摘要:

This special section is the outcome of a symposium held at the Lamont‐Doherty Geological Observatory November 17–19, 1982, on the origin and evolution of seamounts. The topic for the symposium arose from the realization that although there is now a wealth of new ideas on the geology, geophysics, and geochemistry of the ocean floor, the study of seamounts has been relatively neglected despite their great importance to plate tectonics.One of the most interesting features of the ocean floor is the large number of small volcanoes or seamounts. Usually, these features are circular in plan view and have a sharp summit. There has been extensive debate in the literature about the significance of the different morphological types of seamounts. A traditional view has been that flattopped seamounts (e.g., guyots) formed as a result of subaerial erosion when the volcanoes were above sea level. Seamounts covered by fringing reefs or sediments (e.g., atolls), on the other hand, are believed to have formed when the original volcanic foundation subsided below sea le

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11066

年代:1984

数据来源: WILEY

摘要:

A northwest trending group of seamounts located just west of the East Pacific Rise (EPR) axis has been mapped using high‐resolution Sea MARC I side‐scan sonar and Sea Beam multibeam bathymetry. These seamounts have 9–13 km basal diameters, and the shapes of the volcanos change from conical domes near the EPR axis to truncated cones and larger, elliptical truncated cones farther away from the rise axis. The detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions (craters to large complex calderas) and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow. Our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse. The creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera. The concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau. The steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera. The steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain. As a seamount evolves, the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits. The final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit pl

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11069

年代:1984

数据来源: WILEY

摘要:

Multibeam bathymetric charts (100 fm = 183 m contour interval) are presented and analyzed for the ∼1100‐km‐long, 120° trending Geisha chain of eight guyots and at least 13 other seamounts (>1 km height) in the northwest Pacific; these guyots are compared with 23 others in the North Pacific. Edifice distribution is nonrandom and nonuniform and is best described as clusters (200 km apart), some of which are composed of subclusters (30–50 km apart) and edifices 10–20 km from their neighbors. Published radiometric dates from the two ends of the Geisha chain (94 Ma for Makarov in the southeast and 102 Ma for Seiko in the northwest) are consistent with rapid (14 cm/yr) plate motion over a fixed hotspot, but other mechanisms cannot be excluded. Crustal age (from magnetic lineations) increases south westward from 141 to 156 Ma along the chain, i.e., from 39 to 62 Ma at times of seamount volcanism. Total edifice volume is small (2×104km3), with Makarov guyot at 6000 km3and the other guyots 700–1500 km3each. Summit plateau depths (averaging 1420±60 m minimum depth and 1600±120 m break depth) are remarkably uniform along the chain; plateau areas (87±68 km2, ranging from 14 to 246 km2) are the smallest for any guyot chain. Original island heights (0.5–1.3 km) and minimum volumes eroded (2–100 km3) are estimated from the height/area relation of modern volcanic islands. Summit plateau relief (∼100–200 m) is explained by simultaneous shoreline erosion (∼1 km/Ma) and subsidence of thermally rejuvenated lithosphere. There is no bathymetric evidence for guyot volcanism or faulting postdating subsidence below wave base. The Geisha guyots rise 4.5–4.9 km above the regional basement, implying a hotspot‐generated swell height of 0.5–1 km and lithosphere thermally reset to about 45% of its age at time of volcanism. Average upper guyot slopes (21°±3°) exceed those of other guyots (13°+3°) but resemble small seamount slopes, suggesting that flank slopes decrease, above about 5000 km3edifice volume, due to higher effusive rates and larger flows. The Geisha guyots exhibit prominent but short radial protuberances interpreted as flank rift zones (FRZ). A relation between base‐edifice differential magma pressure and FRZ length suggests three regimes: FRZ's are absent on seamounts<2 km in height, increase slowly (0.2 km/MPa) for large seamounts and small guyots like the Geishas, and above 100 MPa increase rapidly (3 km/MPa) with large scatter. FRZ's exceeding 100–150 km length on large edifices suggest hot, low‐viscosity rift conduits along which

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11085

年代:1984

数据来源: WILEY

摘要:

Digital marine bathymetric data are rapidly increasing in both volume and variety. These data provide new perspectives on the morphology of the seafloor. They also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data. We have used digital image processing techniques to reduce and display Seabeam multibeam data for three young (less than 5 Ma) seamounts on the eastern flank of the East Pacific Rise. Navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track. Images of the seamounts were then generated using spatial filtering techniques. The images reveal more information than contour maps because (1) more depth intervals can be displayed, and (2) the continuous tone nature of the images shown in shaded relief or color‐coded form more readily conveys morphologic information. The images suggest a significant degree of structural control on seamount shape, more so than can be delineated by examination of contour maps alone. In one case, a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction, although each seamount within the chain is elongate in a direction parallel to the hills. In another case, a polygonally shaped, flat‐topped seamount overlies a region of the crust where the abyssal hills display two strike directions. Images of the third seamount demonstrate a complicated structural control on morphology, including a control on the distribution of satellite constructs. Finally, the image presentations clearly depict regions where cross tracks could not be registered because of (1) residual navigation errors and (2) the presence of steep slopes, where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the sl

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11108

年代:1984

数据来源: WILEY

摘要:

Thermal subsidence would have drowned ancient volcanic islands (DAVI). Ancient volcanoes, if active at present rates in the main ocean basins, would have produced 1.5–3 times the observed number of DAVI. Shallow‐water indicators have been collected from 11% of guyots, and another 3% appear to be drowned barrier reefs or atolls on reflection profiles. A minimum of 65% of DAVI lie downdrift from hot spots that generate volcanic islands. The DAVI have the same 3 to 4‐km relief as the islands would have if truncated. Nonetheless hypotheses of a constructional origin of guyots continue to be proposed. Relative to these hypotheses are the following: (1) all four large, active submarine volcanoes that have been surveyed lack flat tops, and (2) many small, deep, inactive, relatively flat‐topped volcanoes are constructional forms but none with more than 2 km of relief are known to exist. Thus it is probable that all guyots with more than 2 km of relief are DAVI, but the possible existence of larger constructional forms is not excluded. There is no evidence that guyots large enough to be of interest in studying the epeirogenic history of midplate swells are n

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11117

年代:1984

数据来源: WILEY

摘要:

We have extracted the intermediate‐wavelength magnetic anomaly field from the marine survey data of the central Pacific in the band pass of 4000–400 km (spherical harmonic degree 10–100). Our technique minimizes the effects of external field sources, secular variation, and strike aliasing. The objective of this paper is to demonstrate this capability, compare the derived sea surface field to the equivalent MAGSAT data set, and demonstrate that anomalies observed in both fields are correctable to geologic features within the oceanic lithosphere. As expected, the sea surface field exhibits a strong spatial correlation to and a better resolution than the MAGSAT field. However, the MAGSAT field also displays a diminished resolution for wavelengths less than 1900 km when compared to the upward continued sea surface data. Two reasons are likely for this diminished resolution: (1) differences in the resolution of the spherical harmonic reference fields and (2) an increased noise level in the MAGSAT data at the shorter wavelengths. Other sources of error are also considered. We also demonstrate that the remanent magnetization of the central Pacific seamounts produces negative magnetic anomalies which are observed at satellite altitude. Other sources including the thickened crust of the oceanic plateaus and regional petrologic variations can be associated with intermediate‐wavelength anomalies observed in both the MAGSAT and the sea surface

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11124

年代:1984

数据来源: WILEY

摘要:

Seamount magnetic anomaly studies have been conducted on eight volcanoes in the Line Islands seamount chain. The magnetic anomalies of these seamounts were inverted to obtain paleomagnetic information. Four of these gave paleomagnetic pole positions consistent with other Pacific Cretaceous pole positions. Of these, three have reliable radiometric ages ranging from 72 to 85 Ma. The remaining four seamounts gave geomagnetic poles that agree with other Eocene‐Early Oligocene Pacific paleomagnetic data, and one has a reliable radiometric age of 39 Ma. All of the apparently Eocene volcanoes occur in the central Line Islands between 8°N and 3°S. We suggest that the Eocene volcanic event was approximately coincident with the profound change in Pacific plate motion, recorded by the bend in the Hawaiian‐Emperor seamount chain, that occurred at approximately 43 Ma. This volcanism may have resulted from the large change in intraplate stress accompanying the change of plate motion. The change in stress might have pulled apart a zone of weakness caused by the original formation of the Line Islands. Detailed paleomagnetic modeling of the Line Islands seamounts was necessary to obtain good results, as seven of the eight seamounts appear to be inhomogeneously magnetized to some extent. Despite these inhomogeneities the high goodness‐of‐fit parameters and the excellent agreement of these results with other Pacific paleomagnetic data suggest that the paleopoles derived from these seamounts are reliably d

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11135

年代:1984

数据来源: WILEY

摘要:

The sea surface height has now been mapped to an accuracy of better than ±1 m by using radar altimeters on board orbiting satellites. The major influence on the mean sea surface height is the marine geoid which is an equipotential surface. We have carried out preliminary studies of how oceanic volcanoes, which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges, contribute to the marine geoid. Simple one‐ and two‐dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum. Previous studies based on gravity and bathymetry data and uplift/subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thicknessTeincrease with the age of the lithosphere at the time of loading. The models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid‐ocean ridge crest will be associated with relatively low amplitude geoid anomalies (about 0.4–0.5 m/km of height), while seamounts formed on relatively old lithosphere, on ridge flanks, will be associated with much higher amplitude anomalies (1.4–1.5 m/km). Studies of the Seasat altimetric geoid prepared by NASA's Jet Propulsion Laboratory support these model predictions; geoid amplitudes are relatively low over the Mid‐Pacific Mountains and Line Islands, which formed on or near a mid‐ocean ridge crest, and relatively high over the Magellan Seamounts and Wake Guyots, which formed off ridge. Direct modeling of the altimetric geoid over these features is complicated, however, by the wide spacing of the satellite tracks (which can exceed 100 km) and poor bathymetric control beneath individual satellite tracks. In regions where multibeam bathymetric surveys are available, models can be constructed that fit the altimetric geoid to better than ±1 m. Studies of geoid anomalies over the Emperor seamount chain, for example, suggest that these seamounts formed on 20–30 Ma old oceanic lithosphere, while anomalies over the Louisville ridge and the Hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 Ma. This indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting. We examine the implications of these results to studies which attempt (1) to separate temporal variations of the sea surface due to oceanographic effects, (2) to predict seamounts directly from satellite‐derived gravity and geoid data, and (3) to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due t

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11152

年代:1984

数据来源: WILEY

摘要:

Geoid heights over the Louisville Ridge seamount chain, deduced from the Seasat satellite altimeter data, have been analyzed and interpreted. Two uncharted seamounts belonging to the chain have been detected. The elastic thickness of the oceanic lithosphere has been determined along the chain. This was done rigorously using the three‐dimensional geoid and bathymetry maps and was also computed approximately using spectral methods on geoid and bathymetry profiles. A slightly increasing elastic thickness from the southeast to the northwest, in the range 15.5–22.5 km has been estima

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11171

年代:1984

数据来源: WILEY

摘要:

Plots of effective elastic thickness, Te, of the oceanic lithosphere as a function of age when elastically deformed show a general increase in Tewith the square root of age, although there is much scatter. Some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks, which makes Tefor more sharply bent plates underestimate the true depth Tmto the rheological boundary at the base of the high strength mechanical lithosphere. After converting effective values Teto true thicknesses Tm, it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones. From several other lines of evidence, such as depth anomalies, seamount subsidence, and heat flow data, it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere. The reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature. This dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere. Since the mechanical plate thickness depends principally upon the temperature at one depth, specifically the depth at which the temperature reaches 550–600°C (depending on strain rate), flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating, compared to quantities dependent on depth integrals of temperature, such as swell height and subsidence. The thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first ∼10 Ma after loading if initially reheating is confined to depths close to and below the 550°C isotherm. The flexural data combined with depth anomaly information for Hawaii point to extremely rapid reheating localized in the lower lithosphere. Rejuvenation in the Society Islands, on the other hand, is compatible with slower methods of heat injection involving the entire lithospheric c

ISSN:0148-0227    

DOI:10.1029/JB089iB13p11180

年代:1984

数据来源: WILEY