摘要:

For most oceans the basic feature of the elevation of ridges is a uniform relation between subsidence and age directly associated with the creation and cooling of the oceanic plate. In the North Atlantic we show that superimposed upon this subsidence curve are long‐wavelength (>400‐km) residual elevation anomalies up to 1200‐m amplitude between 50°N and 10°N that are linearly related to regional variations in the free air gravity field. It is unlikely that this relation can be accounted for by sources in the lithosphere and hence it may be evidence of mass excesses or flow in the upper mantle. Further, the correlation between free air gravity and residual depth is two dimensional and passes through the origin with the same sign and gradient as were predicted by investigations of convection in a Newtonian fluid. We assume a correlation between long‐wavelength residual elevation and free air gravity anomalies. Hence where the free air gravity field is close to zero, there should be no residual elevation anomaly. Thus the absence of a large free air gravity anomaly in the North Pacific, other than close to the Hawaiian Island chain, permits us to extend the uniform depth versus age relation from 80 to 150 m.y. B.P. The very small increase of depth with age in the Cretaceous and the uniform heat flow through old oceanic crust are evidence that the oceanic plate has a finite thickness, which can be detected in crust older than 80

ISSN:0148-0227    

DOI:10.1029/JB080i008p01031

年代:1975

数据来源: WILEY

摘要:

The focal mechanisms of 40 earthquakes in Peru and Ecuador, together with the seismicity of the region, indicate particular features of the subduction of the oceanic plate beneath this portion of South America. At shallow depths near the coast and at foci along the contact between the subduction zone and the continental plate the focal mechanisms indicate an underthrust of the continent by the oceanic plate on a thrust plane dipping 10°–15° beneath the continent. Near this same depth but at foci within the oceanic plate, normal faults occur that correspond both to flexure of the plate and to downdip axial tension. At intermediate depths the plate continues to act as a stress guide, the axis of tension being down about 30° from the horizontal and trending to the ENE. The dip of the Benioff zone steepens notably in southern Peru near the Peru‐Chile corner, and the motion of the descending slab relative to the continental plate is in a direction N40°E. Deep‐focus earthquakes indicate a vertical segment of plate under axial compression at depths of 550–600 km. Numerous earthquakes also occur interior to the continent and within the continental lithosphere at depths down to 90 km. Both strike slip and reverse‐type faults are found, but in either case the stress system corresponds to an E‐W horizontal compression. Comparison with the seismicity is consistent with the model of an oceanic plate moving almost horizontally under the continental lithosphere in northern and central Peru and a separate, more steeply plunging segment of plate moving normal to the coast under southern Peru.Equal‐area projections are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D. C. 20006. Document J75‐003; $1.00. Payment m

ISSN:0148-0227    

DOI:10.1029/JB080i008p01053

年代:1975

数据来源: WILEY

摘要:

Velocities for both compressional and shear waves in an Alpine gneiss have been determined along its principal fabric directions under dry, saturated, and high‐pressure conditions. Both saturation and pressure introduce large anisotropic changes in velocities, suggesting that the rock contains microcracks that are preferentially parallel to mesoscopic fabric directions. The ratio between compressional and shear velocities in all directions increases with saturation, as can be expected. The changes due to pressure are more complicated: along the direction perpendicular to the oriented microcracks the ratio increases significantly; along directions parallel to the oriented microcracks, however, the ratio changes only slightly, and it either increases or decreases depending on the direction of polarization of the shear wave

ISSN:0148-0227    

DOI:10.1029/JB080i008p01065

年代:1975

数据来源: WILEY

摘要:

The purpose of this note is to demonstrate that recentVpandVsprofiles are consistent with finite strain extrapolations of reasonable mineralogies. The difference in the size of the discontinuities in theVpandVsprofiles is due to the relative pressure insensitivity of shear velocity in spinel structures. Most of the structure in the upper mantle can be attributed to the collapse of forsterite rich olivine to the β spinel and then the γ spinel structures. The 640‐km discontinuity on the basis of recent work appears to be due to the disproportionation of γ spinel to a mixture of magnesiowustite (Mg, Fe)O and stishovite. The present results indicate that this postspinel phase in the lower mantle is enriched in SiO2relative to the upper mantle. The 640‐km discontinuity is therefore a chemical as well as a phase bo

ISSN:0148-0227    

DOI:10.1029/JB080i008p01070

年代:1975

数据来源: WILEY

摘要:

Amplitude spectra of the spheroidal modes0Sl(l= 2–40) excited by the 1960 Chilean earthquake and observed at Pasadena, Los Angeles, and Isabella show distinct holes at0S10(T= 580 s) and0S21(T= 336 s). These holes can be explained as an interference pattern caused by a composite source consisting of a finite propagating source (main shock) and a slow precursory source, 15 min before the main shock. A total seismic moment of 4–5 × 1030dyn cm is required to explain the observed ampli

ISSN:0148-0227    

DOI:10.1029/JB080i008p01075

年代:1975

数据来源: WILEY

摘要:

Observations of striking and consistent differences in the attenuation ofpPphases produced by mantle earthquakes and recorded by the World‐Wide Standard Seismograph Network (WWSSN) provide data for mapping variations in the attenuation of high‐frequency (0.5‐ to 2‐Hz) compressional waves in the wedge of mantle above nearly all of the inclined seismic zones on earth. The data reveal several zones of high attenuation that in nearly all cases correspond to major presently active tectonic features above or near the inclined seismic zones. Zones of high attenuation behind the Tonga, New Hebrides, Mariana, and Japanese island arcs coincide with zones of presently or recently active crustal extension and creation of marginal basins. In western South America a zone of high attenuation underlies most of the broad uplifted terrain of the Andean Altiplano. In contrast to the zones beneath active marginal basins this zone is not related to crustal spreading. A high‐attenuation zone exists beneath the Sea of Okhotsk that, although it is near, is offset from the Kuril‐Kamchatka arc and thus may not be directly related to that system. This zone may instead be related to possible offshore continuations of the Baykal‐Aldan rift system or the postulated plate boundary between Asia and North America. Where they are determined, the high‐attenuation zones seem to exist in the upper 200–300 km of the mantle. Wherever data are available, they show that the zones of high compressional wave attenuation correspond to zones of high attenuation for shear waves, low seismic wave velocities, and high heat flow. These properties, in addition to the geological evidence of crustal extension or uplift, suggest that the compressional wave attenuation results from high temperatures and/or partial melting of the upper mantle material. Although all the high‐attenuation zones occur near inclined seismic zones that reach depths greater than 300 km, the absence of such zones in the Indonesian, Philippine, Izu‐Bonin, and New Britain‐Solomon regions shows that lithospheric subduction to great depths is not a sufficient condition for their occurrence. The zones of high attenuation are also not systematically related to the distributions of active volcanos; this condition, together with other data, indicates that large bodies of partially melted material probably do not exist beneath the

ISSN:0148-0227    

DOI:10.1029/JB080i008p01079

年代:1975

数据来源: WILEY

摘要:

A seismic refraction profile was recorded along the southern Wasatch front in Utah to investigate crustal structure in the transition zone between the Great basin and Colorado plateau provinces. Interpretation of both refracted and reflected phases indicates a thin (∼25‐km) crust, a lowPnvelocity (∼7.5 km/s), and a crustal low‐velocity layer in the approximate depth range of 8–15 km. Correlation with previous geophysical studies suggests that these features are present throughout the transition zone and that a mantle upwarp extending at least 50 km east of the boundary between these provinces (Wasatch front) is present. This upwarp may be of considerable tectonic significance, since it approximately coincides with a zone of high seismicity, the intermountain sei

ISSN:0148-0227    

DOI:10.1029/JB080i008p01093

年代:1975

数据来源: WILEY

摘要:

The occurrence of leucocratic rocks in ophiolites, herein collectively named oceanic plagiogranites, indicates that in some instances, differentiation has taken place during the development of oceanic crust. These plagiogranites consist primarily of quartz and plagioclase with only minor amounts of ferromagnesian minerals. High silica, moderate alumina, low iron‐magnesium, and extremely low potassium characterize oceanic plagiogranites and distinguish them from continental granophyres developed from differentiated mafic rocks. The87Sr/86Sr ratios of the mafic and silicic rocks of the ophiolite suite of Troodos are similar. This similarity is consistent with the idea of low‐pressure differentiation of subalkaline basaltic magma to form K poor leucocratic rocks. Slow‐spreading centers may favor the formation of the oceanic plagiogranites because crystal fractionation is more likely to occur in such situa

ISSN:0148-0227    

DOI:10.1029/JB080i008p01099

年代:1975

数据来源: WILEY

摘要:

The cross sections for21Na production induced by 10‐, 12‐, 14‐, and 16‐MeV protons on natural Mg targets have been measured as 0.21 ± 0.08,17.0 ± 2.5, 45.2 ± 6.8, and 44.0 ± 6.6 mb, respectively. These values imply that just below the surface the22Ne/21Ne ratios found in extraterrestrial samples would be lowered by an excess of low‐energy protons. Unfortunately, the lowering will in most cases be masked by

ISSN:0148-0227    

DOI:10.1029/JB080i008p01109

年代:1975

数据来源: WILEY

摘要:

Airborne coherent radar observations of ice fields conducted in the last 4 years are presented and discussed. These observations contain radar imagery of glaciers in southeast Alaska, imagery of coastal and sea ice in northern Alaska and the Beaufort Sea, and sounding of layered continental ice in Greenland.

ISSN:0148-0227    

DOI:10.1029/JB080i008p01113

年代:1975

数据来源: WILEY