摘要:

To test hypotheses relating the absolute velocities of the plates to driving force models for the present plate system, we have determined the absolute plate motions in the early Tertiary for a number of force models using the assumption that these forces exert no net torque on the lithosphere. All absolute motion models are based on a self‐consistent set of relative plate velocities obtained from published finite plate rotations and on the Jurdy and Van der Voo map of plate boundaries at 55 m.y.B.P.The derived pattern of plate velocities in the early Tertiary is consistent with paleomagnetic and paleosedimentation data and differs in important respects from the present pattern: (1) The clear separation of the plate population at present into fast (7–10 cm/yr) oceanic plates and slow (generally 0–2 cm/yr) continental plates does not occur 55 m.y. ago. The rms absolute velocities of subducted oceanic plates at 55 m.y. are neither uniformly fast (4–10 cm/yr) nor consistently greater than those for dominantly continental plates (0–5 cm/yr). (2) There is a tendency at 55 m.y., as at present, for lithosphere in the equatorial half of the earth to move faster than in the polar half, though at 55 m.y. the coordinate pole giving the highest equatorial rms velocity is far from the geographic pole. (3) The predicted motions do not agree with the hypothesis of fixed hot spots and the observed trends of seamount chains and aseismic ridges, the poorest agreement being in the Pacific. From point 1 we infer that viscous drag beneath continents is not significantly greater than that beneath oceanic lithosphere and that forces at subduction zones are not the sole regulators of the velocities of subducted oceanic plates. Observation 2 weakens the case for a possible connection between plate speeds and earth rotation. From point 3 we concur with others that either substantial relative motion among the asthenospheric roots of hot spots is required or an origin unrelated to hot spots for at least some of the seamount chains and aseismic ridges must be postulated. The bends in the Pacific seamount chains may be associated with a reorientation of stress in the Pacific plate as its northern boundary switched from a ridge to a subduc

ISSN:0148-0227    

DOI:10.1029/JB082i002p00203

年代:1977

数据来源: WILEY

摘要:

A relocated earthquake sequence in the western Aleutian arc lasting from February 1965 through December 1968 is shown to map an episode of subduction of the Pacific plate. The February 4, 1965, Rat Island main shock (Ms= 8.1) was a complex rupture involving greatest underthrusting at lock zones beneath the Aleutian arc transverse canyons. The primary tectonic consequences of the main shock rupture were (1) the downslab diffusion of a compressional pulse, (2) the oceanward diffusion of an extensional pulse in the oceanic lithosphere, and (3) the slow rebound of the continental plate. The velocities of these strain pulses (actual plate motions) are determined by the viscosity of the constraining mantle material; observed strain velocities indicate a Newtonian mantle viscosity of 6×1019P. The extensional pulse that propagated oceanward is shown to determine the locations and normal‐faulting mechanisms of the trench earthquakes of this sequence. It is inferred that the entire subducting oceanic lithosphere contains a sequence of near‐vertical faults that strike parallel to the trench axis, reflecting former trench‐related normal‐faulting earthquakes. This condition is supported by considerable data and suggests a slabbing, escalatorlike descent mechanism for the upper 100 km of the subducting Pacific plate. A working hypothesis is presented that relates the primary features of the typical volcanic arc and interarc basin to episodic downslab compressional pulses. This hypothesis involves a sequence of high‐loading‐rate mantle compression, viscoelastic rebound, pressure reduction, increased partial melt concentrations, and counterflow in a thin tabular zone above the subd

ISSN:0148-0227    

DOI:10.1029/JB082i002p00213

年代:1977

数据来源: WILEY

摘要:

Magnetic anomalies over ocean crust with ages between 0 and 3.5 m.y. B.P. have been identified on the Mid‐Atlantic Ridge crest at 26°N. These anomalies indicate a spreading rate of 1.1 cm/yr on the west flank of the ridge and 1.3 cm/yr on the east flank. Computer modeling of the anomalies has shown that simple blocks of uniform thickness, whose width and direction of magnetization are obtained from the reversal time scale and whose intensities are chosen for best visual fit of the anomaly, are not capable of explaining the details of the observed magnetic anomalies. Shorter‐wavelength variation in the intensity of magnetization parallel to the ridge crest can, however, explain the observed anomalies. A possible degradation of the intensity of magnetization within the axial anomaly is associated with large faulted blocks in the wall of the rift, which is the site of the Trans‐Atlantic Geotraverse (TAG) hydrothermal field. Magnetic anomaly data suggest that extensive rotation of blocks within the rift valley is not

ISSN:0148-0227    

DOI:10.1029/JB082i002p00231

年代:1977

数据来源: WILEY

摘要:

Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000Pwave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500‐km depth; the average perturbation level increases in the lowermost mantle. The region between 1100 and 1500 km may represent a transition zone with respect to the dimensions of anomalies. We present statistical evidence for a negative correlation between the long wavelength gravity anomalies observed at the surface and those computed from velocity anomalies at depths greater than 1100 km under the assumption of proportionality between velocity and density perturbations. The proportionality coefficient Δυ/Δρ has been determined by using two different methods: the values are −4.45 and −6.02 (km/s)/(g/cm3). Only minor changes in the velocity distribution are needed to satisfy the long wavelength gravity field exactly. Possible origins of the correlation are (1) sinking of eclogite‐rich material into the lower mantle from regions of lithospheric subduction, (2) chemical plumes of light high‐velocity material originating near the core‐mantle boundary, (3) temperature differences and perturbations of the core‐mantle boundary and the earth's surface associated with mantle‐wide convection, or (4) static chemical heterogeneities in a nonconvecting mantle. The first three alternatives, all involving flow in the lower mantle, may be complementary but act on a different time scale. There appears to be a westward or northwestward translation of some anomalies with respect to the pattern obtained for the innermost shell. In particular, the direction of translation of a large negative anomaly under the Pacific is in agreement with the sense of motion of the Pacific plate. We must caution the reader, however, that this is a highly speculative interpretation. If correct, it would favor the co

ISSN:0148-0227    

DOI:10.1029/JB082i002p00239

年代:1977

数据来源: WILEY

摘要:

The analysis of seismic moments of intermediate and deep‐focus earthquakes in four island arc regions supports the hypothesis that these shocks are the result of shear stresses caused by phase transitions in the descending slab at depths near 150, 350, and 600 km. Cumulative seismic moments ∑M0for earthquakes listed by Gutenberg and Richter were found to exhibit four maxima when they were plotted as a function of depth; the maxima occur within the depth ranges 0–100, 100–250, 250–450, and 450–700 km. The uppermost range corresponds to the zone of underthrusting. The seismic moments of earthquakes in the three lower depth ranges are related to phase transitions in the slab by ∑M0= μLT∣ΔV/V∣ υsub, where μ is the modulus of rigidity,Lis the length along strike of the subduction zone,Tis the slab thickness, ΔV/Vis the fractional volume change associated with a particular phase transition, and υsubis the subduction velocity. In the range 100–250 km the earthquakes result from volume decreases associated with the melting of the oceanic crust. In the depth range 250–450 km the volume change is due mostly to the transformation of olivine to spinel structure with a density increase of about 8%. In the lowest depth range the decrease in volume is due to the change of spinel to postspinel phases with a density increase of about 8%. For any island arc region, up to four independent estimates of υsubcan be calculated by using Brune's method for the zone of shallow underthrusting and the relationship between ∑M0and volume changes for the three deeper zones. As an example, the four estimates of υsubfor the Tonga arc, in order of increasing depth, a

ISSN:0148-0227    

DOI:10.1029/JB082i002p00256

年代:1977

数据来源: WILEY

摘要:

Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2∼ 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68–Fo86, An58–An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more ‘primitive’ tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pac

ISSN:0148-0227    

DOI:10.1029/JB082i002p00265

年代:1977

数据来源: WILEY

摘要:

A new three‐dimensional earth modeling is proposed as a framework to obtain more detailed and accurate information about the earth's interior. We start with a layered medium of classic seismology but divide each layer into many blocks and assign a parameter to each block which describes the velocity fluctuation from the average for the layer. Our data are the teleseismic P travel time residuals observed at an array of seismographs distributed on the surface above the earth's volume we are modeling. By isolating various sources of errors and biases we arrive at a system of equations to determine the model parameters. The solution was obtained by the use of generalized inverse and stochastic inverse methods. Our method also gives a lower limit of the true rms slowness fluctuation in the earth under the assumption of ray theory. Using P wave residual data from the Norwegian Seismic Array (Norsar), we have obtained the map of velocity anomalies at various depths up to a depth of 126 km. The rms slowness fluctuation was found to be at least 3.1%. This is in agreement with estimates obtained from statistical analysis of P time fluctuations based on the Chernov theory. The three‐dimensional velocity anomalies are presented both by the generalized inverse and by the stochastic inverse solutions. We prefer the dual presentation because it gives the reader greater freedom in judging the results than a single ‘optimal’ solution. Both methods gave essentially the same results. The discrepancies, when they existed, were always explainable in terms of differences in the smoothing procedure which is explicitly given in the resolution matrix. The dominant features in the obtained three‐dimensional velocity image of the lithosphere beneath the Norsar array are low velocities to the west and high velocities to the east. The latter feature may be attributed to rocks of the Baltic shield which are undisturbed by the Caledonian orogeny or by Permian volcanism. Our result conclusively demonstrates the existence of strong small‐scale inhomogeneities to the bottom of the lithosphere. More theoretical and experimental works are needed to relate these velocity anomalies with the magma ascent mechanism which caused the

ISSN:0148-0227    

DOI:10.1029/JB082i002p00277

年代:1977

数据来源: WILEY

摘要:

Six nanogravity surveys have been made at two coastal areas of eastern Canada in order to study the gravity changes associated with seasonal groundwater fluctuations. The repeated surveys made with a LaCoste and Romberg model D gravimeter, D6, demonstrate that (1) seasonal changes in gravity differences (∼100 nm/s2, 10 µgal) at Cap Pelé, a region of relatively simple hydrogeology, are in good agreement with well and piezometer measurements, and (2) the corresponding changes (∼120 nm/s2, 12 µgal) at York Point; a region of complex hydrogeology, suggest a different interpretation of seasonal groundwater flow from that inferred from piezometer measurements alone. The results demonstrate that normal variations in groundwater may obscure gravity changes associated with tectonic movements and earthquake precursory effects. When normal variations in groundwater are known or are not significant, the results demonstrate that tectonic gravity changes as small as 20–30 nm/s2(2–3 µgal) may be detected at the 90% confidence level with the LaCoste and Romberg D meter. Such a gravity change corresponds to a ‘free air’ change in ele

ISSN:0148-0227    

DOI:10.1029/JB082i002p00297

年代:1977

数据来源: WILEY

摘要:

Lg propagates efficiently with predominant periods of about 1 to 1 1/2 s across the stable regions of Asia: the Indian shield, the Eurasian platform, the Tarim basin, and (less efficiently) eastern China. This suggests that the structure is relatively homogeneous and Q must be high in the crust in these regions. For paths across the Tien Shan, where the crust is abnormally thick, Lg is observed with somewhat less sharp arrivals, but nevertheless, frequencies are high, and amplitudes large. For paths along the Tien Shan, signals are weaker, and arrivals are still less sharp. Lg was not observed for paths crossing the Tibetan Plateau. We infer that the change in crustal structure on the margins of the Tibetan Plateau and at the Tien Shan may disrupt the wave guide for Lg and may scatter energy out of the relatively homogeneous wave guide of the more stable crust. Alternatively or in addition, Lg may not propagate across the Tibetan Plateau because of an unusual velocity structure or because of high attenuation in the crust.

ISSN:0148-0227    

DOI:10.1029/JB082i002p00307

年代:1977

数据来源: WILEY

摘要:

Multiple‐filter analysis is applied to surface waves with their paths almost entirely inside the Philippine Sea plate. The resulting dispersion data are used to study the crustal and upper mantle structures of the plate. Effort is made to examine the intraplate structural variations. Part of the Philippine Sea plate appears to have a thin plate thickness of about 30 km and is overlying a very low velocity mantle. Lateral structural variations in the plate are indicated by the dispersion data. The plate can be divided into several elongated regions, all of which are parallel to the eastern plate boundary and each having a rather distinct crustal and upper mantle structure. The active marginal basin region, located immediately to the west of the Mariana trench, probably represents an area of lithospheric generation. This is supported by the very low shear wave velocity, high heat flow, and magmatism. The Kyushu‐Palau ridge along the central portion of the plate seems to have a thicker lithosphere, in contrast to its two flank regions, the Philippine basin and the Parece Vela ba

ISSN:0148-0227    

DOI:10.1029/JB082i002p00317

年代:1977

数据来源: WILEY