摘要:

The standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure: the Euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero. Deviations from closure suggest that the data are systematically in error, or that the plate boundaries have been identified incorrectly, or that the plates are non‐rigid. We examine two methods, one based on a chi‐square test and the other based on anF‐ratio test, of testing whether plate motion circuits close. We find the latter test is useful, but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data. We apply theF‐ratio test to Minster and Jordan's data for the boundaries dividing the Pacific, Cocos, and Nazca plates, which prove consistent with plate circuit closure. Application of the test to their data for the boundaries dividing the Australian, Antarctic, and African plates shows nonclosure even after deletion of data from the Carlsberg Ridge, which recently has been suggested to divide the African plate from an Indo‐Arabian plate. This nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the Indian Ocean Triple

ISSN:0094-8276    

DOI:10.1029/GL014i006p00587

年代:1987

数据来源: WILEY

摘要:

The aftershocks that followed three moderate size earthquakes occurring on June 12, 1985, off the east coast of Taiwan were monitored by land and ocean bottom seismographs. Hypocenters of aftershocks located by this sea‐land joint array outline a rupture plane striking northeast and dipping steeply to the southeast. The result agrees well with fault plane solutions of the three main shocks. These results combine to indicate a right‐lateral strike‐slip mechanism for the rupture. The earthquake sequence studied occurred behind an active subduction zone and is explained in terms of a forearc "sliver plate" movement resulting from the strong oblique convergence between the Phillipine Sea plate and the Eurasian plate along the trench just east of T

ISSN:0094-8276    

DOI:10.1029/GL014i006p00591

年代:1987

数据来源: WILEY

摘要:

Commonly it is assumed that the intensity of mid‐ocean ridge hydrothermal activity should correlate with spreading rate, since high spreading rates are an indication of large subcrustal heat sources needed for intense hydrothermal activity. We have tested this hypothesis by modeling the deposition of hydrothermal precipitates from cores from Deep Sea Drilling Project Leg 92, taken on the west flank of the East Pacific Rise at 19°S. Although spreading rates at the East Pacific Rise and its predecessor, the Mendoza Rise, have varied by only 50% in the last 30 million years, we found certain episodes, at about 25, 18, 14, and 9 million years ago, of hydrothermal manganese deposition as much as a factor of 20 higher than equivalent Holocene accumulation. These eposides do not correlate with spreading rate changes and instead seem to occur at times of major tectonic reorganizations. We propose that ridge jumps and changes of ridge orientation may substantially increase hydrothermal activity by fracturing the ocean crust and providing seawater access to deep‐seated heat sou

ISSN:0094-8276    

DOI:10.1029/GL014i006p00595

年代:1987

数据来源: WILEY

摘要:

The distribution of an isotopic species, HDO (deuterium) or H218O (oxygen), in ice formed by the migration of a well‐defined freezing front in water is dependent on the freezing rate. Development of a box diffusion model combined with the boundary layer concept leads to a possibility of prediction of the freezing rate in nature by the determination of the isotopic composition of the ice in δD or δ18O. The isotopic distribution in ice formed during experiments conducted with variable freezing rates gives results in accordance with the model. Investigation on lake ice formed under well‐known climatic conditions provides a further test of the

ISSN:0094-8276    

DOI:10.1029/GL014i006p00599

年代:1987

数据来源: WILEY

摘要:

Twenty‐three SEASAT altimeter profiles crossing the Ascension fracture zone are used together with seafloor ages inferred from a kinematic model to estimate a geoid slope‐age relation between ages of 5 and 50 Ma. The data do not definitely contradict a half space cooling model (slope = −9.5 ± 3 cm/Ma) if biases are accepted, resulting in large scatter. However the data would seem to support a puzzling explanation in terms of a plate cooling model involving two distinct thermal thicknesses, one of ≈70 km for ages35 Ma. Although preliminary, this result is similar to that found in the Pacific with a much larger

ISSN:0094-8276    

DOI:10.1029/GL014i006p00603

年代:1987

数据来源: WILEY

摘要:

Western Aphrodite Terra, Venus, is characterized by a series of parallel linear structural discontinuities 2000‐4000 km in length and 100‐200 km wide, which strike at high angles to the general topographic trend of the highlands. The detailed characteristics of these features are similar to terrestrial oceanic fracture zones and strike‐slip fault zones. Either interpretation implies the presence of large‐scale horizontal tectonic movement o

ISSN:0094-8276    

DOI:10.1029/GL014i006p00607

年代:1987

数据来源: WILEY

摘要:

A vertical profile of Krypton‐85 from 12 to 40 km height was obtained from a balloon‐borne experiment. The results display a clear concentration gradient between the troposphere and 22 km. Above this altitude, the concentration varies very little, in agreement with theoretical predicti

ISSN:0094-8276    

DOI:10.1029/GL014i006p00611

年代:1987

数据来源: WILEY

摘要:

Balloonborne measurements of stratospheric aerosol in the late October ‐ early November period in 1983, 1984 and 1985 indicate that the sulfuric acid aerosol which formed in the stratosphere following the eruption of El Chichón in 1982, and which was subsequently transported to the polar regions, decayed over Antarctica with an exponential decay time of about one year, in reasonable agreement with extensive measurements at Laramie during this period. These results suggest that the increased aerosol burden over Antarctica, which occurred as a result of the El Chichón eruption, cannot be directly involved in the precipitous decline of stratospheric ozone in Antarctica since 1982, although indirect effects cannot be ruled

ISSN:0094-8276    

DOI:10.1029/GL014i006p00614

年代:1987

数据来源: WILEY

摘要:

In the period from mid‐November 1985 through April 1986 the stratospheric aerosol profiles determined by lidar observations at the Mauna Loa, Hawaii Observatory indicated a persistent sequence of profiles characteristic of stratospheric volcanic aerosols. Aerosol layers were observed in two general altitude ranges. A high layer between about 25 and 27 km was detected in late November 1985 and again in late February and late March 1986. A lower layer, generally located between 20 and 23 km was present from early December 1985 through April 1986. These layers appear to have been due to the November 13, 1985 eruption of Nevado del Ruiz in Columbia. On the basis of these lidar data the impact of this volcanic plume on direct solar irradiance at 694 nm in Hawaii was estimated to have been an average diminution of about 0.4 per cent during the period December 1985 through April 198

ISSN:0094-8276    

DOI:10.1029/GL014i006p00618

年代:1987

数据来源: WILEY

摘要:

Ground‐based infrared solar spectra at 0.02 cm−1resolution obtained at the Amundsen‐Scott South Pole station in December 1986 have been analysed for the atmospheric content of HCl. Nonlinear least‐squares spectral fitting applied to the spectra yields a total HCl column amount of (6.4 ± 0.8) × 1015molec/cm², most being stratospheric. This amount is larger than that extrapolated from earlier results on the latitudinal distribution of atmos

ISSN:0094-8276    

DOI:10.1029/GL014i006p00622

年代:1987

数据来源: WILEY