Objective classification of oceanic ridge-crest terrains using two-dimensional spectral models of bathymetry: Application to the Juan de Fuca Ridge

An important application of detailed bathymetric mapping is the interpretation of geological processes based on the nature of the fine-scale morphology of the seafloor. This interpretation is usually accomplished through qualitative analysis of contour maps. In this paper, an objective classification technique, based on a two-dimensional spectral model of bathymetry developed by Fox and Hayes (1985) is applied to detailed Sea Beam data from the Juan de Fuca Ridge. Parameters of the model can be directly related to seafloor properties corresponding to 1) isotropic (non-directionally dependent) roughness; 2) anisotropic (directionally dependent) roughness; 3) orientation of the anisotropic component; and 4) spectral rool-off ( fractal dimension), by fitting the model surface to two-dimensional amplitude spectra of bathymetry determined on a regular grid over the study area. A test area was selected which encompasses the southern volcanic rift zone of Axial Volcano and the northern terminus of the Vance Segment. Parameters of the model clearly define the contrast between the constructional volcanic terrain (rough, isotropic, with high fractal dimension) and the tectonic extensional terrain (smoother, anisotropic, with low fractal dimension). An agglomerative, hierarchical cluster analysis is applied to the data, independent of spatial information, to delineate groups of spectra with similar characteristics. Distinct, mappable regions, corresponding to volcanic and tectonic provinces, are objectively determined. Also, coherent sub-regions of consistent spectral properties occur within the larger volcanic/tectonic divisions. The classification is extended to the Juan de Fuca Ridge system from 44°30 N to 47°20 N through combining these results with an a priori technique (K-means clustering). Broad-scale physiographic regions of the Juan de Fuca Ridge are delineated by the technique, which may aid geologists in the interpretation of crustal accretion processes at seafloor spreading centers.The U.S. Government right to retain a non-exclusive royaltyfree license in and to any copyright is acknowledged.     

Structure of the Northern Symmetrical Segment of the Juan de Fuca Ridge

A seismic refraction profile was shot along the axis of the Northern Symmetrical Segment of the Juan de Fuca Ridge system. Three models of the along-axis crustal structure fit the observed data equally well. One model includes a low-velocity zone, the top of which is at a depth below the seafloor of approximately 3 km, that is continuous along-axis for at least 30 km. A second model includes a low-Q layer, the top of which is also at a depth of approximately 3 km below the seafloor and is continuous along-axis for at least 30 km. Both the low-Q layer and low-velocity zone can be explained geologically by a region of elevated temperatures. The third model is characterized by a homogeneous seismic layer 3. All models contain an ~1 km s^–1 discontinuity at the seismic layer 2/3 boundary; a wide-angle reflection from this boundary is seen on all record sections. Kappel and Ryan (1986) had previously proposed that the Northern Symmetrical Segment was in a stage of volcanic inactivity, and this theory is supported by the seismic observations. Two-dimensional modelling of travel times to ocean bottom hydrophone instruments shows that the amplitude variations in the along-axis depth to intracrustal seismic layers (a few hundred meters) is on the order of the lateral changes in topographic relief. It is suggested that the crustal emplacement processes reflect the deeper style of 3-D mantle upwelling beneath the ridge.     

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.     

Ridge event detection: T-phase signals from the Juan de Fuca spreading center

An analysis of T-phase source locations determined in the mid-1960s for an area of the northeast Pacific Ocean encompassing the Juan de Fuca spreading center reveals that most of the source locations are associated with regions where seamount chains intersect the spreading center and with edifices both along and near the spreading center. The T-phase source locations also tend to cluster on, or near, areas of the most concentrated and vigorous hydrothermal venting along the Juan de Fuca Ridge. Of the 58 T-phase source locations determined for a period from October 1964 through December 1966, only one was found to be associated with an earthquake detected by the National Geophysical Data Center/National Earthquake Information Service because of the characteristic small magnitude of spreading-center seismic events. Monitoring T-phase activity originating along the 80 000 km-long global seafloor spreading-center system offers a practical and unique opportunity to better understand the dynamics and oceanic effects of episodic spreading-center tectonic, volcanic, and hydrothermal processes.     

Abundance,distribution, and feeding patterns of juvenile coho salmon (Oncorhynchus kisutch) in the Juan de Fuca Eddy

The Juan de Fuca Eddy is a seasonal, counter-clockwise gyre off the mouth of the Strait of Juan de Fuca between Washington, USA and British Columbia, Canada that may provide favorable feeding habitat for juvenile coho salmon (Oncorhynchus kisutch) during their early marine existence. In late September 2002, physical and biological sampling was conducted along two transects of the eddy region. Surface rope trawling was conducted to capture juvenile salmon and other nekton, along with bongo and neuston net tows to examine potential mesozooplanktonic salmon prey. Presence of the Juan de Fuca Eddy was confirmed with vertical water profiles. In addition, nutrient and chlorophyll a concentrations collected from 3-m depth were within the range observed in previous studies within the eddy region. In the mesozooplankton community, euphausiids, chaetognaths, and decapod megalopae were common. In the diet of juvenile coho salmon, euphausiids and decapod megalopae were dominant by percent number, and larval and juvenile fish were dominant by percent weight. Feeding intensity (percent body weight) based on stomach contents was variable, but not significantly different among stations. To compare the Juan de Fuca Eddy region with an upwelling area, we sampled along a transect off La Push (LP), Washington, USA which is south of the eddy. The eddy region was found to be less productive than the LP transect. Nutrients were lower, chlorophyll a concentrations were higher, and zooplankton abundance was generally higher along the LP transect than in the eddy region. In addition, more juvenile coho salmon were captured from the LP transect than the eddy region. Prey items in stomachs of salmon from the LP transect were heterogeneous compared to those from the eddy region. Feeding intensity along the LP transect was slightly lower and more variable than in the eddy region, and differences in feeding intensity among LP stations were significant. In addition, feeding intensities among stations nested within regions were significantly different.     

An analysis and interpretation of the current data collected in the strait of Juan de Fuca in 1973

Abstract

We evaluate a set of current measurements done in a section of the Strait of Juan de Fuca. The flow is of estuarine character, the upper layer flow usually being directed seaward. The RMS value of steady current exceeds its mean value appreciably in the upper layer; it also exceeds the mean near the bottom. The near‐surface currents do change their direction on occasions and can run landward for over five consecutive days, especially in the southern part of the channel. The lower layer flow (landward) and the upper layer flow (seaward) varied in magnitude from 90 to 160 thousand m³/sec, and their fluctuations were in phase; their difference, the net flow, is of the order of 5% to 10% of these flows and could not be calculated with any confidence. Tidal motion is barotropic in the section, with some deformation in the vertical caused by bottom friction and internal stresses. The M₂ tide in the system Juan de Fuca‐Georgia Strait can be represented by a standing Kelvin wave influenced by friction. The K₁ tide can also be represented by a standing Kelvin wave if some leakage is allowed in the northern end of Georgia Strait. Topographic effects mask the Coriolis influence on the intensity of tidal currents.     

飓风Juan(2003)路径附近实测飓风浪的谱特性研究

Hurricane Juan provides an excellent opportunity to probe into the detailed wave spectral patterns and spectral parameters of a hurricane system, with enough wave spectral observations around Juan＇s track in the deep ocean and shallow coastal water. In this study, Hurricane Juan and wave observation stations around Juan＇s track are introduced. Variations of wave composition are discussed and analyzed based on time series of one-dimensional frequency spectra, as well as wave steepness around Juan＇s track： before, during, and after Juan＇s passing. Wave spectral involvement is studied based on the observed one-dimensional spectra and two-dimensional spectra during the hurricane. The standardization method of the observed wave spectra during Hurricane Juan is discussed, and the standardized spectra show relatively conservative behavior, in spite of the huge variation in wave spectral energy, spectral peak, and peak frequency during this hurricane. Spectral widths＇ variation during Hurricane Juan are calculated and analyzed. A two-layer nesting WW3 model simulation is applied to simulate the one-dimensional and two-dimensional wave spectra, in order to examine WW3＇s ability in simulating detailed wave structure during Hurricane Juan.     

Morphology of the Blanco Transform Fault Zone-NE Pacific: Implications for its tectonic evolution

The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to kill the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.     

Seafloor gravity evidence for hydrothermal alteration of the sediments in Middle Valley, Juan de Fuca Ridge

Gravity data collected at two different levels, sea-surface and seafloor, are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in Middle Valley, a deeply sedimented spreading center on the Juan de Fuca Ridge. The sea-surface gravity data constrain density variations at depth beneath the seafloor, whereas sea-bottom measurements are more sensitive to shallow sources. At least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data: short-wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments. In Middle Valley, three distinct gravity anomalies were identified. (a) An anomaly over a sulfide mound, south of Bent Hill, shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near-surface conduits. (b) An anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits. (c) An anomaly paralleling an intra-valley fault, that limits the deepest part of the graben, is interpreted as indicating lithification of the upper sediment layer. A high heat flow anomaly is located 1 to 2 km east of this fault, suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area, due to the progressive migration of the hot fluid circulation.     

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a⁴⁰Ar/³⁹ Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.     

胡安·德富卡洋脊因代沃段热液黑烟囱体成矿物质来源:硫同位素的限制

选取胡安.德富卡洋脊(Juan de Fuca Ridge,JDFR)因代沃(Endeavour)段的17个热液黑烟囱体样品对其中的硫同位素进行分析测定,讨论了因代沃段热液活动区内黑烟囱体成矿的物质来源、将硫同位素数据与已发表的热液流体及硫化物数据耦合,并结合前人的成果得到如下认识:(1)因代沃段硫化物的硫同位素组成与其他无沉积物覆盖的洋脊硫化物硫同位素组成相似,然而其相比于南胡安.德富卡洋脊(South Juan de Fuca Ridge,SJFR)硫化物亏损重同位素;(2)结合前人研究成果,如果SJFR硫化物的硫全部来自基底玄武岩的淋洗与海水中的硫酸盐,那么因代沃段硫化物的硫可能有1%～3%来自沉积物的贡献,故提出因代沃段成矿系统中的硫来源主要来自基底玄武岩,同时伴随有少量海水硫酸盐来源及沉积物来源的硫加入;(3)将硫同位素数据与已发表的热液流体及硫化物数据进行耦合发现热液流体中的沉积物信号与硫化物中的硫可能来自不同的源,并提出沉积物端元可能位于下渗区。     

胡安·德富卡洋脊因代沃段热液黑烟囱体成矿物质来源:硫同位素的限制

选取胡安·德富卡洋脊(Juan de Fuca Ridge,JDFR)因代沃(Endeavour)段的17个热液黑烟囱体样品对其中的硫同位素进行分析测定,讨论了因代沃段热液活动区内黑烟囱体成矿的物质来源、将硫同位素数据与已发表的热液流体及硫化物数据耦合,并结合前人的成果得到如下认识:(1)因代沃段硫化物的硫同位素组成与其他无沉积物覆盖的洋脊硫化物硫同位素组成相似,然而其相比于南胡安·德富卡洋脊(South Juan de Fuca Ridge,SJFR)硫化物亏损重同位素;(2)结合前人研究成果,如果SJFR硫化物的硫全部来自基底玄武岩的淋洗与海水中的硫酸盐,那么因代沃段硫化物的硫可能有1%~3%来自沉积物的贡献,故提出因代沃段成矿系统中的硫来源主要来自基底玄武岩,同时伴随有少量海水硫酸盐来源及沉积物来源的硫加入;(3)将硫同位素数据与已发表的热液流体及硫化物数据进行耦合发现热液流体中的沉积物信号与硫化物中的硫可能来自不同的源,并提出沉积物端元可能位于下渗区。     

2002年春季吕宋海峡海流:观测与改进逆模式计算

基于2002年春季航次在吕宋海峡海域锚碇测流站(20°49'57"N,120°48'12"E)200,500与800m处锚碇测流以及CTD观测,采用改进逆方法对调查海域进行海流计算.(1)主要观测的结果:1)在200m处,观测期间海流平均速度为(47.4cm/s,346°).在500m处,海流观测期间平均速度为(20.3cm/s,350°).这些都表明黑潮在吕宋海峡锚碇测流站200和500m处向西北方向入侵南海.2)在800m处,海流观测期间平均速度为(1.2cm/s,35°),它的方向为东北向.比较每层实测流结果,表明800m层海流状况与200和500m层流况不同.3)在观测期间,200,500和800m处,日平均流速在4月皆比3月时要强.4)在调查海区西部的中间区域存在一个高密、冷水中心(HDCW),其中心位置位于断面A的水文站3附近.5)在调查海区东南区域存在一个低密、暖水(LDWW)中心,其中心位置位于断面B的水文站8附近.(2)主要计算结果:1)通过断面B的偏北方向与偏南方向的流量分别为32.48×106m3/s(包括反气旋涡的流量)与3.34×106m3/s.因此通过断面B的净北向流量为29.14×106m3/s.2)通过断面A的东向与西向的流量分别为16.71×106m3/s与8.57×106m3/s(包括气旋涡的流量).因此,通过断面A的净东向流量为8.14×106m3/s.3)通过断面M北向的净流量为24.68×106m3/s.4)黑潮通过断面M后分为主流和一个支流,其主流,流量为16.54×106m3/s,流向断面C的东部分.主流通过断面C的东部分后,最后流向台湾以东海域.而其一个分支,净流量为8.14×106m3/s,在一个高密、冷水中心(HDCW)的区域以东作气旋式弯曲,然后向西北方向通过断面C的西部.因此,黑潮在断面C有两个流核.5)比较计算得到的在锚碇测流站M附近流方向与在200与500m处观测流方向为西北向,它们甚为一致.6)在断面B西侧位于550m以深水层南海水可能缓慢地从西北流向东南,通过断面B的南向流量大约为3.34×106m3/s.     

Currents in the Luzon Strait during the spring of 2002: observation and computation by modified inverse model

On the basis of the current measurements at 200,500 and 800 m from moored current meters with the time series data from March 17 to April 15 at the mooring station (20°49′57″N, 120°48′ 12″E) and the hydrographic data obtained in the Luzon Strait during the spring of 2002 cruise, the circulation in the investigated     

台湾海峡及其邻区岩石圈层下地幔流应力场

本文应用卫星测到的2～30阶球谐函数系数(GEM8) 计算了台湾海峡及邻近地区的岩石圈层下地幔流应力场。在比较了该区域的构造地质以后,发现应力场的方向,尤其是在板块收敛区域的甚高阶场(26～30阶)的方向与在菲律宾海和南海等区域的板块运动方向一致。     

浙江近海潮流和余流的特征

依据近年来浙江海岸带和海岛调查的实测海流数据,研究了浙江近海的潮流、余流特征和分布规律。由实测数据得到调和分析的一些参数,并根据这些参数分析了浙江近海的潮流性质、潮流运动形式、最大可能流速、M2分潮潮流椭圆、潮流的垂直分布及冬、夏季的余流分布,从而对浙江近海整个区域的潮流、余流特性有了一个较为全面的认识。     

Propeller underwater radiated noise: A comparison between model scale measurements in two different facilities and full scale measurements

The prediction of propeller induced pressure fluctuations and underwater radiated noise is a subject of great and increasing interest in marine engineering. Nevertheless, the full-scale prediction of these negative effects, even though based on dedicated model scale tests represents still a challenging task. This is due to different phenomena, among which scale effects on cavitation and ship wake, confined environment and near field effects in model tests play an important role; the analysis of these problems is made difficult by the rather limited amount of available data from sea trials and to the complexities of the phenomena, most of which related to cavitation on the propeller blades, that are present in the measurements carried out in cavitation tunnels, depressurized towing tanks or circulating channels.In the present work, the subject has been studied with reference to a four blades conventional CP propeller of a coastal tanker.Cavitation tunnel tests have been carried out in two rather different facilities, at UNIGE cavitation tunnel and at SSPA large cavitation tunnel.Results from model scale tests processed with different treatments are then compared with full scale measurements performed by SSPA on the same propeller in terms of cavitation extension and radiated noise.The analysis is aimed at assessing the effectiveness of different experimental setups, testing procedures and scaling laws.     

山东半岛东北部海洋牧场海域冬季海水流动的时空分布特征

为丰富山东半岛近岸海洋牧场海域水动力环境研究,本文利用2019年12月3日至2020年1月1日在山东半岛东北部4个海洋牧场获取的海流资料,应用功率谱分析、调和分析、余流主轴分析和相关分析,探讨冬季各海洋牧场的潮流、余流特征及其影响机制.结果表明:(1)各海洋牧场潮流由M2分潮潮流主导,受地形边界限制,各主要分潮潮流均为...