Las Matras Block, Central Argentina (37°S-67°W): the Southernmost Cuyania Terrane and its Relationship with the Famatinian Orogeny

The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain.     

Viscoelastic behaviour and temporal fractal properties of lherzolite and marble: possible extrapolation from experimental results to the geological time-scale

The viscoelastic relaxation behaviour of lherzolite and marble and their temporal fractal properties are studied. Based on the high‐temperature viscoelastic behaviour for these rock types derived from the time–temperature superposition principle, the relaxation function for the viscoelastic behaviour of lherzolite and marble can be expressed as a flow law from a power‐law function of temperature‐reduced time given by normalizing the various temperature behaviours, with a non‐integer exponent. The relaxation function is thus time‐scale invariant, i.e. there is a temporal fractal property to the temperature‐reduced time. Their rheological behaviour over geological time‐scales at low temperature can be extrapolated from the experimental high‐temperature behaviour based on the temporal fractal property. Our derived flow law satisfies not only transient behaviour but also steady‐state behaviour.     

The examination of fracturing process subjected to triaxial compression test in Inada granite

The behavior of a granite subject to a triaxial compression test ranging from the prefailure stage to the postfailure stage was studied using a fluorescent technique from the geological point of view. Microscopic observations of the specimens at different stages showed changes in the failure process. The start of formation of new microcracks paralleled the compression direction through their propagation until the onset of faulting and ended with the failure of shear zones after the strength failure point. Pores chiefly identified in the feldspar increased in length and width in the early stages, but not in number. It seems that the effect of pore spaces did not have any effect of failure. The microcracks generated on angular edges of quartz or feldspar grains and around biotite grains with increasing compression force. The phenomenon appearing on the crystal boundaries among biotite and quartz or feldspar agrees with the result calculated based on the theory on stress fields with ellipsoidal inhomogeneity suggested by Eshelby.     

Origin of some calc-alkalic andesites in the Japanese Islands

Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.     

Measured oxygen fugacities of the Angra dos Reis achondrite as a function of temperature

Measurements of the oxygen fugacity (?O₂) as a function of temperature (T) were made on an interior bulk sample of the cumulate achondrite, Angra dos Reis. Data clustered between the?O₂-T relationship of the iron-wüstite assemblage and 1.2 log atm units above iron-wüstite. Interpretation of the data indicates that, throughout most of the cooling history of the meteorite, ?O₂ values were defined by equilibria involving iron-bearing species at values close to the ?O₂ of the assemblage iron-wüstite. Measured ?O₂ data are compatible with crystallization and cooling at pressures greater than 50 bars.     

Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation

Available NiO analyses of olivine in peridotites of probable mantle origin are consistent in giving values around 0.40 weight per cent. Assuming that basaltic magma forming from the mantle was in equilibrium with such peridotitic olivine, the NiO content of primary basaltic magmas is estimated to be about 0.030–0.050 weight per cent. The fractionation behaviour of nickel in basaltic magma due to the crystallization of olivine has been calculated using constant NiMg and FeMg exchange partition coefficients between olivine and magma. It is shown that the NiO content of both magma and olivine decreases by 50 per cent after fractional crystallization of 6–12 per cent of olivine. The nickel distribution in some basaltic rocks and olivines is examined in the light of these results, and it is suggested that basaltic magmas, such as some of the ocean-floor basalt and the Hawaiian tholeiite and alkali basalts, represent primary magmas from mantle peridotites.     

Onshore tsunami deposits caused by the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes

Onshore tsunami deposits resulting from the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes were described to evaluate the feasibility of tsunami deposits for inferring paleoseismic events along submarine faults. Tsunami deposits were divided into three types, based on their composition and aerial distribution: (A) deposits consisting only of floating materials, (B) locally distributed siliclastic deposits, and (C) widespread siliclastic deposits. The most widely distributed tsunami deposits consist of the first two types. Type C deposits are mostly limited to areas where the higher tsunami runup was observed. The scale of tsunami represented by vertical tsunami runup is an important factor controlling the volume of tsunami deposits. The thickest deposits, about 10 cm, occur behind coastal dunes. To produce thick siliclastic tsunami deposits, a suitable source area, such as sand bar or dune, must be available in addition to sufficient vertical tsunami runup. Estimation of the amounts of erosion and deposition indicates that tsunami deposits were derived from both onshore and shoreface regions. The composition and grain size of the tsunami deposits strongly reflect the nature of the sedimentary materials of their source area. Sedimentary structures of the tsunami deposits suggest both low and high flow régimes. Consequently, it seems very difficult to identify tsunami deposits based only on grain size distribution or sedimentary structure of a single site in ancient successions.