摘要:

Magnetostratigraphic studies of a continental sedimentary sequence in the Clark's Fork Basin, Wyoming and a marine sedimentary sequence at Gubbio, Italy indicate that the Paleocene‐Eocene boundary occurs just stratigraphically above normal polarity zones correlative with magnetic anomaly 25 chron. These data indicate that the older boundary of anomaly 24 chron is 52.5 Ma. This age is younger than the late Paleocene age assigned by LaBrecque et al. [1977] and also younger than the basal Eocene age assigned by Ness et al. [1980]. A revised magnetic polarity time scale for the Paleocene and early Eocene is presented in this paper. Several changes in the relative motion system between the Pacific plate and neighboring plates occurred in the interval between anomaly 24 and anomaly 21. A major change in absolute motion of the Pacific plate is indicated by the bend in the Hawaiian‐Emperor Seamount chain at ∼43 Ma. The revised magnetic polarity time scale indicates that the absolute motion change lags the relative motion changes by only ∼3‐5 m.y. rather than by>10 m.y. as indicated by previous polarity ti

ISSN:0094-8276    

DOI:10.1029/GL008i004p00301

年代:1981

数据来源: WILEY

摘要:

The East Pacific Rise collided with western North America in the last 30 m.y. A model in which the preexisting transform geometry was preserved as the actively spreading ridge was subducted under the continent accounts for the gross features of tectonic geometry of the west coast including the mountain blocks and deep Neogene sediment basins of the Transverse Ranges. The present day geometry in this model has transform motion at the surface along the San Andreas Fault and parallel features with decoupling to the East along the old subduction interface.

ISSN:0094-8276    

DOI:10.1029/GL008i004p00305

年代:1981

数据来源: WILEY

摘要:

The high surface temperature of Venus implies a permanently buoyant lithosphere and a thick basaltic crust. Terrestrial style tectonics with deep subduction and crustal recycling is not possible. Overthickened basaltic crust partially melts instead of converting to eclogite. Because mantle magmas do not have convenient access to the surface the40Ar abundance in the atmosphere should be low. Venus may provide an analog to Archean tectonics on the Earth.

ISSN:0094-8276    

DOI:10.1029/GL008i004p00309

年代:1981

数据来源: WILEY

摘要:

Models indicate that the high heat flow from Io would result in a very thin (approximately 5 km) silicate lithosphere overlying a molten interior, if all heat was transported through the lithosphere via conduction. However, the presence of mountains with relief in excess of 10 km would seem to demand a thick lithosphere, at least locally. A significant fraction of Io's heat flow may be transported via advection through volcanoes. Advective heat transfer permits a thicker lithosphere than does pure conduction, possibly reconciling Io's high heat flow with the rugged topography.

ISSN:0094-8276    

DOI:10.1029/GL008i004p00313

年代:1981

数据来源: WILEY

摘要:

A seismic crisis, with a mb= 5.3 main shock, occured in the Somali Republic, East Africa (10°N, 43°E) from April to November 1980. Up to 2000 earthquakes with ML>2 have been recorded during this period. This earthquake sequence is of particular interest because it occured in a seismically inactive zone and include a rather long aftershock sequence. Two groups of epicenters were identified using a relative location procedure. Aftershocks observed during the first two weeks fall very close to the Borama city, while latter shocks are situated 10km west. This may suggest that the second group of earthquakes has been induced by the first one. These data show that the continental margin between the Somalian Plateau shield and the quasi‐oceanic crust of the Afar‐Gulf of Aden region, remains active to day and is relevant to intraplate seism

ISSN:0094-8276    

DOI:10.1029/GL008i004p00317

年代:1981

数据来源: WILEY

摘要:

Loading machine stiffness is considered in an attempt to simulate strain accumulation processes prior to failure in earthquakes. We impose the similarity condition that the inelastic deformation in a test specimen relative to the loading machine corresponds with that in a spherical seismic region relative to the surrounding earth. From this constraint the appropriate ratio between the stiffnesses of machine and specimen is determined to be 1.5 ∼ 2.3. Preliminary experiments were carried out for sandstone samples. A qualitative difference between acoustic emission sequences was observed depending on whether the conditions were similar or dissimilar to the earth based on the scaling assumption. In the former case, a subsidence in acoustic emission activity prior to the main fractures was clearly and systematically observe

ISSN:0094-8276    

DOI:10.1029/GL008i004p00321

年代:1981

数据来源: WILEY

摘要:

The permeability and strength of San Andreas fault gouge from the Cienega Valley was measured under confining pressures up to 2 kbar. The gouge was composed almost entirely of clay minerals, predominantly montmorillonite and mixed‐layer clays. Permeabilities ranged between 1 and 10 nanodarcies for confining pressures of a few hundred bars to 1 kbar, and less than 1 nanodarcy for pressures greater than 1 kbar. Permeability was sensitive to confining pressure and differential stress, but did not depend significantly on accumulated strain.The low strength of saturated clay is often attributed to the buildup of excess pore pressure as the clay is stressed. These experiments, performed under drained conditions where excess pore pressure was not created, suggest that loosely bonded interlayer water in the hydrated clays can generate a pseudo‐pore pressure that serves to lower the strength of the go

ISSN:0094-8276    

DOI:10.1029/GL008i004p00325

年代:1981

数据来源: WILEY

摘要:

Simple hydraulics of open channel water flow is used to find the water level response to earth tilt along the 200 km long Los Angeles Aqueduct. The water level changes are shown to depend on the length scale of the tilt field. Tilts over lengths greater than 10 km permit the water to assume an essentially equilibrium level throughout the tilt field; change in water depth is therefore directly proportional to the degree of slope change. Tilts occurring on length scales shorter than 10 km require the use of an integral relation between tilt and water depth. The relation is used to model a tilt field corresponding to an anomalous water level event observed on the Los Angeles Aqueduct.

ISSN:0094-8276    

DOI:10.1029/GL008i004p00329

年代:1981

数据来源: WILEY

摘要:

We have developed a rapid method of characterizing the fine‐scale variation (sub‐micron to hundreds of microns, µm) in the grain size of magnetite in long sediment cores. This method is a new rock magnetic tool and can be readily applied to the following disciplines: (1) environmental studies; (2) hydrology; (3) petrology; and (4) geomagnetism.The method consists of a comparative plot of the depth variation of two magnetic parameters. The first parameter, low field (less than 0.1 millitesla, mT, or 1 Oe) susceptibility in a steady field is plotted on the abscissa. The second parameter, anhysteretic remanent magnetization (ARM) imparted by a smoothly decreasing AF of peak value 100 mT (1,000 Oe) in the presence of a weak steady field (0.01‐0.1 mT, or 0.1‐1 Oe) is plotted on the ordinate. For a sediment column in which the magnetic mineral is magnetite, a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points

ISSN:0094-8276    

DOI:10.1029/GL008i004p00333

年代:1981

数据来源: WILEY

摘要:

Waves and currents were observed for a total of 36 days in August 1976 and 1977. Calculations of wave transport of sediment, using a Bagnold equation for transport and second order Stokes theory, show that wave orbital current asymmetry results in landward sediment transport in 10 m or less water depth, but does not significantly effect transport at greater depths. Calculations of sand transport using the Madsen‐Grant formulation show that during a 36 day observation period, sediment in less than 10 m of water generally moved landward into the breaker and then along coast with the wave‐driven littoral flow. The calculations show that below 10 m the generally upwelling coast‐parallel mean flow did not move sediment. However, during two days of a mild summer "northeaster" storm, a downwelling coastal jet developed, with 60 cm sec−1maximum current. The calculations show that if this flow had been combined with the intense wave regime of the succeeding several days, sediment would have been entrained across the lower as well as the upper shoreface. Movement would have been offshore and alongshore. Such storm transport is believed to be typical of the more intense winter storms, and to be responsible for the erosional shoreface retreat that has caused up to a kilometer of shoreline retreat on the Long Island Coast in the last

ISSN:0094-8276    

DOI:10.1029/GL008i004p00337

年代:1981

数据来源: WILEY