenes in chemical systems, approximating upper mantle
chemistry.
Received: 9 August 1999 / Accepted: 29 October 1999 相似文献
15.
Transform and non-transform discontinuities that offset slow spreading mid-ocean ridges involve complex thermal and mechanical interactions. The truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment-end. Likewise, the spreading system is expected to influence the lithospheric plate adjacent to the ridge-end opposite of the discontinuity. Tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside corners and outside corners, along-axis variations in rift valley depth, style of crustal accretion, and ridge segment retreat and lengthening. Along such slow-spreading discontinuities and their fossil traces, oceanic core complexes or mega-mullion structures are rather common extensional tectonic features. In an attempt to understand deformation of oceanic lithosphere near ridge offsets, the evolution of discontinuities, and conditions that may favor oceanic core complex formation, a three-dimensional thermo-mechanical model has been developed. The numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3-D models. The initial suite of results reported here focuses on deformation when axial properties do not vary along-strike or with time, showing the extent to which plate boundary geometry alone can influence deformation. We find that non-transform discontinuities are represented by a wide, oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments. This contrasts with predicted deformation near transform discontinuities, where initial orientation is maintained in time. The boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge–transform intersection. Ridge–normal tensile stresses concentrate in line with the ridge tip, extending onto the older plate across the discontinuity, and high stress amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models. With the tested rheology and boundary conditions, inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases. Additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere. 相似文献
16.
By scaled physical modelling, we have investigated the mechanical response to gravitational forces in an oceanic lithosphere, overlying a less dense asthenosphere. In the models, an upper wedge-shaped layer of sand represented an oceanic lithosphere (0–35 Ma old, with a half-spreading velocity of 3 cm/yr), and a lower layer of polydimethylsiloxane (PDMS), mixed with dense wolframite powder, represented the asthenosphere. In the models, as in nature, isostatic compensation resulted in uplift of ridges and subsidence on their flanks. The resulting relief was responsible for ridge push. We tested two main configurations: straight ridges and offset ridges. In all the models, ridge push was sufficient to cause plate motion, underlying advection, and symmetrical rifting at the ridge axis. There was no need to impose plate motions through external pistons and motors. In models of straight ridges, the style of normal faults in the axial rift zone depended on the local thickness of the brittle sand layer. For thick layers, normal faults rafted out from the active zone of rifting, creating a fossil topography of tilted blocks, between faults dipping toward the ridge. In a model of an offset ridge, with thin lithosphere at the ridge crest and no embedded weakness, ridge push was responsible for a short transform fault, linking en-échelon rifts. In a similar model, but with thick lithosphere, an oblique rift formed at about 20° to the offset trace. We conclude that ridge push was not adequate to create an ideal transform fault. In a model of an offset ridge, with an embedded thin vertical layer of pure PDMS at 90° to the ridge, transform motion concentrated along this weak layer, and the resulting structural style was very similar to that in nature. On the basis of these results, we infer that, in nature, (1) ridge push can indeed drive plate motion, and (2) ridge push can drive strike-slip motion on transform faults, provided that these are weaker than the adjacent oceanic lithosphere and that they form early in the history of spreading. 相似文献
17.
R. Meissner 《GeoJournal》1979,3(3):227-233
Based on the strong heat flow in the Archaean and Proterozoic a concept for dynamical processes is developed that agrees with observation of low and high pressure metamorphism in ensialic belt of the Precambrian in Scandinavia and other old shield areas. Heat release is considered to have taken place predominantly at fast spreading oceanic ridges, some of them certainly adjacent to continental margins. Thinner, warmer, lighter, and faster oceanic plates descended with very small dip below the continent, leading to an underplating, to a decrease of the continental thermal gradient, and to a compressive stress with the tendency to keep the growing Archaean supercontinent together. Ensialic belts may have partly been created by a shallow secondary back-arc creep pattern. The general decrease of heat production and heat release continuously slowed all creep processes. At around 1000 ± 300 Ma, oceanic plates must have reached critical values of thickness, weight, and density to perform a real steep angle subduction, reinforced by sediment loads and phase transition at depth. A well developed plate tectonic pattern with an opening and closing of the lapetus ocean, with subduction, and a subsequent continent-continent collision can be inferred for the forming of the Caledonian-Appalachian mountain range. Even today, dynamical movements at asthenospheric levels below Fennoscandia are still present as indicated by observations of seismic travel time residuals, deep seismic soundings, gravity, compressive stresses, and theoretical calculations of temperature and viscosity. 相似文献
18.
Lili Xu Jörg Renner Marco Herwegh Brian Evans 《Contributions to Mineralogy and Petrology》2009,157(3):339-358
We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under
a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic
pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure.
The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered
electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution
was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm,
increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent
with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively.
We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either
constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus.
The deformation tests took place at between 700 and 800°C and at strain rates between 10−6 and 10−3 s−1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts
coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion
creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with
increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress,
(n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the
stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain
rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with
solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical
composition will be an important element to consider when solid substitution can occur during natural deformation. 相似文献
19.
A world-wide correlation between satellite-derived gravity signatures and the relative abundance of teledetected earthquakes over mid-ocean ridges has yielded some unexpected results. Rift valley disappearances along slow-spreading centres and attendant excess volcanism coincide with seismicity gaps, at times related to nearby hotspots, whereas earthquake clusters along virtually aseismic, faster-spreading centres systematically indicate the presence of active propagating ridge tips. Therefore, at the world scale of investigation, seismicity fairly well predicts ridge morphology and 2nd order axial discontinuities. The occurrence of a certain degree of seismicity along the 'ductile' Reykjanes ridge south of the Iceland hotspot is tentatively explained in terms of prevailing shear stresses due to oblique spreading which accumulate on the available brittle volume on the flanks of the ridge rather than on its crest. 相似文献
20.
Evolution of a classic sand ridge field: Maryland sector, North American inner shelf 总被引：1，自引：0，他引：1
The ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware-Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross-sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression. A second aspect is the change in bottom sediment characteristics that accompanies these large-scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high-frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea-floor. A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large-scale bedforms. Finally, flow was measured and transport calculated on the same ridge during a one-month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43-year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration. Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat. 相似文献