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1.
New paleomagnetic data from 11 sites in layered gabbros and lava flows from the Oman Ophiolite indicate stable, early remagnetizations and suggest that the southern portion of the ophiolite (the Wadi Tayin, Sumail, Nakhl-Rustaq and Haylayn massifs) is relatively unrotated since detachment near the paleoridge. The gabbros possess a magnetization carried by a combination of primary and secondary magnetites derived from hydrothermal alteration. Evidence from positive tilt tests, constancy of remanence directions in differing magnetic mineralogies and agreement with previous paleomagnetic data, however, suggests that this remagnetization was acquired early – analogous to the remagnetization of the V2 volcanic series. Thus, the evidence implies that the southern portion of the ophiolite has been primarily translated from the paleoridge since the time of V2 remagnetization, and 120° of clockwise rotation affecting the northern Oman Ophiolite is internal to the ophiolite, rather than a combination of internal and global rotation as previously hypothesized. Given this evidence, we propose a simplified model of a rapid, active microplate rotation of a portion of the ophiolite resulting from spreading at an EPR-type propagating ridge at a high angle to the previous spreading direction. Paleomagnetic data from this and previous studies can be well explained by a rapidly rotating microplate, similar to the kinematic evolution documented for the Juan Fernandez microplate in the modern setting. 相似文献
2.
Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric
The anisotropy of magnetic susceptibility was measured on 42 gabbros sampled across a complete plutonic sequence from the Oman ophiolite. The rock fabrics, investigated in the field and through plagioclase crystallographic fabric measurements, were compared to the magnetic fabrics. This comparative study reveals that from the paleo-Moho to the top of the foliated gabbros level, 73% of the rocks display a good correspondence in orientation, between the magnetic and rock fabric orientation. In these rocks, the AMS is controlled by secondary magnetites located in the fracture network of the olivines, and probably, but to a lesser extent, by secondary magnetites located in the exsolution lamellae of the clinopyroxenes. The high correlation between the AMS ellipsoid orientation and the rock fabric orientation is explained by the fact that the magnetic foliation is essentially constrained by the orientation of the olivine fracture planes, which is in turn constrained by the orientation of the overall magmatic rock fabric. In contrast to the primary mineral phases, the orientation of magnetite crystals in these gabbros is not due to their alignment in a flowing magma, so their preferred orientation, although usually mimicking that of the rock fabric, does have not the same origin. Furthermore, given that the preferred orientation of the anisometric secondary magnetites is much less perfect than the preferred orientation of the plagioclases, no correlation between the shape and magnitude of the AMS and plagioclase fabrics can be established. In the uppermost levels of the sequence there is no correspondence between the magnetic and rock fabric orientation. The magnetism of these rocks is mainly carried by primary magnetite and ilmenite grains. These minerals occur as small and scattered interstitial grains that exhibit neither alignment nor parallelism with the pre-existing rock fabric. Hence, the anisotropy, shape and orientation of the AMS ellipsoid are independent of the rock fabric ellipsoid. Although in the Wadi Al Abyad gabbros, just like in other magnetite bearing rocks (Rochette et al., 1992; Archanjo et al., 1995), the AMS cannot be used to evaluate the shape and strength of the finite strain ellipsoid, it can be reliably used to get the orientation of the rock fabric ellipsoid when the AMS is controlled by secondary magnetites. 相似文献
3.
The Semail ophiolite in Oman is one of the few ophiolitic complexes that may display locally magmatic sulphide ores in layer 3 of the crustal section. The ores found in the wadi Haymiliyah plutonic sequence, are composed of low Ni-pyrrhotite, chalcopyrite, pyrite and pentlandite. They are located at the bottom of a thick two-pyroxene and noritic gabbro unit, the Main Laminated Noritic Gabbro Unit (MLNGU) that crystallized from evolved tholeiites according to a calc-alkaline liquid line of descent. The MLNGU rests on coarse-grained layered gabbros of the Main Layered Gabbro Unit (MLGU) crystallized from more primitive MORB-like magmas. A detailed mineralogical study coupled with analyses of S, Se, chalcophile transition metals (Cu, Ni, Platinum-group elements, PGE) and Au allows two stages to be distinguished in the precipitation of the sulphides. Sulphide modal abundances start to increase at the top of the MLGU (up to 4%) where orthopyroxene becomes a major phase. Sulphides in the MLGU are only intercumulus and Cu-rich, indicating a S-undersaturated regime. The fine-grained two-pyroxene gabbros at the bottom of the MLNGU contain up to 16% sulphides (monoclinic pyrrhotite, pyrite, chalcopyrite). However, their precious metal contents are very low and far from economic grade ( PGE+ Au <15 ppb except one concentration at 230 ppb in the richest layer). Sulphide droplets in cumulus silicates indicate that sulphides precipitated in the fine-grained gabbros from a S-saturated magma. Sulphide liquid immiscibility was likely triggered by a set of unusual circumstances, such as a progressive S enrichment in high fO2 environments resulting from the closure of the Haymiliyah magma subchamber, the lack of a strong Fe-enrichment trend, and possibly a sudden temperature drop. The abundance of disseminated ores suggests that gravity segregation was inefficient; sulphide droplets probably nucleated at the crystallization front of the silicates. This hypothesis (of in-situ crystallization of sulphides) would also account for the very low precious metal contents. All the S present in the magma chamber was mobilized into the ores since the underlying MLNGU and isotropic gabbros are almost devoid of sulphides. 相似文献
4.
Daniel A. Walker Stephen R. Hammond 《Deep Sea Research Part II: Topical Studies in Oceanography》1998,45(12):2531-2545
The U.S. Navy’s Sound Surveillance System (SOSUS) hydrophone arrays are extemely efficient receptors of a high-frequency earthquake energy phase known as the t(ertiary)-wave, or t-phase (Fox et al., 1994). After a nearly 30-year hiatus in such studies, SOSUS arrays are again being utilized to detect t-phases and to locate seismic and volcanic events occurring along the Gorda seafloor spreading center (Fox et al., 1995; Fox and Dziak, 1998). Earlier, Northrop et al. (1968) also used other military arrays to infer tectonic structure along the Gorda Ridge. From October 1964 through December 1966, over 600 low-magnitude earthquakes occurred along the Gorda Ridge. Nearly all of these events had magnitudes below the detection thresholds of land-based seismic networks. Northrop et al. (1968) interpreted the geographic distribution of these events as evidence for a nascent fracture zone near the midpoint of the ridge. In the present study, the spatial distributions of these older data and, for the first time, their temporal distributions as well, were examined with respect to detailed bathymetry of the ridge that was acquired in the early 1980s. This analysis, of 570 on-axis and 74 off-axis events, led to the following observations: (1) nearly all of the Gorda Ridge t-phase events occurred in discreet swarms centered about the ridge axis, (2) most of the events within each of 8 (of 9) observed swarms occurred mainly along single ridge segments, and, (3) reconfirming the earlier Northrop et al. (1968) conclusion, most of the events originated in the region of a major change in the strike of the ridge axis. During the 27-month interval that the ridge was observed, relatively few t-phase events took place along the northernmost segment of the Gorda Ridge where the 1996 eruption occurred. However, a unique sequence of small events which visually resemble the events associated with a Juan de Fuca Ridge eruption in 1993 (Fox et al., 1995) and a Gorda Ridge eruption in 1996 (Fox and Dziak, 1998) may have been associated with an eruption on the ridge during 1965. 相似文献
5.
Based on the extended mild-slope equation, the wind wave model (WWM; Hsu et al., 2005) is modified to account for wave refraction, diffraction and reflection for wind waves propagating over a rapidly varying seabed in the presence of current. The combined effect of the higher-order bottom effect terms is incorporated into the wave action balance equation through the correction of the wavenumber and propagation velocities using a refraction–diffraction correction parameter. The relative importance of additional terms including higher-order bottom components, the wave–bottom interaction source term and wave–current interaction that influence the refraction–diffraction correction parameter is discussed. The applicability of the proposed model to calculate a wave transformation over an elliptic shoal, a series of parallel submerged breakwater induced Bragg scattering and wave–current interaction is evaluated. Numerical results show that the present model provides better predictions of the wave amplitude as compared with the phase-decoupled model of Holthuijsen et al. (2003). 相似文献
6.
大西洋洋中脊海底表层热液沉积物的铅同位素组成及其地质意义 总被引：5，自引：0，他引：5
新测得 TAG热液区中 6件海底表层热液沉积物样品的铅同位素组成 ,其变化范围不大 ,具有均一性的特征。在 Pb- Pb图解上 ,热液沉积物的铅同位素数据大多落在 MARB的铅同位素组成范围内 ,与大西洋沉积物和 Fe- Mn结核相比明显具较少的放射成因铅 ,反映其上部洋壳岩石为该区热液沉积物的形成提供了铅。对比研究表明 ,因不同地质 -构造环境中的海底热液区为热液沉积物形成提供物源的情况不同 ,是导致有沉积物覆盖洋中脊中热液沉积物的铅同位素组成与无沉积物覆盖洋中脊不同的主要原因。海底扩张中心的扩张速率与热液沉积物的铅同位素组成有一定的对应关系 ,但其并不是热液沉积物铅同位素组成的唯一控制因素 相似文献
7.
An experimental study of stratified mixing caused by internal solitary waves in a two-layered fluid system over variable seabed topography 总被引：4，自引：0，他引：4
Stratified mixing is observed in a wave flume on an internal solitary wave (ISW) of depression or elevation type propagating over a submarine ridge. The submarine ridges, which comprise the seabed topography, are either semicircular or triangular. Tests are performed in a series of combinations of submarine ridges with different heights and ISW in different amplitudes within a two-layer fluid system. When the thickness of the top layer is less than that of the lower layer (i.e., H1<H2), a depression-type ISW may produce a strong hydraulic jump with downwards motion and continuous eddy diffusion. During diffusion, the leading profile of the ISW transforms a wrapped vortex on the front face of the ridge, and a vortex separation at the apex of the ridge. Meanwhile, an elevation-type ISW causes a vortex in the lee of a submarine ridge, which resembles a surface solitary wave in terms of wave transmission process. The degree of wave-obstacle interaction is determined by energy loss, which is induced by submarine ridge blockage. The experiment results suggest that degree of blocking can be applied to classify various degrees of ISW-obstacle encounter in the stratified two-layer fluid system. 相似文献
8.
近23年的调查研究，使我们认识到分布于洋中脊、弧后盆地、岛弧和热点等环境的海底热液活动发育在多种围岩类型之上，包括超基性岩石、基性岩石、中性岩石、酸性岩石和沉积物。海底热液活动经历了岩浆去气作用、流体-岩石/沉积物相互作用和流体-海水混合，获取了岩浆、岩石、海水和沉积物的物质，构成了热液循环，产生了高温、低氧、高或低pH值、富含Fe、Mn、Cu、Zn、Pb、Hg、As等元素以及气体组分（甲烷、氢等）的喷口流体，影响了海水、沉积、岩石和生物环境，形成了热液柱、硫化物、含金属沉积物和蚀变岩石等热液产物，组成了海底热液系统。未来，促进海底热液活动探测技术和热液产物测试方法的发展，对海底热液区的岩石、喷口流体、热液柱、硫化物、含金属沉积物以及热液循环、生物活动的持续观测与研究，无疑将为人类探知海底地质过程及生命活动、保护海底热液环境和合理开发利用海底资源提供有力的工作支撑。 相似文献
9.
James R. Hein Ann E. Gibbs David A. Clague Michael Torresan 《Marine Georesources & Geotechnology》2013,31(2):177-203
This is the first article to describe mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and bimessite. Thin Fe‐Mn crusts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of δ‐MnO2. The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material (in Mn‐cemented sandstone), and seawater that mixed with the hydrothermal fluids. Mineralization on Haleakala Ridge occurred sometime during the past 200 to 400 ka, when the summit was at a water depth of more than 1,000 m. Hydrothermal circulation was probably driven by heat produced by intrusion of dikces, magma reservoirs, and flow of magma through axial and lateral conduits. The supply of seawater to ridge interiors must be extensive because of their high porosity and permeability. Precipitation of Mn oxide below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks. 相似文献
10.
Direct numerical simulations are performed to study the transformation of internal solitary waves (ISWs) of depression type propagating over an underwater ridge in a two-layer fluid system. Bottom ridges with relatively smooth vertex are employed to represent sills in natural lakes and oceans. Consistent with previous experiments, three interaction types (weak, moderate and strong) are observed to be based on the energy loss. In addition, the moderate interaction are found to be categorized into transmitted and reflected type according to their distinct transformation process. General flow characteristics for ISW–ridge interaction in the benthic boundary layer and in the pycnocline is monitored and analysed. A modified degree of blocking Bm considering both the nonlinear effect of incident ISWs and the blockage effect of the submerged ridge is proposed. Different ISW–ridge interactions are discovered to be linked with Bm. Maximum wave-induced velocities, wave energy losses, reflected and transmitted wave amplitudes are found to have a self-similar feature with Bm. The maximum energy loss is up to 35% and the maximum wave-induced velocity can reach 1.8 times of the phase speed of the incident ISW. Empirical equations are obtained based on the data fitting to predict some useful physical parameters during ISW–ridge interaction. 相似文献
11.
We analyzed seafloor morphology and geophysical anomalies of the Southeast Indian Ridge(SEIR) to reveal the remarkable changes in magma supply along this intermediate fast-spreading ridge. We found systematic differences of the Australian-Antarctic Discordance(AAD) from adjacent ridge segments with the residual mantle Bouguer gravity anomaly(RMBA) being more positive, seafloor being deeper, morphology being more chaotic, M factors being smaller at the AAD. These systematic anomalies, as well as the observed Na_(8.0) being greater and Fe_(8.0) being smaller at AAD, suggest relatively starved magma supply and relatively thin crust within the AAD.Comparing to the adjacent ridges segments, the calculated average map-view M factors are relatively small for the AAD, where several Oceanic Core Complexes(OCCs) develop. Close to 30 OCCs were found to be distributed asymmetrically along the SEIR with 60% of OCCs at the northern flank. The OCCs are concentrated mainly in Segments B3 and B4 within the AAD at ～124°–126°E, as well as at the eastern end of Zone C at ～115°E. The relatively small map-view M factors within the AAD indicate stronger tectonism than the adjacent SEIR segments.The interaction between the westward migrating Pacific mantle and the relatively cold mantle beneath the AAD may have caused a reduction in magma supply, leading to the development of abundant OCCs. 相似文献
12.
洋中脊和弧后盆地热液区热液流体B同位素组成的系统性差异 总被引：1，自引：0，他引：1
硼(B)是流体迁移元素，趋向于在热液流体中富集而成为常量元素。不同来源的B其同位素组成有着明显的区别。因此，B的含量及其同位素组成可标识热液流体(元素)的物质来源、水–岩反应程度及沉积物(元素)混入等重要过程，对海底热液活动及其成矿作用过程具有重要的示踪意义。迄今，对全球主要热液活动区热液流体中B的含量及同位素组成特征已做了大量的测试分析及研究工作，积累了丰富的资料和重要研究成果。但是，对不同地质背景(构造环境)条件下热液流体中B的含量及同位素组成特征尚缺乏系统性的对比分析，进而对造成不同环境热液流体中元素及其同位素组成的系统性差异的原因或机制尚缺乏深入的认识。本文在获取了洋中脊和弧后盆地主要热液活动区热液端元流体中B的含量及其同位素组成数据的基础上，定量估算了热液流体中B的主要来源，并对洋中脊和弧后盆地热液端元流体中B同位素组成的系统性差异进行了分析及成因探讨。结果表明，不同热液活动区热液端元流体的δ11B值都具有较大的变化范围，水–岩反应过程中不同来源B的混合是热液流体B同位素组成变化的主要原因。无沉积物覆盖的洋中脊和弧后盆地热液区热液流体中的B主要为海水与基底岩石来源B的混合，弧后盆地岩浆挥发性组分对热液系统的直接贡献及两种不同地质背景下基底岩石地球化学组成与水–岩反应程度的差异是其热液端元流体B同位素组成差异的主要原因。在有沉积物覆盖的弧后盆地热液区，热液流体中B的同位素组成与前两者之间存在显著差异，具有异常低的δ11B值，水–岩反应过程中沉积物来源B的加入是导致热液流体中δ11B值系统性降低的主要原因，沉积物的吸附作用也在一定程度上影响了热液流体的B同位素组成。有沉积物覆盖的洋中脊热液区热液流体同样受到了沉积物来源B加入的影响，具有较低的δ11B值，且相对于冲绳海槽受到了更强烈的沉积物吸附作用的影响。基于以上分析，并结合热液流体的Sr同位素组成特征，本文提出了洋中脊和弧后盆地这两大构造环境中热液流体B同位素组成系统性差异的成因模式。 相似文献
13.
冲绳海槽浮岩中碳、氢同位素组成特征 总被引：4，自引：1，他引：3
利用分阶段热解释放气体质谱分析法研究了冲绳海槽浮岩热解释放气中CO2和H2O的碳、氢同位素组成,结果显示:浮岩中原生CO2和H2O主要释放于400～1 000℃,CO2的碳同位素组成介于-6.7×10-3～-22.7×10-3,H2O的氢同位素组成从-45×10-3变到-71×10-3,均落入幔源火山岩的变化范围,而且浮岩的氢同位素组成与海槽区玄武岩的氢同位素组成非常接近,这表明冲绳海槽浮岩与玄武岩之间具有密切的成因联系,浮岩岩浆和玄武岩岩浆是同源岩浆不同程度结晶分异的产物.另外,这些浮岩较洋中脊玄武岩要贫13C,并富集D,同时具有从洋中脊玄武岩向岛弧玄武岩变化的趋势,这表明浮岩岩浆在形成或上升过程中可能受到俯冲板块释放流体的影响. 相似文献
14.
Prediction of phytoplankton growth in a Warm-Core Ring using three dimensional ecosystem model 总被引：1，自引：0，他引：1
Michio J. Kishi 《Journal of Oceanography》1994,50(5):489-498
In order to simulate the response of the primary production dynamics to the decay of the Warm-Core Ring (WCR) 86-B off the east coast of Japan, we made a numerical model which consists of a three dimensional physical model (modified GCM) and we used the same biological model that Frankset al. (1986) did. According to the three dimensional model, the well known sub-surface chlorophyll maximum was reproduced but horizontal distributions of Chl.a and NO3
–-N showed different patterns corresponding to different initial conditions of nutrient. This is because the weak vertical velocity in the WCR does not play an important role on the ecosystem but only the light intensity and the balance between uptake and vertical diffusion of dissolved nutrient is important. This result differs from that of Frankset al. (1986). The two WCRs interaction model suggests that a weak upwelling could exist between two WCRs accompanied by baroclinic instability. 相似文献
15.
Bathmetric highs on the old crust proximal to ridge-transform intersections (RTIs), termed intersection highs, are common but poorly understood features at offsets of fast to intermediate rate spreading centers. We have combined new reflection seismic, photographic, and geochemical data with previously published Seabeam, SeaMARC I, and SeaMARC II data to address the nature of the intersection highs at the Clipperton Fracture Zone. The Clipperton Intersection Highs are both topped by a carapace of young lavas at least 100 m thick. These lavas, which were erupted on the intersection highs, are chemically similar to their adjacent ridge segments and different from the surrounding older crust. At least some of the erupted magma traveled directly from the adjacent ridge at a shallow crustal level. Ridge-related magma covers and intrudes at least the upper 500 m of the transform tectonized crust at the RTI. We suspect that additional magma enters the intersection highs from directly below, without passing through the ridge. The young oceanic crust near the western Clipperton RTI is not thin by regional comparison. The 1.4 m.y. old crust near the eastern Clipperton RTI thickens approaching the transform offset. If the thermal effects of the proximal ridge were negligible, the eastern intersection high crust would appear to be in isostatic equilibrium. We believe that thermal effects are significant, and that the intersection high region stands anomalously shallow for its crustal thickness. This is attributable to increased temperature in the mantle below the ridge-proximal crust. Although ridge magma is injected into the proximal old crust, plate boundary reorganization is not taking place. Intersection high formation has been an ongoing process at both of the Clipperton RTIs for at least the past 1 m.y., during which time the plate boundary configuration has not changed appreciably. We envision a constant interplay between the intruding ridge magma and the disrupting transform fault motion. In addition, we envision a nearly constant input of magma from below the high, as an extension of the magma supply to the ridge from the mantle. Because the proximal ridge profoundly affects the juxtaposed crust at the RTI, sea floor fabric along the aseismic extensions of this fast-slipping transform fault is primarily a record of processes at work at the RTI rather than a record of transform tectonism. 相似文献
16.
The newly developed nearshore circulation model, SHORECIRC, using a hybrid finite-difference finite-volume TVD-type scheme, is coupled with the wave model SWAN in the Nearshore Community Model (NearCoM) system. The new modeling system is named NearCoM-TVD and the purpose of this study is to report the capability and limitation of NearCoM-TVD for several coastal applications. For tidal inlet applications, the model is verified with the semi-analytical solution of Keulegan (1967) for an idealized inlet-bay system. To further evaluate the model performance in predicting nearshore circulation under intense wave–current interaction over complex bathymetry, modeled circulation patterns are validated with measured data during RCEX field experiment (MacMahan et al., 2010). For sediment transport applications, two sediment transport models are applied to predict three sandbar migration events at Duck, NC, during August to October 1994 (Gallagher et al., 1998). The model of Kobayashi et al. (2008) incorporates wave-induced onshore sediment transport rate as a function of the standard deviation of wave-induced horizontal velocities. The modeled beach profile evolution for two offshore events and one onshore event agrees well with the measured data. The second model investigated here combines two published sediment transport models, namely, the total load model driven by currents under the effect of wave stirring (Soulsby, 1997) and the wave-driven sediment transport model due to wave asymmetry/skewness (van Rijn et al., 2011). The model study with limited field data suggests that the parameterization of wave stirring is appropriate during energetic wave conditions. However, during low energy wave conditions, the effect of wave stirring needs to be re-calibrated. 相似文献
17.
Dziak Robert P. Fox Christopher G. Embley Robert W. Nabelek John L. Braunmiller Jochen Koski Randolph A. 《Marine Geophysical Researches》2000,21(5):423-450
Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the 150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector estimates of 43 earthquakes (M
w3.8) that occurred along the eastern BTFZ over the last 5 years reveal that the Blanco Ridge is a high-angle right-lateral strike-slip fault, with a small component of dip-slip motion, where the Juan de Fuca plate is the hanging wall relative to the Pacific plate. Furthermore, the Cascadia and Gorda basins are undergoing normal faulting with extension predominantly oblique to the transform trend. Seafloor submersible observations agree with previous hypotheses that the active transform fault trace is the elongate basin that runs the length of the BR summit. Brecciated and undeformed basalt, diabase, and gabbro samples were collected at the four submersible survey sites along the Blanco Ridge. These petrologic samples indicate the Blanco Ridge is composed of an ocean crustal sequence that has been uplifted and highly fractured. The petrologic samples also appear to show an increase in elevation of the crustal section from east to west along the Blanco Ridge, with gabbros exposed at a shallower depth farther west along the southern (Pacific plate side) BR ridge flank. Further supporting evidence for BR uplift exists in the seismic reflection profiles across the BR showing uplift of turbidite sequences along the north and south ridge base, and gravity and magnetics profiles that indicate possible basement uplift and a low-density zone centered on the ridge's Pacific plate side. The BR formation mechanism preferred here is first, uplift achieved partially through strike-slip motion (with a small dip-slip component). Second, seawater penetration along the fault into the lower crust upper mantle, which then enhanced formation and intrusion of a mantle-derived serpentinized-peridotite diapir into the shallow ocean crust, causing further uplift along the fault. 相似文献
18.
Nicolas A. Ildefonse B. Boudier F. Lenoir X. Ben Ismail W. 《Marine Geophysical Researches》2000,21(3-4):269-287
Mafic dikes and dunite veins are observed in the mantle section of the Oman – United Arab Emirates (O-UAE) ophiolites, as well as diabase dikes and hydrothermal veins in the crust section. They have been systematically measured during the mapping of this ophiolite and are represented by their trajectories in the folded map 3 in the back of this volume, and by local stereoplots included in this study. Mafic dikes in the mantle section correspond to basaltic melt being injected at decreasing temperatures from above or at peridotite solidus, down to below 450°C. Hydrothermal veins associated with dioritic dikes issued from hydrous melting of host gabbros are observed down to the base of the crust, bearing evidence for sea water penetration into basal gabbros at or above 900°C, that is very close to the ridge axis. Dike orientations record the stress field at the time of their injection. In most places, all types of dikes are dominantly parallel to the general trend of the nearest sheeted dike complex; thus the stress field has not visibly changed from melt injection in the asthenosphere below the ridge of origin to injection in a lithosphere up to a few Myr old, at distances beyond 100 km from the axis. Local preferred orientations, when they are considered in the frame of the paleo-ridge system of O-UAE, result in a coherent model throughout the belt: the sheeted dike complex dips moderately away from the presumed ridge axis and the mantle dikes, toward this axis. These opposite directions are explained by the presumed effect of subsidence toward the axis for the sheeted dikes and by the central feeding from an asthenospheric uprise for the mantle dikes. 相似文献
19.
The Wuqbah peridotite, central Oman Ophiolite: Petrological characteristics of the mantle in a fossil overlapping ridge setting 总被引：2，自引：0，他引：2
Jacques Girardeau Christophe Monnier Laurent Lemée Frédéric Quatrevaux 《Marine Geophysical Researches》2002,23(1):43-56
The Wuqbah peridotites (Wuqbah massif, central Oman Ophiolite) constitute the mantle part of a complete ophiolitic sequence and their field deformation geometry is thought to reflect mantle dynamics in a fossil overlapping ridge settings (Girardeau et al., 2002). These peridotites comprise dominantly residual harzburgites and dunites. Nearly 70% of the harzburgites are clinopyroxene-free, and the rest contains less than 1%. The mineral chemistry of olivine, pyroxenes and spinel, and whole rock major and rare-earth element data, indicate that the Wuqbah peridotites are all strongly refractory and that they record a major percolation event, marked by strong enrichments in incompatible elements. At the massif scale, the Central Zone contains rocks with the most refractory features (20% melt extraction), as expected in an area of mantle upwelling. In the overlapping ridge senario, it corresponds to the overlap zone whose formation is discussed. 相似文献
20.
The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr−1, a transitional velocity between the very slow (≤20 mm yr−1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr−1) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam,
magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is
partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order
discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating
complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first
and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace
of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to
lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the
ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to
the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief
of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments
at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit
along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the
segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively
low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and
is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the
ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken
together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining
segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this
contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather,
there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge
segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent
with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused
along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate
that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking
morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as
well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the
morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies
the segment. 相似文献