Measurements of Relative Motion Between Ocean Ridges and Its Analysis
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This paper develops new absolute models of plate motion relative to ocean ridges: PRF-2000VEL model with respect to the fixed Pacific Ridge, SRF-2000VEL model with respect to Southern mid-Atlantic ridge and NRF-I2000VEL model with respect to Northern mid-Atlantic ridge, analyze and compare the absolute motions of lithospheres relative to Pacific Ridge, Southern mid\|Atlantic and Northern mid-Atlantic Ridge, respectively, which shows the Eastern Pacific mid-ridge and the mid-Atlantic Ridge is extending at the even 10.9mm/a rate.Keywords:
Mid-Atlantic Ridge
Ridge push
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One hundred and seventy-nine Lamont Geological Observatory heat-flow measurements in the Atlantic Ocean and the Caribbean Sea are presented; their reliability is carefully estimated. Together with 197 other measurements, they are used to describe the broad regional pattern of heat flow in the Atlantic Ocean. The average heat flow over the mid-Atlantic ridge is within 20% of the heat flow of the basins, and the absence of a wide heat-flow maximum in the observed values precludes the possibility of continuous continental drift during the Ceno-zoic by the spreading-floor mechanism in the Atlantic Ocean. In contrast, the excess of heat flow measured over the East Pacific rise is consistent with the existence of large convective transfer of heat in the underlying mantle.
Seabed
Heat flow
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Non–hot spot, intraplate volcanism is a common feature near the East Pacific Rise or Pacific‐Antarctic ridge. Volcanic ridges and seamount chains, tens to hundreds of kilometers long, are asymmetrically distributed about the ridge axis, with most volcanic features occurring on the Pacific plate. Their origins remain controversial. We have analyzed off‐axis volcanic ridges near the Pacific‐Antarctic ridge from bathymetry, backscatter, gravity, and geochemistry data of the Pacantarctic 2 cruise. K/Ar dating of samples dredged on these structures reveals a contrast of up to 3 Ma between the volcanoes and the underlying crust. The volcanic activity, as suggested by the strong backscatter in sonar images, appears to be limited to areas of seafloor younger than about 3 Ma. All surveyed ridges north of the Menard transform fault (TF) show recent activity close to the ridge axis and are not affected by faults. The off‐axis volcanic ridges south of the Menard TF show recent volcanic flows in their center and are affected by N‐S extension. Two different types of volcanoes can be characterized: conical, flat‐topped ones and rough, elongated ones associated with narrow, E‐W trending volcanic ridges, some of them showing strong backscattering on the EM12 imagery. From the morphology of the seamount chains and the ages of the lava samples, we infer that the magma source for the volcanic ridges is related to the feeding of the ridge axis through three‐dimensional mantle convective circulation. We suggest that a change in the relative plate motion since about 5 Ma might have induced an offset of the mantle upwelling circulation under the ridge axis so that anomalously hot mantle rises under the Pacific ridge flank. The kinematic change is also likely responsible for the tectonic deformation in the young lithosphere south of Menard TF.
Seamount
Seafloor Spreading
Pacific Plate
Submarine volcano
Volcanic cone
Transform fault
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Mid-Atlantic Ridge
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Seafloor Spreading
Mid-Atlantic Ridge
Rift valley
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Rift valley
Mid-Atlantic Ridge
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Seismic refraction
Crest
Oceanic basin
Seismometer
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In this study we estimate the statistical properties of abyssal hill morphology adjacent to the Southeast Indian Ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate. We explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate. Two cruises aboard the R/V Melville collected Sea Beam 2000 multibeam data along the Southeast Indian Ridge, providing continuous multibeam coverage of the axis from ∼89°W to ∼118°W, and ∼100% coverage within four survey regions extending out to ∼45 km (∼1.2 Ma) from the axis [ Sempéré et al., 1997; Cochran et al ., 1997]. We apply the statistical modeling method of Goff and Jordan [1988] to gridded data from the four survey areas, examining in particular estimates of abyssal hill rms height, characteristic width and length, aspect ratio, and skewness. Two analyses are performed: (1) comparison of the along‐axis variation in abyssal hill characteristics to ridge segmentation, and (2) a calculation of population statistics within axial high, intermediate, and axial valley data populations of this study, and comparison of these results to population statistics derived from studies adjacent to the Mid‐Atlantic Ridge and East Pacific Rise. We find that abyssal hills generated along axial high mid‐ocean ridges are very different from those generated along axial valley mid‐ocean ridges, not only with respect to size and shape, but also in their response to such factors as spreading rate and segmentation.
Seafloor Spreading
Abyssal zone
Abyssal plain
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Bowers ridge, which is submerged in the deep-water part of the Bering Sea, has the geophysical and structural characteristics of an island arc-trench system. Three crustal structure sections of the strongly curved aseismic Bowers ridge based on seismic, magnetic, and gravity data, indicate that Bowers ridge is a volcanic ridge characterized by large-amplitude, short-wavelength magnetic anomalies and bordered on its convex side by a sediment-filled trench. Positive gravity anomalies greater than 200 mgal are associated with Bowers ridge; negative anomalies with amplitudes greater than 100 mgal are associated with the bordering trench. The geophysical and structural characteristics of the linear, also entirely submerged, Shirshov ridge in the Bering Sea differ significantly from the characteristics of Bowers ridge. Over Shirshov ridge the gravity anomalies are of small amplitude and there is no geophysical evidence of a buried trench. A straight NE-SW trending chain of elongated seamounts, possibly located on an old transform fault zone, connects Bowers ridge and Shirshov ridge and may indicate the direction of late Mesozoic-earliest Tertiary subduction from the northeast, which would have led to the construction of the Bowers arc-trench system.
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Asthenosphere
Seafloor Spreading
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A method is developed to quantify the relationship between the ridge axial topography and gravity and the spreading rate along the Mid‐Atlantic Ridge between 22 and 38°N. This relationship reflects the variations of slope of the best‐fit line of topography and gravity spectra with the spreading rate of the ridge segments. The slope of the best‐fit line of topography spectrum becomes smaller as the spreading rate increases, indicating that with increasing spreading rate more energy of the ridge axial topography shifts into high‐frequency bands. The spreading rate dependence of the ridge axial topography may be explained by an anomalous thermal structure beneath the ridge. No significant correlation was found between the slope of the best‐fit line of gravity spectrum and the spreading rate in this region. The lack of spreading rate dependence of the ridge axial gravity may be attributable to the isostatic compensation of the spreading center.
Ridge push
Line (geometry)
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