摘要:

The results of a line of seismic reflection measurements and 9 two‐ship refraction profiles made on the Eauripik‐New Guinea rise and in the Caroline basins on either side are compared with preliminary results from Joides boreholes. The combined data indicate that the rise is a thickened and raised section of oceanic crust and has a fairly uniform thickness of calcareous pelagic sediment over basaltic basement of Oligocene age. Sediments deposited above a prominent reflecting horizon in the sediment body thin with increasing distance from the rise crest, apparently because of greater solution of calcium carbonate in the basins, whereas those below are distributed differently, indicating as yet unknown changes in the depositional environm

ISSN:0148-0227    

DOI:10.1029/JB076i020p04711

年代:1971

数据来源: WILEY

摘要:

Deep seismic reflection profiling has been used to outline the main features of the basement topography of the Labrador Sea. Two major sedimentary reflectors, tentatively dated as Paleocene and Eocene, are correlated throughout the survey. A large basement feature extends southwest below a sedimentary ridge from Cape Farewell. It is interpreted as a transform fault created by the movement of the tip of Greenland during the early stage of opening. On the basis of the sedimentary history of the sea, of transform fault geometry, and of magnetic anomaly pattern, a preliminary history of the opening of the Labrador Sea is given in terms of plate tectonics. Two main episodes are proposed. The first one, from Upper Cretaceous to Paleocene, would correspond to an opening rate of slightly less than 1 cm/yr accompanied by rapid subsidence of the margins. The second one, from Paleocene to Eocene, would correspond to the opening of the final third of the basin at a rate of about 0.5 cm/yr. Very slow movement would have continued up to the present.

ISSN:0148-0227    

DOI:10.1029/JB076i020p04724

年代:1971

数据来源: WILEY

摘要:

In the model whose implications are investigated here, the lithosphere is assumed to be 100 km thick and the asthenosphere 300 km thick. A linearized viscosity is assumed, its mean value for the lithosphere being 1023poises and for the asthenosphere 1019poises. The mesosphere, lying below 400 km depth, is assumed to be more viscous again than the asthenosphere, but no specific values are required. The viscosity contrast of 104between lithosphere and asthenosphere is the dominant feature of this model. The model reproduces a surprisingly large number of observed features in a qualitative manner and without the need for subsidiary assumptions of an essential kind. The existence of a mathematical theorem due to Helmholtz is recalled: it says that for small Reynolds number the actual flow is such as to minimize viscous dissipation. This theorem suggests that only extremely simple flow patterns can be used in describing convection in the mantle. In particular, the shallowness of upper‐mantle convection is in part due to the shallowness of the asthenosphere, in part to the mechanical advantage obtained by the flow's hugging a free boundary during nearly half a closed circuit. It is next pointed out that the asthenosphere appears as the uniquely determined locus of the chemical differentiation of sial from primary mantle material. The shortness of vertical distance from asthenosphere to the lithosphere facilitates the progressive incorporation of sialic material into the continental plates, especially into their roots, as seems to be observe

ISSN:0148-0227    

DOI:10.1029/JB076i020p04744

年代:1971

数据来源: WILEY

摘要:

Palynological evidence suggests that the North Florida Lower Paleozoic block has arrived at its present position with respect to the Appalachians from a Silurian Pangean position inside the divergence of the 1000‐fathom contours of the African and South American continental block

ISSN:0148-0227    

DOI:10.1029/JB076i020p04754

年代:1971

数据来源: WILEY

摘要:

Forty‐one measurements of heat flow, including three close‐spaced groups of measurements, were taken at positions extending from the continental slope off Oregon to the east flank of the Juan de Fuca ridge. Individual values ranged from 1.08 μcal/cm2sec in 445 meters of water on the continental slope off Oregon to 7.21 μcal/cm2sec measured in 2740 meters of water near the Juan de Fuca ridge crest. The mean of the 41 heat‐flow values is 2.86 μcal/cm2sec, which is considerably higher than the approximate global mean of 1.5 μcal/cm2sec. This high average heat flow fits the previously deduced world heat‐flow pattern, which includes a high over the eastern Pacific. The distribution of 16 heat‐flow measurements taken from near the base of the continental slope off Oregon westward to Cascadia channel has a broad peak between 1.50 and 2.99 μcal/cm2sec. The distribution of 15 heat‐flow measurements taken from west of Cascadia channel to near the Juan de Fuca ridge crest has a relatively sharp peak between 3.50 and 3.99 μcal/cm2sec. The results of this research, combined with existing heat‐flow data for the west flank of the Juan de Fuca ridge, show that the heat‐flow profile across this ridge is not symmetrical. Unusually low values predominate on the west‐flank, whereas above average values dominate the east flank of

ISSN:0148-0227    

DOI:10.1029/JB076i020p04758

年代:1971

数据来源: WILEY

摘要:

A technique for computing bistatic‐radar echo characteristics from arbitrary scattering functions is developed and applied to several well‐known laws. For a Gaussian scattering law, the bandwidth of the echo spectrum is nearly proportional to surface slopes, whereas the total received power is only weakly dependent on slopes. The contribution to radar cross section of a diffuse scattering law under oblique bistatic geometries can be markedly less than for monostatic geometry at large ranges. Numerically obtained dependences of bistatic cross section on mean square surface slopes support previous inferences from Explorer 35 data that lunar highlands are less reflective than the s

ISSN:0148-0227    

DOI:10.1029/JB076i020p04775

年代:1971

数据来源: WILEY

摘要:

Paleomagnetism of 91 samples (370 specimens) and petrochemical analyses of 37 samples from 10 different dikes of Mysore state, India, indicate that these dikes belong to two different groups. The poles of four dikes correspond with the pole position of Deccan traps of Cretaceous‐Tertiary age that were presumably erupted when India was in the southern hemisphere. The pole positions of two of the dikes, however, suggest that at the time of their intrusion India was probably in the northern hemisphere. Petrochemically, the former are tholeiitic like the Deccan traps, whereas the latter are alkalic basalt

ISSN:0148-0227    

DOI:10.1029/JB076i020p04786

年代:1971

数据来源: WILEY

摘要:

A detailed shipborne magnetic survey in the western North Atlantic consisted of forty east‐west lines, spaced 20 miles apart, extending between 23°N and 34°N and from 75°W to 60°W. Several magnetic provinces are evident. From west to east, there is the magnetically smooth zone previously described, a 100‐km‐wide rough‐smooth transition, then a 300‐km‐wide band of correctable sea‐floor spreading anomalies (the USNSKeathleysequence), whose eastern boundary (called the Bermuda discontinuity) is characterized by anomalies of rather high amplitude, and finally a region in which no certain anomaly correlations could be found. A model is derived for theKeathleyanomalies, which, based on three Joides deep drill holes, are supposed to have been generated between approximately 155 and 135 m.y. B.P. at a rate of at least 2.4 cm/yr. This value, based on Joides drilling results, is the average spreading rate between 155 and 120 m.y. B.P. as measured parallel to transform faults discovered by an unpublished airborne survey. Closely spaced fracture zones between 120 and 135 m.y. B.P. suggest slow spreading during that time. This implies a rate exceeding 2.4 cm/yr for theKeathleyanomalies between about 34°N, 63°W and 36°N, 67°W. An assumed spreading rate of 3 cm/yr for theKeathleysequence near 30°N leads to a reversal frequency of 3 per million years, approximately the upper Tertiary value. Possible left‐handed transform fault offsets occur in theKeathleysequence at about 23°N and 28°N. Certain resolution of smaller offsets must await more detailed surveys. As none of the observed offsets approach the 180‐km offset of the present ridge at 24°N, it is concluded that the present ridge trace is not an exact image of the early break. The paleospreading pole for theKeathleysequence is found from the rate of northward narrowing of the anomalies and from possible transform faults. The rate of northward increase of anomaly amplitude is compared with models based on various magnetic poles along the North American polar wandering curve. The agreement seems best for a pole between the Triassic and the Jurassic poles, the latter being taken near the present geographic pole, but scatter in the data is too great to be certain. If the magnetized layer is 0.5 km thick, its magnetization intensity is about ±0.005 km. Using linear cross correlation, predominate northeast, northwest, and north trends are found in the magnetic fabric of the uncorrelatable region. Comparison with bathymetric studies suggests that the first two are ‘ridge’ and ‘fracture zone’ trends, respectively. We speculate that the rift between South America and Africa developed about 135 m.y. B.P., causing spreading rates to fall and directions to change in the North Atlantic, thus producing the Bermuda discontinuity, a band of high, rough basement and high‐amplitude magnetic anomalies. The high basement ridge formed 135 m.y. B.P. apparently affected the sedimentation pattern in the western North Atlantic throughout the Cretaceous and most of the Tertiary. The uncorrelatable region is characterized by closely spaced fracture zones and perhaps slow spreading. A reconstruction of the North Atlantic and surrounding areas is made

ISSN:0148-0227    

DOI:10.1029/JB076i020p04796

年代:1971

数据来源: WILEY

摘要:

A saturation survey of 800 km2of ridge flank topography was made near 47°N, 132°W. Echo sounding depths at 12 kHz and total‐field magnetic intensity were measured from a surface vessel on a dense track line pattern. Thirteen long gravity cores, three seismic profiles, and two camera stations provided additional data. Two abyssal hills 700 meters high are topographically and magnetically delineated in considerable detail: one is smooth and associated with a negative magnetic anomaly; the other is of complex morphology and is associated with a positive anomaly. Neither anomaly can be due to simple bulk magnetization of the seamounts. The southeast seamount shows massive flows, cliffs, steep pseudomassive faces, and rubble slide, in sequence downward from the summit. Smooth sediment slopes on the skirt are broken by almost bare rock outcrops, with sharp rock‐sediment contacts. The undulating basement plain in the area is covered by up to 70 meters of sediment containing internal acoustic reflectors that persist for 10 km or more. A corresponding correlation is observed in the layering of the surface cores. Most contain low carbonate gray clay layers that appear to be similar to Tufts Plain turbidite material but are not concentrated in topographic lows. Low‐density turbid water may occasionally overflow from the abyssal plain and intersperse with the climatically controlled pelagic s

ISSN:0148-0227    

DOI:10.1029/JB076i020p04824

年代:1971

数据来源: WILEY

摘要:

The early paleomagnetic observations from the Permian period of the western United States were not subjected to demagnetization studies. Observations on the Lower Permian Cutler formation of western Colorado suggest that many of these older poles are in error by 10 or more degrees. The new pole from the Cutler formation is at 42.7°N, 121.9°E (α95=6.0) and is almost identical with the demagnetized results from eastern North America; it provides an excellent test of the dipolar hypothesis. The results of this study emphasize the need for additional study of the older paleomagnetic data for North Ameri

ISSN:0148-0227    

DOI:10.1029/JB076i020p04842

年代:1971

数据来源: WILEY