摘要:

Much of Utopia Planitia is underlain by polygonal terrane. Most of the troughs defining the polygons are 250 m to 1 km wide; typical diameters of polygons are 4‐5 km. These dimensions are orders of magnitude greater than the corresponding dimensions for familiar polygonal structures on Earth; consequently, the Earth structures are not valid mechanical analogues. Crater and superposition relationships indicate that trough formation was coeval with deposition of the polygonal terrane material. Based on crater age correlations with Elysium volcanism and outflow‐channel activity, either a volcanic or a sedimentary source for this material is possible. Tensile stresses responsible for the troughs were produced by cooling or dessication shrinkage plus differential compaction over a rough buried surface that is the structurally disrupted remnant of the cratered highlands. Shrinkage alone will produce polygons orders‐of‐magnitude smaller than those present, but differential compaction will reflect the scale and pattern of the buried topography, and thus control the scale and pattern of the giant polygons of

ISSN:0094-8276    

DOI:10.1029/GL013i008p00705

年代:1986

数据来源: WILEY

摘要:

The probability of the occurrence of an earthquake of M≥6.5 is estimated by applying logistic models to seismicity in the Hokkaido‐Tohoku region, northeast Japan. It is found that the probability of an earthquake occurrence near the arc junction off northeast Japan is high when a specific seismicity pattern appears; that is, seismic activity increases 7∼10 years or decreases 3∼6 years before an event of M≥6.5. Because seismic activity is currently extremely low near the arc junction, it is possible that an earthquake of M≥6.5 may occur there within a few years. Specifically, the probability estimated on the basis of logistic models is higher than 0.9 for the y

ISSN:0094-8276    

DOI:10.1029/GL013i008p00709

年代:1986

数据来源: WILEY

摘要:

The seismic slip rate along the Chile Trench estimated from the slip in the great 1960 earthquake and the recurrence history of major earthquakes has been interpreted as consistent with the subduction rate of the Nazca plate beneath South America. The convergence rate, estimated from global relative plate motion models, depends significantly on closure of the Nazca ‐ Antarctica ‐ South America circuit. NUVEL‐1, a new plate motion model which incorporates recently determined spreading rates on the Chile Rise, shows that the average convergence rate over the last three million years is slower than previously estimated. If this time‐averaged convergence rate provides an appropriate upper bound for the seismic slip rate, either the characteristic Chilean subduction earthquake is smaller than the 1960 event, the average recurrence interval is greater than observed in the last 400 years, or both. These observations bear out the nonuniformity of plate motions on various time scales, the variability in characteristic subduction zone earthquake size, and the limitations of recurrence time es

ISSN:0094-8276    

DOI:10.1029/GL013i008p00713

年代:1986

数据来源: WILEY

摘要:

A model is proposed to understand the magma genesis beneath the volcanic front which overlies the dipping seismic zone with a constant depth of about 110 km in most subduction zones. It is suggested that the constant depth of 110 km is governed by the decomposition of amphibole in the mantle wedge. Hydrous phases in the subducted slab decompose at levels shallower than 100 km, that is, beneath the fore‐arc region. The slab‐derived H2O, enriched in incompatible elements with larger ionic radii, reacts with the overlying mantle wedge materials to form polluted amphibole peridotite. The polluted peridotite is transported downward on the slab by the induced convection in the mantle wedge. Amphibole in the dragged peridotite decomposes at a depth of about 110 km, just beneath the volcanic front. H2O released migrates upward to reform amphibole peridotite in the higher‐temperature and lower‐pressure region. When the front of amphibolization reaches a level at which the solidus temperature of amphibole peridotite is distributed, initial magmas are produced and rise as a form of mantle diapir. The mantle diapir stops rising to segregate a primary magma for lavas on the volcani

ISSN:0094-8276    

DOI:10.1029/GL013i008p00717

年代:1986

数据来源: WILEY

摘要:

The March 3, 1985 central Chile earthquake (Ms=7.8) ruptured a well studied seismic gap along the Chilean subduction zone. The epicenter of this event is located near the center of an approximately 300 km long region which ruptured in a great event in 1906 (Mw=8.2). The northern portion of the 1906 zone has since ruptured in 1971 (Ms=7.5) and 1973 (Ms=6.7). We have determined the rupture history of the 1985 central Chile earthquake from the deconvolved source functions of body waves (both P and PP). The source functions show one major pulse of moment release, the onset of which occurs about 16 seconds after an initial small pulse. The azimuthal directivity in the P waves indicates that the rupture front propagated from the epicenter southward and that the moment release of the major pulse is concentrated in a region between the epicenter and approximately 75 km south of the epicenter. We conclude that the region of high moment release is the dominant asperity and was loaded by slip in the relatively weaker northern region.

ISSN:0094-8276    

DOI:10.1029/GL013i008p00721

年代:1986

数据来源: WILEY

摘要:

Convective plumes that rise above Hawaiian‐style fire fountains consist of volcanic gases, aerosols, fine ash, and entrained heated air. Plume theory has been applied to observational estimates of the rate of thermal energy release from large fire fountains. The theoretically predicted heights of maintained plumes agree very well with the heights found from actual observations. Predicted plume heights for both central‐vent (point‐source) and fissure (line‐source) eruptions indicate a stratospheric penetration by plumes that form over vents with very high magma‐production rates. Flood basalt fissure eruptions that produce individual lava flows with volumes>100 km³ at very high mass eruption rates are capable of injecting large quantities of sulfate aerosols into the lower stratosphere, with potentially drastic short‐term atmospheric consequences, like acid precipitation, darkening of the sky, and clim

ISSN:0094-8276    

DOI:10.1029/GL013i008p00725

年代:1986

数据来源: WILEY

摘要:

Volcanic glass separates (colorless to dark brown) from the KBS tuff of northern Kenya have been studied with a combination of transmission electron microscopy and low‐temperature ac susceptibility and dc magnetization experiments. The darker of these glasses exhibit classic superparamagnetic behavior, the origin of which lies in a spatially‐uniform precipitate of magnetite, which is present as ∼1% by weight in glass shards with the highest susceptibility. In any given glass separate (obtained by magnetic separation) this precipitate has a surprisingly narrow size distribution. A theory for the origin of the precipitate is nucleatation and growth in quenched glasses at temperatures of ∼1000‐1300°K; an experiment demonstrates the feasibility of this idea. These glasses provide us with a sample for experimental investigations of physical properties of naturally‐occurring dispersed magnetic phases at the lower limit of physical dimension that can be attained in the cryst

ISSN:0094-8276    

DOI:10.1029/GL013i008p00729

年代:1986

数据来源: WILEY

摘要:

Detailed demagnetization experiments isolated a characteristic remanent magnetization in ten stable sites from the upper Cretaceous Winthrop and Midnight Peak Formations in the Methow‐Pasayten belt of north‐central Washington. This remanence agrees best between opposite limbs of a fold (the Goat Peak syncline) when corrected for 46% of tilt. This is consistent with magnetization acquired during deformation. Synfolding magnetization may have been facilitated by a thermo‐chemical event associated with synkinematic intrusions along the axis of folding. The mean direction (D=12.0°, I=61.1°, Alpha‐95=4.8°) is highly discordant with respect to the expected direction for north‐central Washington. This discordance points to about 1,400 km of poleward transport and 48° of clockwise rotation between 93

ISSN:0094-8276    

DOI:10.1029/GL013i008p00733

年代:1986

数据来源: WILEY

摘要:

A new mechanism by which viscous remanent magnetization, VRM, can be acquired is proposed. This VRM probably is the major cause of overprinting during long magnetic polarity intervals.

ISSN:0094-8276    

DOI:10.1029/GL013i008p00737

年代:1986

数据来源: WILEY

摘要:

The "Msissi norite" in Morocco (30.93°N, 4.71°W) from where one of the paleomagnetic reference pole for Africa in Devonian times has been derived, has been revisited and dated. Petrological examination shows that the rock is an alkaline gabbro (teschenite). K‐Ar analyses on fresh biotite yield a 136‐139 Ma age, which infirms the supposed Devonian age. The paleomagnetic study displays several distinct magnetic compo

ISSN:0094-8276    

DOI:10.1029/GL013i008p00741

年代:1986

数据来源: WILEY