摘要:

A Rb‐Sr and Sm‐Nd study was carried out on five ultrabasic inclusions, three alnoite host rocks, and one sample of basalt from Malaita in an effort to determine the isotopic stratigraphy of the mantle in this area. The results of this study show the presence of relatively undepleted mantle segments underlying relatively older depleted upper mantle. The alnoite samples, three megacrysts and one Iherzolite inclusion, and the basalt are relatively uniform isotopically and yielded εNd(T) ≈ +3.5 and εSr(T) ≈ −6 to +17 (T = 100 m.y. for the basalt and Iherzolite and 34 m.y. for the rest of the samples). It is suggested that a mantle segment of that isotopic composition exists at a depth of more than 100 km. This mantle segment is relatively undepleted. A Iherzolite inclusion with εNd(T) = 6.6 is included in the alnoite and indicates the location of a light rare earth element depleted layer overlying a less depleted mantle at a depth of approximately 100 km. The mantle source for the alnoite and basalt cannot be old, depleted oceanic mantle or subducted old continental crust but may come from a relatively young, slightly depleted mantle segment. The isotopic results are compatible with a mixture of about 99% depleted oceanic mantle homogenized with 1% continental crust possibly by metasomatic processes. Such a mixture is only slightly fractionated and has ƒSm/Nd≈ 0.13 and ƒRb/Sr≈ 0. Assuming that the basalt is characteristic of the Ontong Java Plateau, a volume greater than 108 km3of mantle source with εNd∼ 3.5, εSr∼ 0 was involved. This corresponds to a block of continental crust of ∼ 106km3which had to reach a depth greater than 100 km to produce the proposed nearly homogeneous isotopic mixture. Alternatively, the source of the Ontong Java Plateau may be a segment of a type of young continental crust with the appropriate isotopic signature which was subducted below 100 km. This would not require homogenization with a large

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02415

年代:1984

数据来源: WILEY

摘要:

An unusually large number of off‐ridge earthquakes have occurred within a broad region astride the Southeast Indian Ridge. The source mechanisms of nine of these earthquakes, in lithosphere from 4 to 27 m.y. old, have been determined with a formal inversion technique based on matching observed and syntheticPandSHwaves. All events occur in the oceanic mantle, from 8 to 22 km below the seafloor. The seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events. All of the off‐ridge earthquakes are characterized by normal‐faulting mechanisms, with theTaxes nearly parallel to the nearby ridge axis, in sharp contrast to typical mid‐ocean ridge or intraplate earthquakes. A variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes, including bending stresses, thermal stresses of various origins, differential subsidence at a fracture zone, off‐ridge volcanism, latitudinal plate motions on a nonspherical earth, basal shear stress from small‐scale convection, and plate‐wide perturbations of the stress field due to the continental collision between India and Asia. No single mechanism is adequate to explain all the observed off‐ridge normal faulting earthquakes. An apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the Antarctic and Indian plates are caused by different processes. The Antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the Kerguelen Plateau and the Southeast Indian Ridge, a region which has been suggested as the site of a horizontal “pipe” for flow in the asthenosphere from the Kerguelen hot spot to the vicinity of Amsterdam and St. Paul islands on the Southeast Indian Ridge. The off‐ridge earthquakes in the Indian plate are speculatively attributed to part of the plate‐wide response of the lithosphere to the ongoin

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02425

年代:1984

数据来源: WILEY

摘要:

The focal mechanism and depth were determined for nine small earthquakes (M0<1025dyn cm, M<5.5) that occurred in southern Pakistan and the northern Arabian Sea from an analysis of the vertical component of Rayleigh waves in combination with limited first‐motion data. Focal parameters were determined from the Rayleigh waves by using an event‐pair method of analysis. For earthquakes that are located very close to each other (<≈ 50 km), the event‐pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest (20–50 s). For events separated by more than ≈ 100 km the propagation effects are reduced for only the longer periods (≈ 40–50 s). The earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern Pakistan triple junction. The Owen fracture zone is a transform fault that accommodates right‐lateral motion between the Indian and Arabian plates. The plate boundary in the vicinity of the Murray ridge is also partially made up of transform segments that strike subparallel to the Owen fracture zone. Spreading centers may also exist in the vicinity of the Murray ridge but were not documented by seismic or other evidence. The slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion. The Arabian plate is being subducted beneath the Eurasian plate along the southern coast of Pakistan. Slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions. Left‐lateral motion is documented along the boundary between the Indian and Eurasian plates in southern Pakistan. The predicted direction of relative motion between these plates is not significantly different from that observed. Two of the earthquakes studied appear to be intraplate in nature. The depth and focal mechanism of one intraplate event, which may have occurred within the accretionary prism along the Makran coast, however, are based on limited data. The other intraplate earthquake occurred within the sedimentary deposits along the western edge of the Indian subcontinent near the Indian‐Eurasian plate boundary.Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐009; $2.50. Payme

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02444

年代:1984

数据来源: WILEY

摘要:

Focal mechanisms and depths for seven earthquakes in central Pakistan were determined from an analysis of Rayleigh waves of 20‐to 50‐s periods. In east‐central Pakistan, the nodal planes for some solutions strike obliquely to the grain of surface structures. This observation supports the contention of other workers that a thin surficial unit, decoupled from the basement along a surface of decollement, characterizes this region. In west‐central Pakistan, relative movement between the Indian and Eurasian plates is at least partially accommodated by seismic slip along the Chaman fault. Other faults, which are situated to the east of and lie subparallel to the Chaman fault, may also take up some of the relative plate motion. Observed activity within the zone of convergent‐type structure in central Pakistan may be a result of the greater component of convergence across the Indian‐Eurasian plate boundary north of Quetta, Pakistan.Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐010; $2.50. Payment must ac

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02459

年代:1984

数据来源: WILEY

摘要:

The Tehachapi trilateration network spans the intersection of the San Andreas and Garlock faults in southern California in the “Big Bend” region of the San Andreas fault. Analysis of data from 1973–1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults. The Palmdale network, spanning the San Andreas fault entirely within the Tehachapi network, showed increases of about 1 μstrain in both east‐west and north‐south extension in late 1979. The Tehachapi strains also jumped at this time, but the magnitude of the increase was only about one third that of Palmdale. The principal strain rates for Tehachapi over the time interval 1973–1983 areε˙1=0.08±0.01μstrain/yr andε˙2=‐0.12±0.01μstrain/yr, with the 1 axis directed N76°E. Strains were also computed for two subregions. The principal strain rates for the southeast Tehachapi region areε˙1=0.14±0.02μstrain/yr andε˙2=‐0.16±0.02μStrain/yr, with the 1 axis directed N73°E. This result differs significantly from the principal strain rates at Palmdale (ε˙1=0.21±0.02μstrain/yr,ε˙2=0.17±0.01μstrain/yr, with the 1 axis directed N71°E). For the northwest Tehachapi region,ε˙1=0.11±0.02μstrain/yr andε˙2=‐

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02471

年代:1984

数据来源: WILEY

摘要:

The short‐range geodetic data from northern Baja California, Mexico, for the period 1974–1982 are carefully analyzed. These data contribute to an understanding of the complex pattern of faulting associated with the Pacific‐North American plate boundary in this region. Survey precisions are evaluated and significant systematic errors are found to exist. A technique of studying a scale‐free displacement solution is developed as an aid to interpreting the data. We conclude that (1) the motion on the San Miguel‐Vallecitos fault system is presently in a right‐lateral sense and at a level that warrants trilateration surveys at least annually, (2) present geodetic data permit no statement about movement on the Agua Bianca fault, and (3) the mesa, whose flank delineates the Cerro Prieto fault in the Valle de Mexicali, lies in a zone which has undergone significant horizontal areal compression, at a rate of 14±5 ppm/yr, in addition to a right‐lateral tensor shear at a rate of 4±1 ppm/yr, oriented N(;33°±9°)W, bet

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02478

年代:1984

数据来源: WILEY

摘要:

The slopes of near‐infrared spectra between ∼1 and ∼2 μm from quartz‐bearing plutonic rocks are strongly correlated with rock chemistry determined by X ray spectrometry. The empirically derived predictive equations provide compositional data of adequate precision and resolution to discern patterns of regional geochemical variation in granitic batholithic rocks of southern California. As an analytical method, infrared spectrometry is rapid and inexpensive, and the method has potential in applications to direct field measurements and to data from aircraft and spacecraft scanner systems of relatively low spectral and spatial resolution, provided vegetative cover and surface alteration are not prohibitively

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02491

年代:1984

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02497

年代:1984

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02501

年代:1984

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB089iB04p02505

年代:1984

数据来源: WILEY