摘要:

We have applied40Ar‐39Ar dating, using stepwise thermal extraction of Ar, to five potassium‐rich granitic xenoliths and their host basalt from Sawmill Canyon in the Sierra Nevada, California. Previous K/Ar analyses showed the age of the basalt to be roughly 100,000 years or less. The xenoliths, which had accumulated large amounts of radiogenic40Ar since their crystallization ∼100 m.y. ago, were partially degassed upon their inclusion in the basaltic magma. Ar released from the xenoliths in the laboratory at temperatures substantially below the melting temperature of the basalt, was created since the host magma cooled. Isotopic compositions of Ar released from the xenoliths in several extraction steps at temperatures below ∼900°C were colinear in36Ar/40Ar versus39Ar/40Ar diagrams and defined isochrons giving a mean age of degassing of 119,000±7000 (2σ) years.40Ar extracted at higher temperatures included ancient radiogenic40Ar that was never diffused from the xenoliths during immersion in the magma. This40Ar caused an increase in the apparent age for the high‐temperature extractions. The high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional K/Ar dating of sanidine.40Ar‐39Ar analysis of granitic xenoliths to date young basaltic lava flows may prove to yield results superior to those found from analysis of the lava itself. Establishing the age of eruption of the basalt flow in Sawmill Canyon establishes age limits for two Sierran glaciations which left moraines stratigraphically above and below the lava. Thus the younger glaciation must be Wisconsin; the older must be pre‐

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01033

年代:1984

数据来源: WILEY

摘要:

A detailed record of the lower Jaramillo (reversed to normal) polarity transition was obtained from a southern hemisphere, deep‐sea sediment core (latitude = 35.91°E, longitude = 59.97°E) The record consists of over 850 samples taken across 140 cm of section. The transition itself is recorded across approximately 70 cm and is represented by more than 475 specimens from about 160 levels giving intermediate directions. The transition is identified by a nearly 180° shift from directions in good agreement with a reversed, axial dipole field to those closely aligned with a normal, axial dipole field for the core site latitude. The inclinations shallow gradually early in the reversal and pass through very steep negative values (−80°) late in the transition. The declinations show little appreciable variation until the inclinations have moved through the near vertical, and then slowly approach values in agreement with a normal polarity field. An intensity low accompanies the directional change during which the intensity drops to less than 15% of the maximum values observed in this sample interval. The intensity fluctuation spans a wider interval than the directional change, decreasing prior to any systematic change in the directions and then increasing to pre‐transition levels by the same depth at which the directions have stabilized. The VGP path constructed for this reversal is longitudinally constrained to a certain extent, between 140° and 230° for intermediate VGP latitudes and is roughly centered 120° from the site longitude. This path is therefore a far‐sided VGP path in Hoffman's [1977] terminology. Assuming a constant sedimentation rate (67m/Ma) through the Jaramillo Subchron, the duration of the transition is estimated to be 11,200 years to 4,500 years (depending on the criteria) for the directional change, whereas the associated intensity variation occurred over 15,000 to 20,000 years. Considered together with records of the most recent reversal(Matuyama/Brunhes) in light of current transitional field models, this record strongly suggests that the lower Jaramillo transitional field was dominated by different harmonics than the Matuyam/Brunhes tran

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01049

年代:1984

数据来源: WILEY

摘要:

Paleomagnetic data demonstrating three late Tertiary excursions in the direction of the geomagnetic field recorded in sequences of basaltic lavas on the island of Oahu, Hawaii were published by R. R. Doell and G. B. Dalrymple in 1973. We have determined geomagnetic paleointensities by the Thelliers' method for 14 lavas from the three sites. During these experiments, considerable difficulty was encountered because of the presence of titanomaghemite in many lavas and the contamination of natural remanent magnetization by lightning in many others. Moreover, we often observed the production of spurious high‐temperature chemical remanent magnetization during the Thellier experiments. An analysis of this particularly troublesome problem is presented. Two of the sites showed low paleointensities associated with angular departures of the paleomagnetic field direction from that of a geocentric axial dipole, which suggests that these excursions represent aborted reversals or fragments of reversals. At the third site, however, the paleointensity did not become low as the field diverged. This excursion may reflect the variation of a large nondipole source near Hawai

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01059

年代:1984

数据来源: WILEY

摘要:

A spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of Magsat data. Global maps representing the contribution due to terms of the expansion aboven= 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar, even in polar regions. An expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high‐order field structure at a constant altitude than do component maps derived independently.The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐008; $2.50. Payment must accompany or

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01070

年代:1984

数据来源: WILEY

摘要:

Postglacial rebound appears to have been observed gravitationally by the Lageos satellite. Sixty‐four observations of the orbital node made over a 5‐year time interval reveal an acceleration of ( −8.1±1.8)×10−8arc sec d−2due to a source which is not presently modeled in the GEODYN orbit determination computer program. This acceleration cannot be explained by the ocean tide with an 18.6‐year period, assuming it to be an equilibrium tide. Instead it seems to be due to postglacial rebound, which changes theJ2coefficient in the spherical harmonic expansion of the earth's gravitational field at the rate of ( −8.2±1.8)×10−19s−1this, in turn, accelerates the node. This rate does not agree with the −32×10−19s−1predicted by Wu and Peltier's L2 model, which has upper and lower mantle effective viscosities of 1021and 1022Pa s, respectively. It does agree well with their L1 model, which gives about −10×10−19s−1. Since the effective viscosity is 1021Pa s throughout the entire mantle in the L1 model, the results support the contentions that (1) the effective viscosity is near 1021Pa s everywhere in the mantle, and (2) this relatively low value for the effective viscosity may have permitted several degrees of polar wander due to glaci

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01077

年代:1984

数据来源: WILEY

摘要:

Images of sea surface undulations in the South Pacific have been constructed from GEOS 3 and SEASAT altimeter data. Height discrepancies at crossover points, associated with long‐wavelength radial orbit error, were suppressed by taking along‐track derivatives of the ascending and descending profiles. These geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical. Finally, the results are displayed by using the hill shading technique, where gray‐tone images represent the innner product of the deflection vector with an assigned sun vector. Less apparent sea surface undulations can be enhanced by varying the sun's zenith and azimuth. Shorterwavelength sea surface undulations reflect seafloor topography. For instance, fracture zones (FZ's) appear as elongated sharp steps in the sea surface, while seamounts produce circular bumps. Since large areas of the South Pacific are unsurveyed, many previously undetected features appear on the images. Comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to Easter Island's expression. The dominant features in the images, however, are the large age‐offset FZ's such as the Eltanin and Udintsev FZ's. The images reveal that the Eltanin FZ is connected to the Louisville Ridge; combined they produce a continuous geoid signature across most of the South Pacific. This supports the hypothesis of Hayes and Ewing (1968) that the Louisville Ridge is the northwest extension of the Elt

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01089

年代:1984

数据来源: WILEY

摘要:

Geoid undulations from satellite Doppler stations have been compared to geoid undulations computed using three procedures. The first procedure combines potential coefficients with uncorrected 1°×1° free air anomalies in a 10° cap surrounding the computation point. The second procedure uses the Helmert second condensation procedure for anomaly reduction where terrain corrections and topographic indirect effects are considered. The third procedure uses a set of potential coefficients to degree 180. After correcting the original Doppler undulation for a −0.4‐ppm scale change, and a 4‐mZaxis bias, comparisons were made at 10 sites located in mountainous areas of the western United States and 10 sites in the eastern United States where the topography was relatively flat. The mean difference (Doppler minus gravimetric) for the 10 western stations was 1.65 when no corrections were applied to the gravity data, while the difference was reduced to 0.10 m when the terrain corrections and indirect effects were included. For the 10 eastern stations the systematic difference (excluding the negligible terrain effects ) was 0.42 m. For western sites the standard deviation of the differences was ±0.96 m (with no gravity corrections) and ±0.93 m with the correction applied. In the eastern sites the standard deviation was ±0.55 m. Similar results were found with the potential coefficients to degree 180. Our major conclusion is that the neglect of terrain corrections, and to a lesser extent the indirect effect, can cause systematic errors of the order of 1.5 m (for a cap size of 10°) in geoid undulations for stations in moun

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01105

年代:1984

数据来源: WILEY

摘要:

Two adjacent geodetic networks span a 105‐km extent of the Wasatch fault zone in north‐central Utah. One of the networks, located in the vicinity of Salt Lake City, was observed by the National Ocean Service/National Geodetic Survey (NOS/NGS) in 1962–1963 (triangulation mode) and again in 1973–1974 (combined triangulation‐trilateration mode). To the north, in the vicinity of Ogden, the U.S. Geological Survey (USGS) performed five separate trilateration surveys of the second network during the 1972–1981 interval. When fit to a model assuming both temporal and spatial homogeneity for the strain rate, the two independent data sets yield mutually consistent strain rate tensors. Subsets of the NOS/NGS data, however, suggest spatial variation in the strain field, and a subset of the USGS data indicates a possible temporal variation in the strain rate. The quality of the data is such that we are uncertain whether or not these apparent variations represent actual tectonic signals. The strain rate tensors for the two complete data sets suggest an underlying deformational pattern characterized by east‐west (E‐W) maximum principal strain (extension positive). This pattern may also be inferred from regional seismicity and Quaternary geology. The suspected variations would imply that the area to the north of Salt Lake City experienced significant E‐W compression preceding 1978, in contradiction to the underlying strain pattern. This compression may be caused by the mountainous block, eastward adjacent to the fault zone, displacing westward along a shallow‐angle, west dipping detachment surface. We speculate on the existence of an E‐W trending zone of weakness, for example, secondary faults, located north of Salt Lake City to explain how the stress field to the north may be decoupled from that to the south where E‐W compression has not been detected. While seismic and geologic information contribute some plausibility to our speculations, the evidence for the existence of the suspected var

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01113

年代:1984

数据来源: WILEY

摘要:

Correlations of level changes with topography demand an assessment of the effect of an irregular free boundary on surface deformation. This is examined through a plane strain elastic model with topography of small slope, subjected to a change in the far‐field horizontal stress. To leading order, vertical surface displacements due to the topographic perturbation are proportional to the local relief. Elevation‐dependent uplift results from a compressional change, and downdrop results from a tensional change. The model further predicts that the ratio of elevation change to elevation is proportional to and of the same order of magnitude as the regional strain. Horizontal strains are locally perturbed by topography as well, with the magnitude scaling with the local slope. The predicted localization of level changes is very small in aseismic regions and cannot contribute significantly to measured correlations. A test case in southern California bears this out, with strains of order 10−6accompanied by elevation change to elevation ratios of order 10−4. Releveling following the Nankaido‐Tonankai earthquakes, which induced large coseismic and postseismic strains, reveals scattered examples of elevation‐dependent level changes. However, when compared to modeled strains, the correlations are again at least an order of magnitude larger than the localization effect predicted by the elastic model. Although the topographic perturbation of vertical displacements appears to be unmeasurably small, local variations in horizontal strain or borehole dilatation across steep relief may be discernible with current

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01123

年代:1984

数据来源: WILEY

摘要:

A general technique is proposed for estimating rupture geometry from measurements of body wave durations, wherein measurements of the overall signal duration or the duration of a single pulse identified in all of the arrivals are inverted to estimate the geometry of the overall rupture or of a specific subevent, respectively. The complexity of a waveform is determined by comparing the overall signal duration to the duration of the most prominent pulse in the waveform; this complexity is used to estimate the ratio of the average rupture velocity to the subevent rupture velocity. The shear waves radiated by eight moderate (3.6≤ML≤4.9) multiply recorded aftershocks of the 1975 Oroville, California, earthquake are analyzed using measurements of the corner frequency, the characteristic frequency, the pulse duration, and the signal duration. Comparison of these four different estimates of source size indicates that the corner frequency and the characteristic frequency are more sensitive to the dominant subevent size than to the overall event size. The difference between the estimates of source size determined from the signal duration measurements and the spectral measurements increases as the rupture complexity increa

ISSN:0148-0227    

DOI:10.1029/JB089iB02p01132

年代:1984

数据来源: WILEY