摘要:

Magnetic, bathymetric, and seismic reflection data obtained aboard the M.V.RSAduring the past 5 years provide new evidence for the evolution of the Mozambique basin in the southwest Indian Ocean. A major fracture zone, here identified with the Prince Edward fracture zone, has been traced over 2000 km from its active transform section across the southwest Indian Ocean ridge (SWIOR). Clearly lineated magnetic anomalies have been mapped and identified. The ridge itself, in the vicinity of the Prince Edward Islands, has been a spreading center since at least Cretaceous times. Spreading rates have varied considerably with two discontinuities which may be related to cessations in spreading or ridge axis jumps. A late Cretaceous change in spreading direction has resulted in opening (leaking) of transform faults and is the apparent cause of the complicated ridge axis morphology in this part of the Indian Ocean. Magnetic anomalies mapped within the Prince Edward fracture zone provide unique information on the process of adjustment to a spreading direction change. Fracture zone and magnetic anomaly trends in the Mozambique basin are different from those reported in the Madagascar basin and thus provide evidence for a plate boundary having existed between them.

ISSN:0148-0227    

DOI:10.1029/JB081i029p05221

年代:1976

数据来源: WILEY

摘要:

Thirteen new gravity profiles, 16,000 km in total extent, cover all the major tectonic regions of Melanesia. The isostatic balance of individual features is judged from comparison of free air to Bouguer anomalies. Isostatic anomalies are also shown for two representative profiles. Topography is the key factor producing free air anomalies over the trenches and adjacent ridges despite variations in tectonics. The Manus, South Mussau, and Vityaz trenches along the northern border of Melanesia are less deep than the Tonga and New Hebrides trenches and accordingly show smaller amplitude free air anomaly minima. Long‐wavelength free air highs south of the New Hebrides Trench, over Solomon Sea, and probably west of North Mussau Trench, support the idea of lithospheric uparching suggested by A. B. Watts and M. Talwani. A large amount of our data delineates predominantly positive free air anomalies over the Bismarck and Fiji plateaus, the result being consistent with earlier measurements. A representative profile across the Lord Howe Rise indicates a significant correlation of free air and isostatic anomalies with topography. Gravity anomalies over some local topographic features are discusse

ISSN:0148-0227    

DOI:10.1029/JB081i029p05240

年代:1976

数据来源: WILEY

摘要:

The Cape Verde Islands are emerged portions of a Mesozoic‐Cenozoic volcanic accretion in the form of a westward‐opening horseshoe along fracture zones converging from the mid‐Atlantic ridge toward Africa. An interior abyssal plain slopes westward, increasing in depth from 2.7 to 4.5 km. The plain is underlain by low relief on acoustic basement that is associated with a 300‐gamma negative magnetic anomaly. The flanks of the Sal‐Maio ridge appear bounded by large‐displacement normal faults; superficial slumping is common. The trends of magnetic anomalies are linear N‐S north of the islands and less linear within the islands and may change coincident with E‐W bathymetric trends south of the islands. A triangular pattern of reversed refraction lines 200–250 km long along the north and east ridges and NW‐SE across the interior abyssal plain indicated 2–3 km of semiconsolidated sediments underlain by 3–6 km of basalt and 6–8 km of plutonic rocks. The depth of the Moho is between 16 and 17 km. A deep NW‐SE trending fault intersects the Sal‐Maio ridge near Boa Vista. The consistent depth to Moho and the regional Bouguer anomaly indicate lack of local relief at the base of the crust. The crustal load of the entire archi

ISSN:0148-0227    

DOI:10.1029/JB081i029p05249

年代:1976

数据来源: WILEY

摘要:

Seismic refraction measurements show that in the western part of the Bering Sea (the Kamchatka or Komandorski basin) the sediment is thinner than in the eastern basins of the Bering Sea but much thicker than the Pacific basin average. The basement velocity (V= 5.5 km/s) is within the normal range for oceanic stations, and the basement is thicker than normal (3.1 km). The principal crustal layer (V= 6.8 km/s) is thin (2.6 km), and the mantle velocity (V= 8.0 km/s) is low. A small anisotropy of mantle velocity is observed when the high‐velocity direction is nearly east‐west, perpendicular to the trend of the magnetic anomalies. East‐west spreading is indicated by both the anisotropy measurements, which measure lineations in the uppermost mantle, and the magnetic measurements, which measure lineations in the basement. If there has been extension in this marginal basin, the extension has been parallel to the bordering trench rather than perpendicular as in other marginal b

ISSN:0148-0227    

DOI:10.1029/JB081i029p05260

年代:1976

数据来源: WILEY

摘要:

Richter and Parsons (1975) in their two‐scale model of mantle convection predict that if the depth of the convective layer is about 600 km, then for a plate moving at 10 cm yr−1, longitudinal convective rolls will be produced in about 50 m.y., and the strike of these rolls indicates the direction of motion of the plate relative to the upper mantle. In the case of the Pacific plate there should then exist a series of longitudinal convective rolls probably striking WNW. These predictive features of two‐scale mantle convection are tested by examining a new global free air gravity model which is complete to the thirtieth degree and order. To isolate only those anomalies with wavelengths of probable interest, the low degree and order field (degree of the fieldn≤ 12) is subtracted from a field complete to the twenty‐second degree and order. The resulting free air gravity map shows a series of linear positive and negative anomalies spanning the Pacific Ocean. Anomalies of this type are found only in the Pacific Ocean area. These anomalies cross the east Pacific rise and strike parallel to the Hawaiian seamounts. Their transverse wavelength is about 2000 km. Visual correlations between residual depth anomalies and free air gravity in the central Pacific and along the east Pacific rise are fair. We suggest that the long linear pattern of free air gravity anomalies may indicate the presence of longitudinal convective rolls beneath the Pacific plates. In order for these rolls to have been developed in the course of 43 m.y., the age of the bend in the Hawaiian‐Emperor seamount chain, for an absolute plate velocity of about 10 cm yr−1the convective depth must be about 600 km or less. These results tend to support the predictions of the two‐scale model of Rich

ISSN:0148-0227    

DOI:10.1029/JB081i029p05267

年代:1976

数据来源: WILEY

摘要:

Paleomagnetic samples of basalt and sediment from four Deep‐Sea Drilling Project sites in the Nazca plate were studied in order to detect any possible paleolatitude changes and to evaluate the contribution of different portions of the igneous basement to the overlying magnetic anomalies. Penetration of the basement basalts yielded both coarse‐grained massive flow units and fine‐grained pillow basalts. The massive flow samples had titanomagnetite grains that were relatively unoxidized, while all the pillow basalts had been extensively subjected to low‐temperature oxidation. Average intensity of magnetic remanence in the massive flow units (99.4×10−4emu/cm3) was approximately an order of magnitude higher than that found in the pillow basalts (10.3×10−4emu/cm3). All coarse‐grained massive flow samples had large components of soft magnetization that was probably due to a viscous remanent magnetization (VRM) acquired both in situ and during the drilling process. In some cases this VRM component dominated the initial thermal remanence and could be responsible for the inability of some areas of the oceanic crust to record observable magnetic anomaly patterns. Large deviations in stable inclination from that expected from an axial centered dipole field were recorded by both basalt and sediment samples. Tightly grouped and anomalously high inclinations in the basalt samples from the younger sites indicate that the upper part of the oceanic igneous crust was formed in less than 102yr, and this formation may be episodic. Paleomagnetic inclinations from the sediments, averaged over roughly 106yr for each site, indicated no latitudinal change for the Nazca plate between 15 and 40 m.y. B.P. and are consistent with a southward paleolatitude movement of 3°–5° dur

ISSN:0148-0227    

DOI:10.1029/JB081i029p05281

年代:1976

数据来源: WILEY

摘要:

The East Pacific Rise crest between the Clipperton and Siqueiros fracture zones is characterized by a 10‐ to 20‐km‐wide axial block. A sonobuoy seismic refraction study of this feature indicates that it is a region of profound change in the geophysical nature of the crust and upper mantle. At or near the summit of the block, crustal velocities of about 5.2, 6.0, and 7.0 km/s are observed. Mantle velocities are observed on most summit profiles, but they are anomalously low (V¯= 7.8 km/s) and are associated with large intercept (delay) times, which yield apparent crustal thicknesses of up to 14 km. Proceeding down the flank of the axial block, the 6.0‐km/s refractor disappears, and mantle arrivals occur progressively earlier in the travel time plots, producing crustal thickness values that approach the regional norm (i.e., 5 km from sea floor to Moho). These changes are accompanied by a general downslope decrease in the attenuation of both crustal and mantle refractions. The occurrence of a supramantle shadow zone on the summit record sections indicates the presence of a crustal low‐velocity zone that thins away from the summit. We believe that the large mantle delay times, which are responsible for the negative correlation between crustal thickness and water depth (age), are due to this wedge of low‐velocity material. True depth to Moho is interpreted to be nearly constant throughout the cr

ISSN:0148-0227    

DOI:10.1029/JB081i029p05294

年代:1976

数据来源: WILEY

摘要:

In part 1 (Rosendahl et al., 1976) of this study it was found that a wedge‐shaped low‐velocity zone underlies the axial block of the East Pacific Rise. This low‐velocity zone, which occurs within the oceanic crust, is thought to represent a magma reservoir containing approximately 30% melt by volume. One consequence of this interpretation is that isostatic uplift alone can produce the axial block morphology observed along much of the East Pacific Rise crest. Based upon mantle intercept time anomalies, it appears that the axial block of the Reykjanes Ridge also could be a morphologic expression of an underlying magma reservoir. Because the Mid‐Atlantic Ridge axial valley is about an order of magnitude larger than the probable width of any underlying crustal zone of partial melt, the morphology there could not be generated by volumetric changes (e.g., via magma withdrawal) in a shallow magma chamber. Another consequence of the magma reservoir interpretation is that it leads to new inferences on the constitution of ocean crust. In particular, it is suggested that crust generated at spreading systems with axial block morphology may show both chemical and stratigraphic differences from crust generated at axial valley spreading

ISSN:0148-0227    

DOI:10.1029/JB081i029p05305

年代:1976

数据来源: WILEY

摘要:

The two Hyuganada, Japan, earthquakes of 1961 and 1968 (M = 7.0 and 7.5, respectively) each broke 80‐km segments of the thrust zone abutting to the southern end of the Nankaido 1946 rupture. Mean annual sea levels (period 1942–1973) at eight tide gage stations along a 400‐km segment of the Japanese coast centered in the epicentral area were compared to each other. It was found that mean annual sea level differences between the northernmost and southernmost stations (Kochi and Kagoshima) remained constant over a 16‐year period, with a standard deviation from the 16‐year mean of 1.2 cm. Compared with these northern and southern reference stations, four central tide gages located within 50 km of the two Hyuganada ruptures showed the following changes: (1) At the station between the two earthquakes (Hosojima), sea level dropped by about 5 cm in 1957, and it rose by about 5 cm after the earthquake in 1968. (2) At two stations north of the ruptures (Tosashimizu and Uwajima), sea level rose by about 4 cm in 1957. (3) At a station close to the southern end of the ruptures (Aburatsu), sea level remained constant with a standard deviation of about 1.1 cm between 1950 and 1966. These sea level changes are interpreted to reflect local crustal uplift at Hosojima and subsidence in the Tosashimizu‐Uwajima area in 1957. If one assumes that these vertical movements were precursors to the two Hyuganada earthquakes, the data support the dilatancy‐diffusion hypothesis: Near the center of the ruptures, dilatancy may have caused uplift (Hosojima), and beyond the northern end of the aftershock area, subsidence may have occurred due to reduction in crustal pore pressure. Precursor times of these magnitude 7.0 and 7.5 earthquakes may have been approximately 3.5 and 5 years,

ISSN:0148-0227    

DOI:10.1029/JB081i029p05315

年代:1976

数据来源: WILEY

摘要:

A mathematical solution is developed for the steady, quasi‐static, plane strain advance of a shear fault in a fluid‐infiltrated elastic porous material. As revealed through analysis of some elementary fracture mechanics models, the coupled deformation‐diffusion effects in such a material lead to a required ‘force’ to drive the fault that increases continuously with fault velocity up to a maximum value. The nominal fault tip energy release rate required for spreading at this maximum is greater than that for very slow speeds by a factor approaching (1 ‐v)2/(1 ‐vu)2, wherevandvuare the elastic Poisson's ratios under ‘drained’ and ‘undrained’ conditions, respectively. The effect is numerically significant and provides a mechanism by which a spreading shear fault can, within limits, be stabilized against catastrophic (seismic) propagation. Predictions of the model are compared to data representative of creep events on the San Andreas system. It is concluded that the speeds and slipping lengths of the observed events are consistent with their being stabilized by the effect discussed, and hence the model would seem to provide a viable mechanism for fault creep. Similar effects may be operative also in setting the time scale of progressive landslide failures in overconsolidated clay soils, in which rupture occurs by propagation of

ISSN:0148-0227    

DOI:10.1029/JB081i029p05322

年代:1976

数据来源: WILEY