A mechanical model for plate deformation associated with aseismic ridge subduction in the new hebrides arc

Tectonic features associated with a subducting fracture zone-aseismic ridge system in the New Hebrides island arc are investigated. Several notable features including a discontinuity of the trench, peculiar locations of two major islands (Santo and Malekula), regional uplift, and the formation of a basin are interpreted as a result of the subduction of a buoyant ridge system. The islands of Santo and Malekula are probably formed from an uplifted mid-slope basement high while the interarc basin of this particular arc is probably a subsiding basin instead of a basin formed by backarc opening. The situation can be modeled by using a thin elastic half plate overlying a quarter fluid space with a vertical upward loading applied at the plate edge. This model is consistent with topographic and geophysical data. This study suggests that subduction of aseismic ridges can have significant effects on tectonic features at consuming plate boundaries.     

A deep226Ra maximum in the Northeast Pacific

Seven vertical profiles of²²⁶Ra have been measured along an east-west traverse at about 30°N from San Diego to northwest of Hawaii. These profiles show that there is a distinct core of Ra maximum spreading westward as a tongue in the northeast Pacific deep water. This core starts in the east with 21.1 Ra units (1Ra unit= 10^?14g/kg) at 3.9 km depth at about 130°W, and deepens westward to 4.1 km with its Ra reduced to 18.3 units at 150°W. A similar core with some uncertainty due to possible sampling errors extends westward near the bottom at 5.2 km depth from 19.4 Ra units at 150°W to 15.9 units at about 180° longitude. In addition, these profiles appear to be correlated with each other in structure above the cores of Ra maximum. These cores indicate that the Ra input depends locally on the type and composition of sediments and so the flux varies over the ocean bottom. On the basis of a one-dimensional diffusion-decay model, a horizontal diffusion coefficient of 10⁶ cm²/sec has been computed along these cores. Although this value appears to be slightly lower, it is not inconsistent with those derived from other physical methods.     

Source process and tectonic implications of the Spanish deep-focus earthquake of March 29, 1954

The source process of the deep-focus Spanish earthquake of March 29, 1954 (m_b = 7.1, h = 630 km) has been studied by using seismograms recorded at teleseismic distances. Because of its unusual location, this earthquake is considered to be one of the most important earthquakes that merit detailed studies. Long-period body-wave records reveal that the earthquake is a complicated multiple event whose wave form is quite different from that of usual deep earthquakes. The total duration of P phases at teleseismic distances is as long as 40 s. This long duration may explain the considerable property damage in Granada and Malaga, Spain, which is rather rare for deep earthquakes. Using the azimuthal distribution of the differences between the arrival times of the first, the second and later P phases, the hypocenters of the later events are determined with respect to the first event. The focus of the second event is located on the vertical nodal plane of the first shock suggesting that this vertical plane is the fault plane. This fault plane which strikes in N2°E and dips 89.1°E defines a nearly vertical dip-slip fault, the block to the west moving downwards. The time interval and spatial separation between the first and the second events are 4.3 s and 19 km respectively, giving an apparent rupture velocity of 4.3 km/s which is about 74% of the S-wave velocity at the source. A third event occurred about 8.8 s after the first event and about 35.6 km from it. At least six to ten events can be identified during the whole sequence. The mechanism of some of the later events, however, seems to differ from the first two events. Synthetic seismograms are generated by superposition of a number of point sources and are matched with the observed signals to determine the seismic moment. The seismic moments of the later events are comparable to, or even larger than, that of the first. The total seismic moment is determined to be 7 · 10²⁷ dyn cm while the moments of the first and the second shocks are 2.1 · 10²⁶ dyn cm and 5.1 · 10²⁶ dyn cm, respectively. The earthquake may represent a series of fractures in a detached piece of the lithosphere which sank rapidly into the deep mantle preserving the heterogeneity of material property at shallow depths.     

SIMSAG: Integrated computer system for use in evaluation of mineral and energy resources

A system of interactive graphic computer programs for multivariate statistical analysis of geoscience data (SIMSAG)has been developed to facilitate the construction of statistical models to evaluate potential mineral and energy resources from geoscience data. The system provides an integrated interactive package for graphic display, data management, and multivariate statistical analysis. It is specifically designed to analyze and display spatially distributed information which includes the geographic locations of observations. SIMSAG enables the users not only to perform several different types of multivariate statistical analysis but also to display the data selected or the results of analyses in map form. In the analyses of spatial data, graphic displays are particularly useful for interpretation, because the results can be easily compared with known spatial characteristics of the data. The system also permits the user to modify variables and select subareas imposed by cursor. All operations and commands are performed interactively via a graphic computer terminal. A case study is presented as an example. It consists of the construction of a statistical model for evaluating potential areas for explorations of uranium from geological, geophysical, geochemical, and mineral occurrence map data quantified for equalarea cells in Kasmere Lake area in Manitoba, Canada.     

Radium-barium-silica correlations and a two-dimensional radium model for the world ocean

Radium-barium-silica relationships are examined in various regions of the Pacific. Ra, Ba and Si are not linearly correlated except in the Circumpolar region. From surface water to the intermediate water, Ra usually increases linearly with Ba and Si. In the North Pacific where there is a Ra excess in the deep water, both the Ra-Ba and Ra-Si diagrams show curves concave upward with an increasing slope. The relationships are further complicated by a deep gentle Ba maximum and a broad mid-depth Si maximum.In the western boundary where the Antarctic Bottom Water and Pacific Deep Water are separated by the benthic front, the Ra-Ba and Ra-Si plots show “hooks” of various size and shape caused by a discordance in the depths of the Ra, Ba and Si maxima. The hook becomes smaller toward the south and flips from counterclockwise to clockwise in the southern basin.A two-dimensional horizontal model with a geostrophic circulation pattern is employed to calculate Ra distribution in the deep water with a constant upwelling rate but a variable mean input rate for Ra. The mean input rate or fluxF is the sum of the in-situ production rateJ by particulate dissolution in the water column and the bottom fluxQ_b. The global mean fluxF needed to balance the radiodecay in a steady-state distribution is used as a reference for discussion. It is found that anF value set at1.5 ×F fits the observed data in the deep Atlantic. TheF value needs to be about2.7 ×F in order to produce a comparable distribution in the deep Pacific. These model calculations indicate that a Ra source is required in the northeast Pacific. It appears that no uniqueF can produce a global Ra distribution in deep water comparable to both the Atlantic and Pacific observations. This is also true within the Pacific Ocean itself because of the large Ra excess observed in the northeast Pacific. Thus the Ra input rate is quite variable and increases by a factor of two from the Atlantic to the Pacific. This probably reflects the non-uniformity of the bottom fluxQ_b rather than that of the in-situ productionJ in the world oceans.     

Rapid analysis of Florida phosphates by energy-dispersive X-ray fluorescence

A rapid X-ray fluorescence technique has been developed to determine the BPL (Ca₃(PO₄)₂ or Bone Phosphate of Lime) content of various streams in Florida phosphate beneficiation and in supporting laboratory flotation experiments. The method, which requires simple sample preparation, a 100-sec counting, and has an accuracy of better than 2% BPL (0.4% P), employs an Fe⁵⁵ radioactive source and a room temperature detector to analyze for calcium. The BPL values, which can not be obtained readily and directly because of the weak fluorescence of phosphorus, are instead derived from a close correlation found to exist between the phosphorus and calcium concentrations.     

Radon variations at arrowhead and murrieta springs: Continuous and discrete measurements

Two continuous radon monitors (CRMs) have been deployed at Arrowhead Hot Springs along the San Andreas fault near San Bernardino and at Murrieta Hot Springs along the Elsinore fault. The recorded hourly and daily radon variations during 1983 are within ±5% of the mean values. The radon levels increased, however, by about 10–20% above their normal baseline levels in midyear. Several small-magnitude earthquakes (M=3.0–3.5) occurred within 20 km of the Arrowhead site near the end of the period of the radon increase.Discrete radon and helium monitoring at Arrowhead Springs since 1974 has recorded one definite precursory anomaly: a shapr increase of radon and helium (and also other dissolved gases) in 1979 by as much as 60% above their baseline levels. This anomaly was followed by the nearby Big Bear earthquake swarm (main shock,M=4.8) 45 days later. A similar increase was recorded during the first half of 1983, and it was followed by several small earthquakes (M=3.0–3.5) within 20 km of the Arrowhead site. In both cases radon and helium increased proportionally, indicating mixing between the deep-source water and the surface water at variable proportions.Comparison of radon values in gas and in liquid phases indicates that radon is not in equilibrium between the two phases but is distributed preferentially in the gas phase by a factor of 20 to 25. (Only about 5% or less of groundwater radon is in the dissolved phase.) At both sites the dissolved radon is much lower than that expected from solubility.     

Pb in the western Indian Ocean: distribution, disequilibrium, and partitioning between dissolved and particulate phases

²²⁶Ra profiles have been measured in the western Indian Ocean as part of the 1977–1978 Indian Ocean GEOSECS program. These profiles show a general increase in deep and bottom water Ra concentration from the Circumpolar region to the Arabian Sea. A deep Ra maximum which originates in the Arabian Sea and in the Somali basin at about 3000 m depth spreads southward into the Mascarene basin and remains discernible in the Madagascar and Crozet basins. In the western Indian Ocean, the cold Antarctic Bottom Water spreads northward under the possibly southward-flowing deep water, forming a clear benthic front along the Crozet basin across the Southwest Indian Ridge into the Madagascar and Mascarene basins. The Antarctic Bottom Water continues to spread farther north to the Somali basin through the Amirante Passage at 10°S as a western boundary current. The benthic front and other characteristic features in the western Indian Ocean are quite similar to those observed in the western Pacific where the benthic front as a distinctive feature was first described by Craig et al. [15]. Across the Mid-Indian Ridge toward the Ceylon abyssal plain near the triple junction, Ra profiles display a layered structure, reflecting the topographic effect of the mid-ocean ridge system on the mixing and circulation of the deep and bottom waters. Both Ra and Si show a deep maximum north of the Madagascar basin. Linear relationships between these two elements are observed in the deep and bottom water with slopes increasing northward. This suggests a preferential input of Ra over Si from the bottom sediments of the Arabian Sea and also from the flank sediments of the Somali basin.     

Barium and radium in the Dead Sea

We have measured Ba in Dead Sea samples collected before and after the 1979 overturn, and²²⁶Ra in nine samples collected after the overturn. Before this overturn, Ba and the²²⁶Ra data measured by Chung and Craig [4] show that a distinct two-layer structure existed, with higher concentrations in the upper layer. After the overturn, both elements were uniformly distributed in the water column. The inventories of Ba and Ra calculated from these data are the same for the periods before and after the overturn. If the inventories were constant during the last meromictic phase then the input rate must be balanced by the removal rate, and a mass balance model can be constructed to estimate physical parameters based on known or deduced sources and sinks. The sources include inputs from the Jordan River, springs around the Dead Sea, and submerged springs or seepages, etc. The sinks include coprecipitation with aragonite, gypsum, precipitation of barite, coprecipitation of Ra with barite, particulate scavenging, and radioactive decay for Ra. Our data include measurements of Ba and²²⁶Ra in gypsum, aragonite and halite from the Dead Sea, as well as in some of the inflowing rivers and springs.The inclusion of particulate scavenging as a sink is a major element of the model. We find that, without inclusion of a Ba scavenging term in the deep water, the lake volume at the previous overturn as calculated from the Ba data would be unrealistically high in comparison with historical records. The inclusion of particulate scavenging for Ra in the model reduces the calculated duration of the last meromictic phase significantly.Our model excludes internal mixing between the upper and lower water masses. With this restriction, various sets of model parameters were calculated as a function of theRa/Ba scavenging rate ratio. If the ratio is one, the calculated age of the last meromictic phase is about a hundred years. A substantial increase in the Ra input rate is required to balance the removal rate by particulate scavenging as well as decay. If the ratio is zero, i.e. no particulate scavenging for Ra, the age is about 260 years, as obtained by Stiller and Chung [2].