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271.
Analytical results of the relative and absolute abundance of LIL-incompatible trace elements (K, Rb, Cs, Sr, and Ba) and isotopic compositions (
,
, and
) are summarized for fresh samples from active and dormant volcanoes of the Volcano and Mariana island arcs. The presence of thickened oceanic crust (T 15–20 km) beneath the arc indicates that while hybridization processes resulting in the modification of primitive magmas by anatectic mixing at shallow crustal levels cannot be neglected, the extent and effects of these processes on this arc's magmas are minimized. All components of the subducted plate disappear at the trench. This observation is used to reconstruct the composition of the crust in the Wadati-Benioff zone by estimating proportions of various lithologies in the crust of the subducted plate coupled with analyses from DSDP sites. Over 90% of the mass of the subducted crust consists of basaltic Layers II and III. Sediments and seamounts, containing the bulk of the incompatible elements, make up the rest. Bulk Western Pacific seafloor has
, δ 18O +7.2, K/Rb 510, K/Ba 46, and K/Cs 13,500. Consideration of trace-element data and combined
systematics limits the participation of sediments in magmagenesis to less than 1%, in accord with the earlier results of Pb-isotopic studies. Combined
data indicate little, if any, involvement of altered basaltic seafloor in magmagenesis. Perhaps more important than mean isotopic and LIL-element ratios is the restricted range for lavas from along over 1000 km of this arc. Mixtures of mantle with either the subducted crust or derivative fluids should result in strong heterogeneities in the sources of individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations of crustal materials supplied to the subduction zone; and (2) lesser efficiency of mixing processes accompanying induced convection between arc segments (parallel to the arc) as compared to that perpendicular to the arc. The absence of these heterogeneities indicates that either some process exists for the efficient mixing of mantle and subducted material parallel to the arc or that subducted materials play a negligible role in the generation of Mariana-Volcano arc melts.Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition of this arc's magmas, while (2) a mantle similar to that which has produced alkali-olivine basalts (AOB) of north Pacific “hot spot” chains is indistinguishable in many respects spects from the source of these arc lavas. 相似文献
272.
Multifractal analysis of earthquakes 总被引:5,自引:0,他引:5
Multifractal properties of the epicenter and hypocenter distribution and also of the energy distribution of earthquakes are studied for California, Japan, and Greece. The calculatedD
q-q curves (the generalized dimension) indicate that the earthquake process is multifractal or heterogeneous in the fractal dimension. Japanese earthquakes are the most heterogeneous and Californian earthquakes are the least. Since the earthquake process is multifractal, a single value of the so-called fractal dimension is not sufficient to characterize the earthquake process. Studies of multifractal models of earthquakes are recommended. Temporal changes of theD
q-q curve are also obtained for Californian and Japanese earthquakes. TheD
q-q curve shows two distinctly different types in each region; the gentle type and the steep type. The steeptype corresponds to a strongly heterogeneous multifractal, which appears during seismically active periods when large earthquakes occur.D
q for smallq or negativeq is considerably more sensitive to the change in fractal structure of earthquakes thanD
q forq2. We recommend use ofD
q at smallq to detect the seismicity change in a local area. 相似文献
273.
Water masses in the subsurface and the intermediate layer are actively formed due to strong winter convection in the Japan
Sea. It is probable that some fraction of pollution is carried into the layer below the sea surface together with these water
masses, so it is important to estimate the formation rate and turnover time of water masses to study the fate of pollutants.
The present study estimates the annual formation rate and the turnover time of water masses using a three-dimensional ocean
circulation model and a particle chasing method. The total annual formation rate of water masses below the sea surface amounted
to about 3.53 ± 0.55 Sv in the Japan Sea. Regarding representative intermediate water masses, the annual formation rate of
the Upper portion of the Japan Sea Proper Water (UJSPW) and the Japan Sea Intermediate Water (JSIW) were estimated to be about
0.38 ± 0.11 and 1.43 ± 0.16 Sv, respectively, although there was little evidence of the formation of deeper water masses below
a depth of about 1500 m in a numerical experiment. An estimate of turnover time shows that the UJSPW and the JSIW circulate
in the intermediate layer of the Japan Sea with timescales of about 22.1 and 2.2 years, respectively. 相似文献