摘要:

Marine refraction seismic and gravity investigations were carried out during 1965, 1967, and 1968 in the western Canary Islands. They were aimed at defining the crustal structure and contributing to an understanding of the origin of the Canaries. The results show that the depth of the mantle is 12 km west of La Palma‐Hierro, 13.9 km south of Gomera‐Tenerife, 15 km north of Tenerife‐Gran Canaria, and 21–22 km under the continental shelf. A crustal thickening of about 5 km under the Canaries ridge indicates an isostatic compensation of the island group. The crust around Hierro. La Palma, Gomera, and Tenerife is essentially oceanic, whereas Gran Canaria lies in the transitional zone between oceanic and continental crust. It is concluded that the islands do not form a part of the African continent but are independent volcanic edifices that erupted along NE‐SW striking fract

ISSN:0148-0227    

DOI:10.1029/JB075i026p04901

年代:1970

数据来源: WILEY

摘要:

From an analysis of the seismicity of the Mediterranean basin we obtain a model of the deep structure of this region that is a further improvement to the plate tectonic theory. Taking into account the data of seismology, the gravity anomalies, volcanism and the few available data on the heat flow in this region, we conclude that the African plate is wedged under the Euro‐Asiatic plate with a slope of approximately 58° in the Lipari region and 35° in the Aegean region. This model can explain the high seismicity of middle south Italy, the sinking of middle north Italy, and the uplift of south Italy. The last movement is confirmed also by geologic and archeologic data. The archeologic data give an uplift rate of 0.034 cm

ISSN:0148-0227    

DOI:10.1029/JB075i026p04919

年代:1970

数据来源: WILEY

摘要:

Magnetic profiles over the Alpha cordillera have anomalies of wavelength 5 to 30 km and amplitudes frequently exceeding 1000 gammas. Further, the profiles show a reasonable degree of mutual correlation and biaxial symmetry over the ridge crest. Finally, the profiles appear to correlate with those from the flanks of other well‐documented ridges in the Greenland and Norwegian seas and can be matched with a sea‐floor spreading model over the paleomagnetic time span of 40 to 60 m.y.b.p. From this evidence, as well as from newly presented gravity data and analysis of existing additional geophysical and geological information, it is concluded that the cordillera may have been a dormant mid‐ocean ridge active at least between 40 and 60 m.y.b.p. If this hypothesis is correct, the spreading rate from the ridge axis was 1 cm/yr and ended abruptly 40 m.y

ISSN:0148-0227    

DOI:10.1029/JB075i026p04925

年代:1970

数据来源: WILEY

摘要:

We present a new continental drift reconstruction of the universal continent of Pangaea in the Permian plus a series of five world maps to depict the breakup and dispersion of continents with each subsequent geologic period, Triassic to Recent. Plate tectonics and sea‐floor spreading are accepted as the guiding rationale. Also utilized are the morphologic fitting of continental margins and paleomagnetic pole positions. Rigor is imposed by the geometric requirements involved in presenting continental drift dispersion on maps in orderly time sequence and by following certain assumed rules of plate tectonics. The reconstructions were first made on a globe and then transferred to an Aitoff world projection. In the Permian, the Atlantic and Indian oceans were closed so that all the continents were configured into the universal landmass of Pangaea. The reconstruction is based largely on the morphologic best fit of continental margins to the 1000‐fathom isobath, except for India, the east coast of which is placed against Antarctica, as dictated by plate tectonics. In the Triassic the breakup of Pangaea commenced. The southwest Indian Ocean rift was created, which split West Gondwana (South America and Africa) away from East Gondwana while a Y junction lifted India off Antarctica. An independent North Atlantic–Caribbean rift also formed, which lifted Laurasia (North America and Eurasia) off of South America and the bulge of Africa. In the Jurassic, northward and westward sea‐floor spreading further opened the central North Atlantic and the Indian oceans. At the end of the period, a new rift incipiently split South America away from Africa. The Walvis mantle thermal center or ‘hot spot’ formed, which would subsequently provide an absolute geographic reference point for subsequent continental drift. In the Cretaceous, the motions already established continued. The North Atlantic rift grew northward, blocking out the Grand Banks and the western margin of Greenland. Spain rotated sinistrally, forming the Bay of Biscay. An offshoot rift split Madagascar from Africa, dropping off this subcontinent from Africa, which continued its northern flight. The northward trek of India continued, and Australia incipiently split away from Antarctica. During the Cenozoic, Antarctica rotated further westward. Australia experienced a remarkable flight northward, and New Zealand was split away from its east coast. The North and South Atlantic oceans continued to open; the rift that formerly passed west of Greenland now switched to the east and split Greenland away from northern Europe and extended through the Arctic Ocean. Africa moved slightly northward, continuing sinistral rotation. The Tethyan megashear became dextral for the first time, India collided with and un

ISSN:0148-0227    

DOI:10.1029/JB075i026p04939

年代:1970

数据来源: WILEY

摘要:

Models of earthquake sources that have no volume change, no net force, and no net torque as criteria for the radiation of first motions, have five degrees of freedom in their spatial orientation. The usual double‐couple model has only three degrees of freedom. The most general source of high‐frequency seismic motions must be a linear combination of a double couple and another source called the compensated linear‐vector dipole. A radiation pattern of amplitudes of first motions on the focal sphere cannot be uniquely decomposed into the radiation patterns due to the two so

ISSN:0148-0227    

DOI:10.1029/JB075i026p04957

年代:1970

数据来源: WILEY

摘要:

Amplitudes of long‐period pulses are used in an analysis of several intermediate and deep‐focus earthquakes to determine whether the mechanism is of the double‐couple or compensated linear vector dipole type. For most of the shocks, the double‐couple model dominates the linear dipole model, but not overwhelmi

ISSN:0148-0227    

DOI:10.1029/JB075i026p04965

年代:1970

数据来源: WILEY

摘要:

We consider some theoretical models of crater destruction to explain not only the exponent −2 of the equilibrium distribution of crater diameters, but also the density coefficient and its regional variations, Destruction of small craters appears to be governed principally by blanketing or filling by ejecta, and the equilibrium density coefficient decreases with increasing size of the largest crater affecting the region studied. Destruction of large craters seems to depend mainly on destruction of a substantial part of the crater wall by a crater of similar or greater diameter. Only certain south polar and far‐side continental regions appear to be saturated with crat

ISSN:0148-0227    

DOI:10.1029/JB075i026p04977

年代:1970

数据来源: WILEY

摘要:

A paleomagnetic pole position calculated for 131 sites in normally (N) magnetized Keweenawan intrusive rock from Minnesota lies at 34.5°N, 168.5°W. Gabbro and diabase of reverse (R) polarity also are known but only from a limited area in northern Cook County, near the Canadian border.R‐polarity sites yield a pole position of 42.5°N, 156.5°W and probably are older than theN‐polarity sites. Magnetic intensities in these rocks vary by 4 orders of magnitude, in keeping with other strong differences in lithology, which together reflect active igneous differentiation, Dispersion of magnetic directions in the intrusive rocks indicates that the ratio of nondipole to dipole field‐intensity has not changed markedly since Keweenawan time. It is suggested that, on the basis of current evidence, apparent polar wandering relative to North America is best described by a rather vaguely defined southwestwardly trending path several tens of degrees in width. Under favorable circumstances positions within the path of polar wandering can be used for geological correlation. Several episodes of Precambrian basaltic volcanism in North America are used to illustrate long‐distance correlation by paleomagnetic

ISSN:0148-0227    

DOI:10.1029/JB075i026p04985

年代:1970

数据来源: WILEY

摘要:

An earthquake model is derived by considering the effective stress available to accelerate the sides of the fault. The model describes near‐ and far‐field displacement‐time functions and spectra and includes the effect of fractional stress drop. It successfully explains the near‐ and far‐field spectra observed for earthquakes and indicates that effective stresses are of the order of 100 bars. For this stress, the estimated upper limit of near‐fault particle velocity is 100 cm/sec, and the estimated upper limit for accelerations is approximately 2g at 10 Hz and proportionally lower for lower frequencies. The near field displacement u is approximately given byu(t) = (σ/μ) βr(1 ‐ e−t/r) where. σ is the effective stress, μ is the rigidity, β is the shear wave velocity, and τ is of the order of the dimension of the fault divided by the shear‐wave velocity. The corresponding spectrum isΩ(ω)=σβμ1ω(ω2+τ−2)1/2The rms average far‐field spectrum is given by〈Ω(ω)〉=〈Rθϕ〉σβμrRF(ε)1ω2+α2where 〈Rθϕ〉 is the rms average of the radiation pattern;ris the radius of an equivalent circular dislocation surface;Ris the distance;F(ε) = {[2 – 2ε][1 – cos (1.21 εω/α)]+ε2}1/2; ε is the fraction of stress drop; and α = 2.21 β/r. The rms spectrum falls off as (ω/α)−2at very high frequencies. For values of ω/α between 1 and 10 the rms spectrum falls off as (ω/α)−1for ε<∼0.1. At low frequencies the spectrum reduces to the spectrum for a double‐couple point source of appropriate moment. Effective stress, stress drop and source dimens

ISSN:0148-0227    

DOI:10.1029/JB075i026p04997

年代:1970

数据来源: WILEY

摘要:

Normal mode solutions for a spherical earth model are superposed to obtain realistic synthetic surface waves for various force systems. The calculation of excitation functions is greatly facilitated by the variational technique. The effects of attenuation and instrument response are included in the synthesis. The synthesized seismograms are compared with the actual seismograms for wave form, amplitude, radiation pattern, and phase to obtain various source parameters such as force geometry, source‐time function, seismic moment, source dimension, and rupture velocity. The application is made to the Kurile Island earthquake of October 13, 1963 (M= 8.3). The comparison is made for Love wavesG4and Rayleigh wavesR4for a period range 100 to 400 sec. It is found that a reverse dip‐slip fault with a dip angle of 22° and a dip direction of N47°W can explain the observation reasonably well. Other parameters determined are as follows: seismic moment, 7.5×1028dyne‐cm; source‐time function, step function; fault length, 200 to 300 km; rupture velocity, 2.7 to 4.5 km/sec in the direc

ISSN:0148-0227    

DOI:10.1029/JB075i026p05011

年代:1970

数据来源: WILEY