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1.
Heat fluxes are estimated across transatlantic sections made at 4°30′S and 7°30′N in January–March 1993, following Hall and Bryden (1982. Deep-Sea Research 29, 339–359). Particular care is given to the computation of Ekman volume and heat fluxes, which are assessed both (a) from the windstress data for the period of the cruise and (b) from the comparison between geostrophic and Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP) velocities. In contrast with previous studies, the two estimates for Ekman fluxes do not converge for either section: (a) (11.5±0.5 Sv; 1.01±0.05 PW) across 7°30′N and (−9.3±1.2 Sv; −0.85±0.12 PW) across 4°30′S when windstress data at the date of the hydrographic stations are used; (b) (6.3±1.1 Sv; 0.56±0.09 PW) across 7°30′N and (−3.4±3.0 Sv; −0.35±0.24 PW) across 4°30′N when the ageostrophic transport above the thermocline is used. The divergence would have been even greater at 4°30′S if the strong ageostrophic signal beneath the thermocline, which brings a transport of (8.4 Sv; 0.82 PW), had been considered. The corresponding total meridional heat fluxes are: (a) 1.40±0.16 PW and (b) 0.95±0.20 PW across 7°30′N, (a) 1.05±0.12 PW and (b) 1.67±0.14 PW (2.39±0.14 PW when the subthermocline ageostrophic transport is taken into account) across 4°30′S.The estimates based on windstress data are compared with the results from an inverse model (Lux and Mercier, 1999) to show the importance of the heat flux due to the deviation of the local depth-averaged potential temperature from its average over the section, which is neglected in the Hall and Bryden (1982. Deep-Sea Research 29, 339–359) method but is not negligible in our computation in which we do not isolate the transport of the western boundary current east of the 200 m isobath; this corrective flux amounts here to −0.19 PW across 7°30′N and 0.33 PW across 4°30′S.The seasonal variability of the meridional heat flux across 7°30′N is studied through the hydrographic data collected during the ETAMBOT 1–2 cruises, which repeated the 7°30′N section west of 35°W in September 1995 and April 1996. When the section is completed east of 35°W with CITHER 1 data and when windstress data are used for the computation of the Ekman transport, the estimates for the meridional heat fluxes are 0.20±0.14 PW in September 1995 and 1.69±0.27 PW in April 1996. The estimates fit well with results from numerical models.  相似文献   

2.
Swath MR1 data from the remnant Colville and active Kermadec arc margins, south of 33°30 S (SW Pacific), record the structural morphology and evolution of the rifted, and now separate portions, of the proto-Colville–Kermadec arc flanking the actively widening southern Havre Trough back-arc basin associated with Pacific-Australian plate convergence. Both the remnant Colville and active Kermadec arc margins comprise opposing, asymmetric, partially basement exposed, segmented ridges. Differences in morphology between the two ridges are, however, observed. The single, near linear, border fault system, with relief of 1000 m, along the western edge of the Kermadec margin is interpreted to be the exposed fault escarpment of a major, west-dipping, detachment fault. In contrast, two major zig-zag border fault systems along the eastern edge of the Colville Ridge, bounding a back-tilted ridge flank terrace, are interpreted as major antithetic faults between the remnant arc and back-arc region. This contrast in structural morphology coincides with, respectively, lesser and greater degrees of both active tectonism and channel-canyon erosion, on the remnant Colville and active Kermadec margins. These differences are interpreted to reflect the progressive trenchward collapse and associated greater rift flank uplift and incisive erosion of the Kermadec foot-wall contrasting with the non-collapse and relatively lesser rift flank uplift and ridge erosion of the Colville hanging-wall. The data provide further constraints on the early evolution of the Havre Trough in particular, and back-arc basins in general.  相似文献   

3.
The morphotectonic setting of the East Pacific Rise (EPR) between21°12 and 22°40 S and its recent and past hydrothermalactivity were the focus of the Russian R/V Geolog Fersmans expeditionin 1987–1988.The EPR axial zone in the study area is comprised of three segmentsseparated by overlapping spreading centers (OSCs) near 21°44 and22°08 S. The northern segment is the shallowest of three and hasa distinct massive axial ridge, trapeziodal in cross-section, toppedby a very wide flat summit surface and cut by a well-developedcentral graben. These features testify to intense magmatism and to avoluminous crustal magmatic chamber underlying the whole segment.Fine-scale segmentation is most clearly revealed in the structure ofthe central graben within which several 4th-order segments can bedistinguished. This scale of segmentation is also reflected on flanks of theaxis by variations in the character and intensity of faulting.According to structural and petrologic data, the magmatism is mostintense in the central part of the segment which is probably locateddirectly over a magmatic diapir supplying the melt to the whole segment.Magma migration at the subcrustal level from the center towards the ends ofthe segment with discrete injection into the crustal magmatic chamber ispresumed.The central segment is broken into two morphologically distinct partsseparated by a deval. In the subsided northern part, the wide summit of theaxial ridge is cut by a well-developed, intensely fractured axialgraben. In the southern part, the axial ridge is relatively elevated, butnarrow with an ephemeral graben along its crest. The character and intensityof faulting on the axial flanks are also considerably different in thenorthern and southern parts of the segment. Thus, the magmatic supply tothese two parts is thought to originate from two different sources. If so,then at present the magma chamber underlying the southern part of thesegment is probably at the stage of replenishment, while in the north it isat the stage of deep cooling.The southern segment is structurally similar to the central one. Howeverthere is considerably less intensive magmatic activity in this region,especially south of 22°30 S where the axial ridge is narrow, andtriangular in cross-section.Both OSCs studied are marked by abrupt narrowing and sharp subsidence ofthe tips of axial ridges within the northern limbs. The southern OSC limbsare morphologically similar to normal sections of axial ridges. In bothcases the flanks are structurally and morphologically disrupted adjacent tothe OSCs and oblique structures can be traced far southward of the OSCflanks. Due to the spatial position of oblique structures on the the flanksit is presumed that the OSC near 22°07 S is migrating northward.The 21°44 S OSC zone has apparently undergone small spatialoscillations. In spite of the small amplitude of lateral displacement, thiszone is marked by prominent bathymetric anomalies.Numerous massive sulfide deposits were discovered atop the axial ridgealong the entire length of the uplifted and hydrothermally active northernsegment. Ore metal concentrations in near-bottom waters are maximumover the southern part of the northern segment, while maximum concentrationsof the same metals in surficial sediments are confined to the central partof the same segment. We surmise that there has been a recentalong-axis shift of the zone of maximum hydrothermal activity fromthe middle of the segment to its present position in the southern part ofthe segment. Considering sedimentation rates, the age of this shift can beapproximately estimated to be 5 to 10 thousand years before the present.The relatively Mg-enriched basalts of the middle part of thenorthern segment represent a tike of a more primitive pattern, while therelatively Fe-rich rocks of its southern part probably reflect alarge degree of fractionation at shallow crustal levels. Considering thistrend, in addition to morphotectonic data we presume that subaxial magmaflow from the middle to the southern part of the segment is responsible forthe along-axis shift of hydrothermal activity.In the central segment of the study area, massive sulfides have only beendiscovered south of the 21°55 S deval, where the axial ridgeshoals and where the existence of a subjacent magma chamber is presumed.The very weak manifestations of recent volcanism within the southernsegment explain the absence of hydrothermal activity and sulfide depositswithin this segment.  相似文献   

4.
By analyzing a data set collected using a moored instrument array and CTD during TOGA-COARE, it is found that there exist remarkable internal tides in the western equatorial Pacific Ocean around 1°45′S,156°E, whose horizontal wavenumber (wavelength), vertical wavenumber, h 156° orizontal propagation speed and vertical propagation speed are 3.3×10-2km-1(210 km),-1.6×10-3m, 2.0 m/s and -3.8 cm/s, respectively, that is, the waveform propagates downwards slantingly. Moreover, the propagating direction rotates statistically clockwise as the depth increases and its cause is unclear.  相似文献   

5.
1Introduction DuringtheTOGA COARE,extensivejointin ternationaloceanicobservationswerecarriedoutin thewesternequatorialPacificOceanandavasta mountofvaluabledatawereobtained.Themaindata usedhereincludethedatafromasinglemooringin strumentarrayat1°45′S,156°E,deployedbythe WoodsHoleOceanographicInstitution(WHOI)and theCTD(profilerofconductivity-temperature-depth)datacollectedbytheChineseRVXiangy anghongNo.5.Fangetal.(2000)described brieflytheseobservationdataandanalyzedthedis persion…  相似文献   

6.
Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00N and 63°40N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.  相似文献   

7.
A widespread seamount province, the Rano Rahi Field, is located near the superfast spreading Southern East Pacific Rise (SEPR) between 15°–19° S. Particularly abundant volcanic edifices are found on Pacific Plate aged 0 to 6.5 Ma between 17°–19° S, an area greater than 100,000 km2. The numbers of seamounts and their volume are several times greater than those of a comparablysurveyed area near the Northern East Pacific Rise (NEPR), 8°–17° N. Most of the Rano Rahi seamounts belong to chains, which vary in length from 25 km to >240 km and which are very nearly collinear with the Pacific absolute and relative plate motion directions. Bends of 10°–15° occur along a few of the chains, and some adjacent chains converge or diverge slightly. Many seamount chains have fluctuations in volume along their length, and statistical tests suggest that some adjacent chains trade-off in volume. Several seamount chains split into two lines of volcanoes approaching the axis. In general, seamount chains composed of individual circular volcanoes are found near the axis; the chains consist of variably-overlapping edifices in the central part of the survey; to the west, volcanic ridges predominate. Near the SEPR, the volume of nearaxis seamount edifices is generally reduced near areas of deflated cross-sectional area of the axial ridge. Fresh lava flows, as imaged by sidescan sonar and sampled by dredging, exist around some seamounts throughout the entire survey area, in sharp contrast to the absence of fresh flows beyond 30 km from the NEPR. Also, the increases in seamount abundance and volume extend to much greater crustal ages than near the NEPR. Seamount magnetization analysis is also consistent with this wider zone of seamount growth, and it demonstrates the asynchronous formation of most of the seamount chains and volcanic ridges. The variety of observations of the SEPR seamounts suggests that a number of factors and mechanisms might bring about their formation, including the mantle upwelling associated with superfast spreading, off-axis mantle heterogeneities, miniplumes and local upwelling, and the vulnerability of the lithosphere to penetration by volumes of magma. In particular, we note the association of extensive, recent volcanism with intermediate wavelength gravity lineaments lows on crust aged 6 Ma. This suggests that the lineaments and some of the seamounts share a common cause which may be related to ridge-perpendicular asthenospheric convection and/or some manner of extension in the lithosphere.  相似文献   

8.
Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30 N.The two northernmost segments (38°20 N and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and 38°20 N segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.  相似文献   

9.
Sea Beam bathymetry and SeaMARC II side-scan sonar data are used to constrain the width of the zone of active faulting (plate boundary zone) to be 90 km (0.8 Ma) wide along the East Pacific Rise 8° 30N – 10° 00N. Fault scarps, identified on the basis of contoured, shaded relief and slope intensity maps of bathymetry, are measured. These scarp measurements, used in conjunction with data from a separate near-axis study, show that both inward- and outward-facing fault scarps increase in height away from the ridge axis, reaching average heights of 100 m at 0.8±0.2 Ma, 45±10 km from the ridge axis. Beyond this distance, there is no significant increase in scarp height. Earlier studies had suggested that the width of the zone of active faulting for outward-dipping faults might be significantly narrower than for inward-dipping faults. A lower crustal decoupling zone between brittle crust and strong upper mantle is predicted to exist out to 20–200 km from the ridge based on previously published lithospheric models. Such a decoupling zone may explain why outward-dipping faults continue to be active as far off-axis as inward-dipping faults. If the width of the zone of active faulting is controlled by the width of a lower crustal decoupling zone, our observations predict an 90 km wide decoupling zone in the lower oceanic crust at this location.  相似文献   

10.
We present magnetic field data collected over the Mid-Atlantic Ridge in the vicinity of the Atlantis Fracture Zone and extending out to 10 Ma-old lithosphere. We calculated a magnetization distribution which accounts for the observed magnetic field by performing a three-dimensional inversion in the presence of bathymetry. Our results show the well-developed pattern of magnetic reversals over our study area. We observe a sharp decay in magnetization from the axis out to older lithosphere and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is 10 Ma, we observe an abrupt increase in magnetic field intensity which could be due to an increase in the intensity of magnetization or thickness of the magnetic source layer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive layer and a deeper intrusive layer with sloping polarity boundaries can account for the increase in the amplitude of anomaly 5. South of the Atlantis Fracture Zone, high magnetization is correlated with bathymethic troughts at segment end points and lower magnetization is associated with bathymetric highs at segment midpoints. This pattern can be explained by a relative thinning of the magnetic source layer toward the midpoint of the segment. Thickening of the source layer at segment endpoints due to alteration of lower oceanic crust could also cause this pattern. Because we do not observe this pattern north of the fracture zone, we suggest it is a result of the nature of crustal formation process where mantle upwelling is focused. South of the fracture zone, reversals along discontinuity traces only continue to crust 2 Ma old. In crust >2 Ma, we observe bands of high, positive magnetization along discontinuity traces. We suggest that within the discontinuity traces, a high, induced component of magnetization is produced by serpentinized lower crust/upper mantle and this masks the contribution of basalts to the magnetic anomaly signal.  相似文献   

11.
pH and alkalinity measurements from a coastal upwelling area located near 30°S (Coquimbo, Chile), are used to describe the short-term variations of CO2 air–sea exchanges over a period of one week in summer 1996. A 180 km ocean–coastal transect, together with two almost-synoptic grid surveys off Coquimbo covering approximate 2500 km2 each, showed that during and immediately after a 4 day long southwesterly wind event (24–28 January) a large area of cold surface water (≈14°C), highly supersaturated in CO2 (fCO2 up to 900 μatm), was located near the coast. Three days after the end of the event, the second grid survey showed that in most of the study area the surface temperature and pH had increased significantly (by 1–3°C and 0.05–0.2, respectively), and that the surface water was no longer supersaturated in CO2. The CO2-supersaturated water observed in the first grid survey was identified as upwelled subsurface equatorial water, a water mass with its core at about 200 m depth: the depth from which the water upwells is a major determinant of the surface water fCO2. Integrated C fluxes within a 20 km wide coastal strip (1900 km2) indicate a strong outgassing of CO2 from the ocean under upwelling conditions (Grid 1; 121 t C day-1), while the net C exchange was directed to the ocean during the relaxation period (Grid 2; 19 t C day-1). Estimates of CO2 fluxes in upwelling areas based on surface water fCO2 measurements must therefore take into account these short-term variations: reliance on longer-term averages and interpolation will lead to erroneous results.  相似文献   

12.
根据中美海气ＴＯＧＡ调查第１～４航次（１９８６～１９８８年）的ＣＴＤ资料及部分ＡＤＣＤＣＰ资料，分析研究横跨赤道１４１°３０′Ｅ断面的内波及细结构特性，其中包括温度垂直梯度的概率分布及其谱特性，ＣＯＸ数的统计分布及Ｒｉｃｈａｒｄｓｏｎ数的垂直分布等，由此得出；该断面存在入侵性细结构、双跃层及阶梯结构；温度垂直梯度谱在波数０．１附近有一折点Ｋ１，按谱斜率不同将谱分为３段，当０．０３＜Ｋ＜Ｋ１时，β＜１；当Ｋ，＜Ｋ＜０．６时，β为－１．７～－３；当０．６＜Ｋ＜ｌ时，β为－３．５～－５。  相似文献   

13.
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14.
A study of Sea Beam bathymetry and SeaMARC II side-scan sonar allows us to make quantitative measures of the contribution of faulting to the creation of abyssal hill topography on the East Pacific Rise (EPR) 9°15 N–9°50 N. We conclude that fault locations and throws can be confidently determined with just Sea Beam and SeaMARC II based on a number of in situ observations made from the ALVIN submersible. A compilation of 1026 fault scarp locations and scarp height measurements shows systematic variations both parallel and perpendicular to the ridge axis. Outward-facing fault scarps (facing away from the ridge axis), begin to develop within 2 km of the ridge and reach their final average height of 60 m at 5–7 km. Beyond these distances, outward-dipping faults appear to be locked, although there is some indication of continued lengthening of outward-facing fault scarps out to the edge of the survey area. Inward-facing fault scarps (facing toward the ridge axis), initiate 2 km off axis and increase in height and length out to the edge of our data at 30 km, where the average height of inward fault scarps is 60–70 m and the length is 30 km. Continued slip on inward faults at a greater distance off axis is probable, but based on fault lengths, 80% of the lengthening of inward fault scarps occurs within 30 km of the axis (>95% for outward faults). Along-strike propagation and linkage of these faults are common. Outward-dipping faults accommodate more apparent horizontal strain than inward ones within 10 km of the ridge. The net horizontal extension due to faulting at greater distances is estimated as 4.2–4.3%, and inward and outward faults contribute comparably. Both inward- and outward-facing fault scarps increase in height from north to south in our study area in the direction of decreasing inferred magma supply. Average fault spacing is 2 km for both inward-dipping and outward-dipping faults. The azimuths of fault scarps document the direction of ridge spreading, but they are sensitive to local changes in least compressive stress direction near discontinuities. Both the ridge trend and fault scarp azimuths show a clockwise change in trend of 3–5° from 9°50 N to 9°15 N approaching the 9° N overlapping spreading center.  相似文献   

15.
This paper presents the first data on the vertical distribution of chlorophyll and nutrients in the upper layers of the southern southwestern Atlantic and interprets it in relation to frontal systems and the general hydrographic features. The survey covered quasi-synoptically the area between 30 and 62°S, at the beginning of the austral summers of three consecutive years (1993 through 1995). Our results show a rather consistent oceanographic structure, with similar patterns of chlorophyll and nutrient distributions with latitude. Outstanding features present throughout the three seasons are two zones of high phytoplankton production, where chlorophyll concentrations of >1 μg/l seem to occur frequently. The first is the upper euphotic zone of the Brazil–Malvinas Confluence Zone between approximately 36 and 50°S. The second is centered on subsurface layers of the Antarctic Surface Water south of the Polar Front as far as 62°S. The rest of the area covered shows low chlorophyll concentrations (<0.4 μg/l), due either to nutrient limitation in the oligotrophic subtropical waters of the Brazil Current or to excessive turbulence and upper-layer instability in the energetic Malvinas Current. The occurrence of a permanent meander around 38°S coincided with the cyclonic retroflexion of the Malvinas Current, causing upwelling of deep water. The ecological implications of this eddy-induced upwelling are discussed.  相似文献   

16.
《Oceanologica Acta》1999,22(3):249-263
Mean conditions, seasonal, and ENSO-related (El Niño Southern Oscillation) variability in the vicinity of Wallis, Futuna, and Samoa islands (13°–15° S, 180°–170° W) over the 1973–1995 period are analysed for wind pseudo-stress, satellite-derived and in situ precipitation, sea surface temperature (SST) and salinity (SSS), sea level, and 0–450 m temperature and geostrophic current. The mean local conditions reflect the presence of the large scale features such as the western Pacific warm pool, the South Pacific Convergence Zone (SPCZ), and the South Pacific anticyclonic gyre. The seasonal changes are closely related to the meridional migrations of the SPCZ, which passes twice a year over the region of study. During the warm phase of ENSO (El Niño), we generally observe saltier-than-average SSS (of the order of 0.4), consistent with a rainfall deficit (0.4 m yr−1), a hint of colder-than-average surface temperature is also identified in subsurface (0.3°C), a weak tendency for westward geostrophic current anomalies (2 cm s−1 at the surface), a sea level decrease (5–10 cm), together with easterly (5 m2s−2) and well marked southerly (10 m2s−2) wind pseudo-stress anomalies. Anomalies of similar magnitude, but of opposite sign, are detected during the cold phase of ENSO (La Niña). While these ENSO-related changes apply prior to the 1990s, they were not observed during the 1991–1994 period, which appears atypical.  相似文献   

17.
The distribution of pH and alkalinity has been used to calculate the distribution of total inorganic carbon (TC) and fugacity of carbon dioxide (fCO2) in the upper 200 m of the water column in coastal upwelling areas off northern Chile (23–24°S, near Antofagasta) and central Chile (30–31°S, near Coquimbo) during austral summer 1997. In these upwelling areas, colder surface waters were oxygen poor and strongly CO2 supersaturated (100% near Antofagasta and 200% near Coquimbo), although below the pycnocline the CO2 supersaturation invariably exceeded 200% in both areas. The larger surface CO2 supersaturation and outgassing at 30°S were associated with stronger winds that promoted the upwelling of denser water (richer in CO2) as well as a higher air–sea CO2 transfer velocity. The consistent decrease in intensity of the southerly winds (as derived from NSCAT scatterometer data) from 30–31°S to 23–24°S suggests a corresponding decline in the intensity of the CO2 outgassing due to upwelling. Additionally, we suggest here that the intensity of the local upwelling forcing (i.e. alongshore–equatorward winds) plays a role in determining the water mass composition and phytoplankton biomass of the coastal waters. Thus, while deep upwelling of salty and cold water resulted in high fCO2 (up to 1000 μatm) and very low phytoplankton biomass (chlorophyll a concentration lower than 0.5 mg m−3), the shallow upwelling of less salty (e.g. salinity <34.5) and less CO2-supersaturated water resulted in a higher phytoplankton biomass, which further reduced surface water fCO2 by photosynthesis.  相似文献   

18.
Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08N on the Mid-Atlantic Ridge (MAR). The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone. The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2.5 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples. Two of these mounds were previously unidentified. The existence of these mounds extends the length of the Alvin zone by ~0.5 km to the south. Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley. The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned. The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis.  相似文献   

19.
Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00 N and 30°40 N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points.  相似文献   

20.
Variations of water properties in surface and intermediate layers along 32°S in the southern Indian Ocean were examined using a 50-year (1960–2010) time series reproduced from historical hydrographic and Argo data by using optimum interpolation. Salinity in the 26.7–27.3σθ density layer decreased significantly over the whole section, at a maximum rate of 0.02 decade−1 at 26.8–26.9σθ, for the 50-year average. Three deoxygenating cores were identified east of 75°E, and the increasing rate of apparent oxygen utilization in the most prominent core (26.9–27.0σθ) exceeded 0.05 ml l−1 decade−1. The pycnostad core of Subantarctic Mode Water (SAMW) and the salinity minimum of Antarctic Intermediate Water shifted slightly toward the lighter layers. Comparisons with trans-Indian Ocean survey data from 1936 suggest that the tendencies found in the time series began before 1960. Interestingly, cores of many prominent trends were located just offshore of Australia at 26.7–27.0σθ, which is in the SAMW density range. Spectrum analysis revealed that two oscillation components with time scales of about 40 and 10 years were dominant in the subsurface layers. Our results are fairly consistent with, and thus support, the oceanic responses in the southern Indian Ocean to anthropogenic climate change predicted by model studies.  相似文献   

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