摘要:

This paper presents a model for the thermal and chemical evolution of a global magma ocean created by impact heating during the earth's accretion. Detailed calculations are given for a melt depth of 120 km for end member picritic and peridotitic compositions. Application of fluid mechanics and heat transfer principles shows as a first approximation that the chemical evolution can be portrayed by idealized fractional crystallization occurring only on the the bottom of the magma ocean. The fractional crystallization sequence for the picritic model is 90 km of mafics overlain by 15 km of ol‐sp‐gabbronorite and topped by 15 km of iron‐rich leucodiorite; whereas the peridotite sequence consists of 99 km of dunite, harzburgite, and websterite overlain by 5 km of ol‐sp‐gabbronorite and capped by 16 km of ferroleucogabbronorite. Although subsolidus convection would rehomoginize all but the felsic layers of the picritic model, the layers of the peridotite would remain intact owing to their stable density profile. The mineralogy of the felsic layers is largly independent of the basic model assumptions. However, the composition of the mafic layers is model dependent. The most likely results for the mafic mafic layers (lherzolite or primitive mantle peridotite) are similar to mantle xenoliths, suggesting that a terrestrial magma ocean may have existed. However, comparison of the chemistry of the calculated upper felsic layer with that of the Canadian shield shows that crystallization of a global magma ocean would not directly produce the Archean continental crust. Multiple melting and differentiation of the Fe‐leucodiorite to Fe‐leucogabbronorite layer and of the underlying primitive mantle would be required to form an Archean gr

ISSN:0148-0227    

DOI:10.1029/JB088iB06p04963

年代:1983

数据来源: WILEY

摘要:

Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip. Because steady state subduction does not contribute to the deformation at the free surface, deformation observed there is completely equivalent to that produced by the slip cycle alone. The response to that slip is simply the response of a particular earth model to embedded dislocations. For a purely elastic earth model, the deformation cycle consists of a coseismic offset followed by a linear‐in‐time recovery to the initial value during the interval between earthquakes. For an elastic‐viscoelastic earth model (elastic lithosphere over a viscoelastic asthenosphere), the postearthquake recovery is not linear in time. Records of local uplift as a function of time indicate that the long‐term postseismic recovery is approximately linear, suggesting that elastic earth models are adequate to describe the deformation cycle. However, the deformation predicted for a simple elastic half‐space earth model does not reproduce the deformation observed along the subduction zones in Japan at all well if stick slip is restricted to the main thrust zone. As recognized earlier by Shimazaki, Seno, and Kato, the uplift profiles could be explained if stick slip were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km, but independent evidence suggests that stick slip at such depths is

ISSN:0148-0227    

DOI:10.1029/JB088iB06p04984

年代:1983

数据来源: WILEY

摘要:

The appendixes are available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document B83‐003; $2.50. Payment must accompany order.We have applied40Ar−39Ar dating to potassium‐rich granitic xenoliths and host basalt from the Pleistocene Big Pine volcanic field, California. These xenoliths had been partially degassed upon their inclusion in the basaltic lava. Argon released from the xenoliths at extraction temperatures below ∼900°C yielded plateau ages indistinguishable from the total K‐Ar age of the basalt. The best estimate of the age of eruption was 1.18±0.05 (2σ) m.y.40Ar extracted at higher temperatures included radiogenic argon not degassed from the late Cretaceous xenoliths 1.18 m.y. ago, causing an increase in the apparent age for the high‐temperature fractions. The agreement of the low‐temperature xenolith plateau ages and the basalt K‐Ar ages demonstrates that40Ar−39Ar analysis of xenoliths may be used to measure the age of eruption of very young lavas. This is significant because in many instances ages cannot be reliably determined by analysis of

ISSN:0148-0227    

DOI:10.1029/JB088iB06p04997

年代:1983

数据来源: WILEY

摘要:

A mathematical model for historical horizontal crustal deformation is presented for the southern California region that extends northward from Mexico for approximately 100 km to 33.5°N latitude. Model parameters are estimated from triangulation, trilateration, and astronomic azimuth data observed during the past century. Also geologic and seismic data indirectly participate in specifying the model. The model relates crustal deformation to the episodic movement experienced during large earthquakes (M≥ 6), secular slip rates on geologic faults, and secular strain rates over large geographic areas. The episodic movement is modeled in accordance with the theory of dislocation in an elastic halfspace. For the secular motion the modeled region is partitioned into a mosaic of districts that are allowed to individually translate, rotate, and homogeneously deform as a linear function of time. The derived strain pattern supports the hypothesis that the modeled region spans the Pacific‐North American plate boundary. Secular shear strain is maximum in the NW direction parallel to the major faults, and shear strain rates decrease with distance from a NW–SE trending strip. To the region's north this strip is 40 km wide and is bounded on the west by the San Jacinto fault zone and on the east by the southern extension of the San Andreas fault. To the south this strip converges in width onto the Imperial fault. The secular velocity for a point near the city of San Diego relative to a point at the region's northeast corner is 4.3±3.1 cm/yr in the direction N 17°W±5°. This rate provides a rough estimate for the relative velocity between the Pacific and North Ameri

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05009

年代:1983

数据来源: WILEY

摘要:

This paper presents a theory to account for the main features of the tidal response of an isolated plane fracture as revealed by the water level in an intersecting open borehole. According to the theory the diurnal and semidiurnal tidal constituents (both amplitude and phase) of water level variations depend on the fracture aperture, the orientation (dip and strike) and the radial extent of the fracture, and the compressibility of the asperites. The theory is applied to water level observations made in six open boreholes located in crystalline rock at a test site at the Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, in Chalk River, Ontario, Canada. The boreholes were uncased and intersected many open fractures so that a definitive test of the theory could not be made. However, in four cases a fracture model could be found which fitted the observations within the experimental error.

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05025

年代:1983

数据来源: WILEY

摘要:

Two aspects of the brittle deformation of Sioux quartzite have been studied: water‐induced changes in the amount of energy consumed during fracture of the quartzite and the occurrence of microcracking during crack propagation. Wedge‐loaded double cantilever beam test specimens were cracked in several environments (air, water, aqueous solutions of NaOH, HCl, NaCl, methanol) at crack velocities of 4.2×10−2mm/s and 4.2 mm/s and temperatures of 17°–60°C. Scanning electron microscopy and the determination of enhanced permeability were used to detect fracture induced microcracks. The fracture experiments measured the fracture energy of the quartzite, the specific energy required to produce a unit increment of crack extension, using the analysis of linear elastic fracture mechanics. The fracture energy of Sioux quartzite in water is 15% lower than that of quartzite cracked in air (ambient humidity). The dependence of the quartzite fracture energy on crack velocity (time) and on the concentration of water in the test environment is characteristic of the occurrence of stress corrosion as a mechanism of enhanced crack growth in the presence of water. Other theories of the chemomechanical weakening of rock, reduction in surface energy and changes in dislocation mobility, do not explain the observed environmentally induced changes in the fracture energy of the quartzite. Although increased crack velocity in quartzite cracked in hot water is evidence of thermally enhanced water weakening, the fracture energy of quartzite cracked in hot air or hot water (40°–60°C) is 12–13% higher than room temperature values. The increase in fracture energy is attributed to changes (reduction in residual stress, existence of thermal stresses) in the net‐stress field responsible for microcracking and crack propagatio

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05037

年代:1983

数据来源: WILEY

摘要:

All techniques that currently exist to invert arrival time data from earthquake sources for velocity structure rest on a local linearization of the nonlinear equations relating these data to the velocity model. We have studied the limitations imposed by this approximation using synthetic data in which a one dimensional velocity model is the only variable. We have found that this problem is sufficiently nonlinear that some iterative scheme is generally required to obtain results that are quantitatively correct. On the other hand, convergence studies using the algorithm recently developed by Pavlis (1982) suggest that this problem is somewhat better behaved than one might think. Convergence to a solution linearly close to the truth is fairly readily achieved whenever the resolution of the data is sufficient to resolve the structure of the true velocity model. Convergence to solutions that are not linearly close where found to occur, however, when the true velocity model contained significant unresolvable structure.

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05047

年代:1983

数据来源: WILEY

摘要:

Recently it has become apparent that teleseismic detection has decreased substantially in many regions of the world. The major decrease was related to the closure of the VELA arrays in the United States during the late 1960's. This detection decrease has been recognized in South and Central America, Mexico, the Kuriles, the Caribbean, Tonga, and the New Hebrides. In this paper the effect of the closure of these arrays on the reporting of events in the Aleutian Island Arc is examined. In the Aleutians, the detection history is complicated by the short‐term installation of a local network on and near Amchitka Island during the early 1970's. The temporal coincidence of the installation of this network and the closure of the VELA arrays delayed the detection decrease in the central Aleutians until the Amchitka network was closed in early 1973. Reporting in the eastern Aleutians was unaffected by the installation of the Amchitka network. In that region the detection decreased between 1968 and 1970, the time of the closure of the VELA arrays. New techniques have been developed which make it possible to determine the effect of station installation or closure on the reporting in some regions. These techniques rely on plots which show the distribution of an observed seismicity rate change in the magnitude domain. These plots make it possible to recognize probable detection changes and to determine quantitatively magnitude cutoffs which avoid these changes. The magnitude level at which these cutoffs are made is termed the minimum magnitude of homogeneity (mminh). The reporting of events with mb≤4.6 in the Aleutians decreased substantially during the mid‐1970's, somminhin this region is 4.7. This is different from the magnitude of completeness (mminc) which ismb= 5.0±0.1. If one is interested in examining seismicity rates for changes which may be precursors to earthquakes, then awareness of detection‐related changes and magnitude cutoffs which avoid these changes are essential for meaningful

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05056

年代:1983

数据来源: WILEY

摘要:

During the AMAR 1978 Expedition, over 200 separate basalt samples were collected byAlvinand dredge from the Narrowgate and AMAR segments of the Mid‐Atlantic Ridge near 37°N to determine the origin of reversely magnetized rocks previously recognized in the FAMOUS rift area to the north and to study the variations in rock magnetic properties that occur along the strike of the Mid‐Atlantic Ridge. Magnetic intensity determinations of rocks collected in the FAMOUS area indicate that intensity decays with time by a factor of 5 in less than 106years. Rocks with low intensities have high Curie temperatures (Tc) indicating that low‐temperature oxidation of the magnetic minerals takes place soon after the basalts are extruded and is pervasive in crust older than 106years. Magnetic intensity (Jnrm) of the uppermost oceanic crust can be expressed as a function of age (t) by the expression J = 5.82t−1/3. By measuring Jnrmof the surface rocks from the three valley segments, the relative ages of the surface rocks from the FAMOUS, AMAR, and Narrowgate areas can be estimated to be 1×104years, 3×104years, and 4×104years, respectively. One exception is Mt. Mars, located in the center of the Narrowgate area that has high Jnrmvalues (84 × 10−3emu/cm3) and low Tcvalues (165°C), suggesting that this is an area where volcanic activity is becoming reactivated following a period of quiescence. Of the 55 oriented samples collected, eight are reversely magnetized. Although three of the reversed samples have Curie temperatures above 300°C and may have undergone self‐reversal, the other five reversed samples have magnetic properties similar to normally magnetized rocks and may have been misoriented or extruded during a short reversed event within the Brunhes epoch. Because most of the surficial rock samples are normally magnetized, it is suggested that the previously recognized reversed zones, which appear in a deep‐tow magnetometer profile that crossed the Narrowgate area, are located at depth and are the result of the spreading axis moving into older Matuyama crust that is now covered by m

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05065

年代:1983

数据来源: WILEY

摘要:

The low‐degree portion of a number of recent global gravitational fields have been tested for accuracy using precise accelerations measured on 24‐hour satellites. These measurements are independent of the data in the field models. The results show that the most accurate of the recent solutions have degree error variances significantly less than 100 mm (geoid height equivalent) for all degrees less than five. The broad and lasting features of ocean circulation should now be visible when comparing the geoid computed from this model with the average sea surface computed from satellite altimetry. However, before accurate mapping is possible, the equivalent accuracy of the mean altimetric surface at long wavelengths needs to be verif

ISSN:0148-0227    

DOI:10.1029/JB088iB06p05083

年代:1983

数据来源: WILEY