Tetsumaru Itaya

Late cretaceous to early paleogene paleomagnetic results from Sikhote Alin, far eastern Russia: implications for deformation of East Asia

Abstract Welded tuffs in the Bogopol and Sijanov groups were sampled at 27 sites from 12 caldera formations in the Sikhote Alin mountain range around Kavalerovo (44.3°N, 135.0°E) for chronological and paleomagnetic studies. K Ar age dates show that the welded tuffs erupted between 66 Ma and 46 Ma. All sites yield reliable paleomagnetic directions, with unblocking temperatures higher than 560°C. The high-temperature component at 12 sites and the medium-temperature component at 3 sites in the Bogopol Group show reversed polarity ( D = 193.7°, I = −57.6°,α 95 = 8.1°). The high-temperature component at 11 sites in the Sijanov Group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north ( D = −2.9°, I = 59.6°,α 95 = 11.2°). The combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250.5°E, 84.1°N (A 95 = 8.8°), which falls near Cretaceous paleomagnetic poles from Outer Mongolia, Inner Mongolia, the North China Block and the South China Block. The Sikhote Alin area appears not to have been subjected to detectable motion with respect to East Asia since about 50 Ma. This implies that the Sikhote Alin area behaved as an integral part of East Asia during the opening of the Japan Sea at about 15 Ma. However, significant separation between the paleomagnetic poles of East Asia and Europe during the Jurassic-Paleogene implies a major relative movement between these two blocks since the Paleogene.

A geomagnetic excursion in the Brunhes epoch recorded in New Zealand basalts

Abstract Anomalous geomagnetic directions are recorded in the thermoremanent magnetisation (TRM) of alkali basalt lavas from the Auckland volcanic field, New Zealand. The concordance of paleomagnetic directions both in different sites from individual eruptive centres and in sites from several different centres, as well as their relatively short age span compared with that of the field as a whole, indicates that these anomalous directions have a geomagnetic origin. The directions, which are dated between approximately 25 ka and 50 ka, do not represent full reversals of the geomagnetic field and are interpreted as excursions. Very few excursions have been reported from the Southern Hemisphere; this is the first such record known from igneous rocks and further supports the notion that excursions are global phenomena. The excursions here are tentatively correlated with those reported from Lake Mungo, Mono Lake and Laschamp.

Timing of the cataclastic deformation along the Akaishi Tectonic Line, central Japan

This paper describes K-Ar ages of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. Petrological and mineralogical features of these rocks are also examined. Based on the results, we test the hypothesis that the K-Ar ages of these rocks represent the age of hydrothermal alteration associated with the fault movement. Intensity of deformation and alteration increase passing from host rock into cataclasite and finally into fault gouge. This increase corresponds to an increase of the value of crystallinity index (Kübler index) of the micaceous minerals contained in these rocks. Furthermore, the degree of rejuvention of K-Ar ages of the micaceous minerals increases in the same sense. A correlation of K-Ar ages (plotted on the y axis) versus Kübler index (plotted on the x axis) yields a concave curve asymptotically parallel with the x axis at approximately 15 Ma. This curve is interpreted to represent the mode of decrease of the relative amounts of inherited argon in the K-Ar system of the micaceous minerals, corresponding to an increase in the intensity of deformation and alteration. Inclination of the curve becomes zero if all the inherited argon is lost from the K-Ar system of the micaceous minerals. Thus, it is concluded that the hydrothermal alteration occurred at approximately 15 Ma. The strike-slip basin along the ATL formed the middle Miocene. The K-Ar dating of ATL gouges indicates that the ATL was active simultaneously with formation of the strike-slip basin along its trace during the middle Miocene.

Argon release systematics of hypogene and supergene alunite based on progressive heating experiments from 100 to 1000°C

Abstract Few minerals have as many different geological applications for K Ar or 39 Ar/ 40 Ar dating as alunite (KAl 3 (SO 4 ) 2 (OH) 6 ), but a basic understanding of the systematics of argon release from this mineral is lacking. Progressive heating experiments from 100–1000°C at 100°C intervals were conducted in this study to obtain argon release patterns from hypogene and supergene alunite. The samples analyzed were selected from well understood deposits and are representative of the principal natural environments of acid-sulfate generation and related advanced argillic alteration and alunite formation. For gas extraction and purification, a low-blank metallic extraction-purification system used for dating silicates was complemented with a quartz-Pyrex glass line that included an external resistance furnace. The resulting patterns of argon release vs. temperature are well defined and similar for all the samples. Quantitative release of radiogenic 40 Ar occurs during the dehydroxylation reaction of alunite at temperatures >400 and 800°C is mainly derived from the extraction system. The results indicate that the retentivity of Ar in fine-grained supergene alunite is similar to that of coarsely crystalline hypogene alunite. The data support the reliability of K-Ar dating of hypogene and supergene alunite and provide practical information to conduct routine high precision analyses.