摘要:

Data for numbers of seamounts on North Pacific Ocean crust of different age indicate that the production rate of new off‐ridge seamounts (volcanoes) is proportional to the inverse of the square root of the age of the lithosphere. This observation is consistent with several hypotheses which have been offered to explain the origin of off‐ridge oceanic volcanism and, in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes, leads to a new self‐consistent model for off‐ridge volcanism. In this model, the production rate of off‐ridge volcanoes is controlled primarily by the availability of fracture‐zone conduit systems in the thickening lithosphere. In contrast, mantle plume or "hotspot" volcanoes may punch through the oceanic lithosphere. Decreasing production rate of off‐ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous ma

ISSN:0094-8276    

DOI:10.1029/GL008i008p00853

年代:1981

数据来源: WILEY

摘要:

The observed quasi‐rigid behaviour of surface plates in the course of their relative motion is a consequence of the high viscosity which obtains in the cold near surface region. By assigning a particular constant velocity as the upper boundary condition in a numerical model of mantle convection, we have investigated the effect of lithospheric rigidity on the variation of oceanic bathymetry and heat flow as a function of ocean floor age. Predicted variations of both bathymetry and heat flow at the surface of mantle wide convection cells which are partially heated from within, exhibit the same qualitative behaviour as data compiled for the major ocean basins. The bathymetry varies with distance x from the spreading centre initially as x½but subsequently flattens with respect to an x½reference curve whereas the heat flow closely follows an x−½decay over most of the convection cell. Consequently the viability of the mechanism for sea floor flattening proposed by Jarvis and Peltier (1980) is increased when a constant surface velocity, characteristic of rigid plates, is incorporated in the model. This model successfully predicts both that the bathymetry should flattenandthat the heat flow shou

ISSN:0094-8276    

DOI:10.1029/GL008i008p00857

年代:1981

数据来源: WILEY

摘要:

SS phases from earthquakes on fracture zones near the Easter Island Cordillera and the West Chile Rise which are recorded in the United States have reflection points on either side of the East Pacific Rise (EPR) near the equator. The east‐west records from seven WWSSN stations of seven events in this region were used to obtain spectral amplitudes of horizontally polarized S and SS waves. SS‐to‐S amplitude ratios were formed, and differential attenuation (δt*) computed within the frequency band 0.01 to 0.11 Hz. The values of δt* vary between −0.1 sec and +35.8 sec for the 23 station‐event pairs used. However, the change in δt* with distance from the axis of the EPR does not reflect the smooth variation expected using a model of a simple

ISSN:0094-8276    

DOI:10.1029/GL008i008p00861

年代:1981

数据来源: WILEY

摘要:

Seismic anisotropy in the upper 1500m. of oceanic basement has been observed by polarization analysis of three‐component borehole seismometer records. The most convincing evidence for the anisotropy is shear wave splitting for explosive sources at four azimuths. Compressional wave particle motion deviations suggestive of anisotropy are also observed but they may be caused by lateral inhomogeneities. The anisotropy was not resolved by travel‐time analysis. The observed velocities and particle motions in the horizontal plane can be modelled to within a standard deviation by assuming a perfectly elastic, homogeneous, anisotropic layer 2 with hexagonal symmetry and a horizontal symmetry axis. The most probable cause of the anisotropy is preferred crack orientat

ISSN:0094-8276    

DOI:10.1029/GL008i008p00865

年代:1981

数据来源: WILEY

摘要:

Seismic data can be transformed to the domain of intercept time and ray parameter without the artifical suppression of high frequencies caused by finite temporal and spatial sampling. These data can be analyzed to derive velocity‐depth information using the loci of the arrivals in the τ‐p plane. For limited source‐receiver offsets, the parameters of the best single ellipse approximating the true τ‐p reflection trajectory can be used to derive an approximate velocity‐depth function. We show that this is equivalent to Dix's (1955) hyperbolic traveltime approximation. When large aspect ratios are involved, the path formed by the postcritical reflection and refraction arrivals can be used directly to derive the velocity function. We consider both of these inversion methods as the initial step required to implement an inversion procedure that utilizes both pre‐ and postcritical reflection and refraction arrivals by an exact "normal moveout" correction in τ‐p. This approach is computationally efficient and is the equivalent to a ray traced normal moveout c

ISSN:0094-8276    

DOI:10.1029/GL008i008p00869

年代:1981

数据来源: WILEY

摘要:

Bottom waters from five hydrographic stations, spaced equally along a 400‐km section of the Juan de Fuca Ridge north of the Blanco Fracture Zone, contain anomalously high concentrations of total dissolvable manganese (TDM). The two bottom‐most bottles from each hydrocast (∼ 100 to 200 m above bottom) range from 8.3 to 108.9 nmol kg−1with a median value of 12.0 nmol kg−1. This median is nearly a factor of 10 higher than similar near bottom samples obtained from two off‐axis locations. Analogy to work on the East Pacific Rise implies that hydrothermal activity along the ridge axis is contributing substantially to the manganese content of the water column in this portion of the northeast

ISSN:0094-8276    

DOI:10.1029/GL008i008p00873

年代:1981

数据来源: WILEY

摘要:

The free‐air gravity anomaly observed over the Darfur Swell is explainable by local isostatic balance with a root approximately 50 km deep on average. This root depth is similar to that inferred beneath other African domes and beneath oceanic midplate swells, suggesting that the Darfur Swell is a hotspot uplift created by lithospheric reheatin

ISSN:0094-8276    

DOI:10.1029/GL008i008p00877

年代:1981

数据来源: WILEY

摘要:

The Neda Iron Formation (latest Ordovician) is a hematite and goethite rich oolitic ore which occurs in lens shaped deposits on top of the Maquoketa shale (Late Ordovician), at only a few locations in Wisconsin, Iowa and Illinois. Thermal demagnetization of samples from these three states, indicates the remanence is carried by hematite. Chemical demagnetization suggests the remanence is produced by the interstitial material rather than the oolites.The paleopole averaged for all sites from 25 samples of the Neda ore is at S45.4°, W48° (α95 = 16°). This pole position is similar to late Mississippian to early Permian of North America rather than latest Ordovician. The difference between the known age of the Neda and the time of magnetization suggests that the hematite in the ore was produced from dehydration of goethite probably as a result of tectonic uplift in Wisconsin during late Mississippian time. In addition, these results add evidence to the theory that the Neda ore was formed in two stages. The oolites were produced in a near shore environment during the shoaling of the Ordovician seas. The oolites were then incorporated in iron rich muds. Burial and later uplift dehydrated the iron hydroxide to hemat

ISSN:0094-8276    

DOI:10.1029/GL008i008p00880

年代:1981

数据来源: WILEY

摘要:

The Barton garnet deposit contains large garnet crystals (2.5 ‐ 25 cm) disseminated through a gabbro‐derived garnet amphibolite. These crystals are characterized by two sets of parting planes which intersect at about 80° and have a constant attitude throughout the amphibolite. The results of X‐ray diffraction studies on the parting planes of the garnet crystals by the back‐reflection Laue method and by X‐ray diffractometry show that the parting‐plane sets are not crystallographically controlled by the lattice of the garnet and that the garnets are not oriented in the amphibolite. Each parting set in the giant garnets and its corresponding parallel joint set in the amphibolite matrix are interpreted as brittle fractures which developed in response to a discrete tect

ISSN:0094-8276    

DOI:10.1029/GL008i008p00883

年代:1981

数据来源: WILEY

摘要:

The concept of apparent fracture energy for the shear failure process is employed by many authors in modeling earthquake sources as dynamically extending shear cracks. Using records of shear strain and relative displacement from stick‐slip events generated along a simulated, prepared fault surface in a large (1.5m × 1.5m × 0.4m) granite block and a slip‐weakening model for the fault, direct estimates of the apparent shear fracture energy of the stick‐slip events have been obtained. For events generated on a finely ground fault surface, apparent fracture energy ranges from 0.06 J/m² at a normal stress of 1.1 MPa to 0.8 J/m² at a normal stress of 4.6 MPa. In contrast to estimates for tensile crack formation, we find that the apparent fracture energy of stick‐slip events increases linearly with normal stress. The results for the slip‐weakening model for the stick‐slip events are generally consistent with constitutive fault models suggested by observations of stable sliding in smaller sc

ISSN:0094-8276    

DOI:10.1029/GL008i008p00887

年代:1981

数据来源: WILEY