The Vredefort Dome, located in the central part of the Witwatersrand Basin in South Africa, is the type locality for pseudotachylite. Pseudotachylite at the Vredefort Dome is generally regarded to be of impact origin. Pseudotachylites which are closely associated with faults are, however, also known to be common along the northern and northwestern edges of the Witwatersrand Basin. In order to compare pseudotachylites from the Vredefort Dome and from the surrounding Witwatersrand Basin, different studies were undertaken in the past. Mode of occurrence, microscopic textures, geochemical analyses and chronological measurements of pseudotachylites are briefly reviewed in this paper. In the Vredefrot Dome, pseudotachylites are commonly observed except in the central part of its core. In the surrounding Witwatersrand Basin, they are reported from drill core sections and in underground workings. The matrix in pseudotachylite from the Vredefort Dome is mostly a recrystallized melt phase, while those from the surrounding Witwatersrand Basin seem to be composed of clastic material. Pseudotachylites both from the Vredefort Dome and the surrounding Witwatersrand Basin are geochemically closely related to their host rocks. Although evidence for more than one generation of pseudotachylite has been presented, both in the Vredefort Dome and the surrounding Witwatersrand Basin, it is widely believed that most of them were formed as a result of the Vredefort impact event (ca. 2.0 Ga). Other fault rocks reported from the surrounding Witwatersrand Basin are older than the pseudotachylites and therefore not related to their formation.
Several fluvial terraces and other flat surfaces are well developed in Wangdi Phodrang District along Chang Chhu (Puna Tshan Chhu) in central Bhutan. The low terrace of 5 m to 18 m in relative height is widely distributed, preserving the river flat reliefs on the surface. The middle terrace of 20 m to 50 m in relative height is well developed from Wangdi Phodrang to the upper reaches. The high surfaces of 110 m in relative height are distributed only in Wangdi Phodrang Town and Bajo Monastery Hill, which is 2 km upstream from the former. Geology of the middle terrace is mainly composed of a well rounded and imbricated boulder bed with several sand beds of lesser amount. Geology of the high surface in Wangdi Phodrang Town is composed of two members : the underlying 50 m-thick massive huge boulder bed and the 60 m-thick mudflow deposits overlying the former horizontally. The huge boulder bed continues to the imbricated boulder bed of the middle terrace, suggesting that the mudflow forming the high surface is younger than the middle terrace. On the opposite bank of Wangdi Phodrang Town, there are huge mudflow deposits, which have flowed down towards Wangdi Phodrang Town. Judging from the stratigraphic relation with middle terrace deposits, flow direction, elevation, size, and lithology, the mudflow can be safely correlated with the mudflow at Wangdi Phodrang Town. Then, the mudflow may once have formed natural dam by covering the river flat gravel bed of the middle terrace. Estimated height of the dam is 1310 m a.s.l., and the crest was almost 1000 m wide. While Bajo Monastery Hill is composed of two geological members, the northern major portion of the hill is brown-colored massive sand of over 20 m thick, which are delta fan deposits of a tributary named Limte Chhu, while massive white sand distributed locally at southern downstream side of the hill is abutting on the former. Groundwater drilling on the middle terrace revealed that the well rounded boulder bed is thinly overlaid by white silty sand, which continues to the massive white sand at Bajo Monastery Hill. This evidently suggests that the massive white sand bed is younger than the middle terrace boulder bed. Judging from massive appearance without stratification and mixed occurrence with blocks and fragments of varved clay, the genesis of the massive white sand at Bajo Monastry Hill without doubt resulted from turbidity current of the Glacier Lake Outburst Flood (GLOF). When the GLOF flowed into the deep lake formed by the mudflow dam, the suspended sand, as well as transported blocks of varved clay, may have been settled in the lake. Concurrently, the GLOF may have destroyed the natural dam and drained the lake. Only a small portion of the GLOF sediments remained at the riverside stagnate concavity in the Bajo monastery area. The above-mentioned Quaternary geo-history constructed from field evidence is identical to the legend of the Wangdi Phodrang district. The legend is that an ancient lake was drained in one night by God, who was angry with the impious inhabitants. The upper reach of the lake in the legend is Punakha Town, which is at an elevation of 1340 m. Therefore, the size of the lake in the legend is as same as the size estimated by geology. The legend may suggest that the GLOF event was observed by ancient inhabitants settled in this area.
In order to solve a mechanical inconsistency that the Osaka Bay basin exists between the Median Tectonic Line (MTL) and Arima-Takatsuki Tectonic Line (ATTL) (right-lateral left-stepping faults), we attempted to investigate and evaluate the basin forming mechanism at the termination of right-lateral left-stepping faults by means of the dislocation modeling. The results of the numerical simulations show that the sedimentary basin can be formed at the termination of the right-lateral left-stepping faults develop, if the secondary fault caused by the rightlateral motion is a reverse fault and its displacement is larger than 20% of the lateral motion. We applied this model to the fault distribution in the Kinki district, and found that tectonic structures around the Osaka Bay can be explained by combination of 1) the right-lateral motion of the MTL and ATTL and 2) the reverse motion of their secondary faults, i.e., Nara-Toh'en Fault, Ikoma Fault and Rokko-Awaji Fault Systems.
In order to evaluate the capability of one-dimensional microgravity investigations, we carried out two test surveys across the Katagihara fault in the southwest of Kyoto basin and the Fumotomura fault at the foothills of Suzuka Range. Since seismic reflection survey had already been carried out in these faults, the Bouguer anomaly due to the fault structures was expected in advance at an order of 0.1 mgal across the Katagihara fault and less than 0.1 mgal across the Fumotomura fault. It was also estimated that the spacing of the gravity points should be less than 50 meter to reveal the structures. We therefore conducted precise gravity measurements using a LaCoste & Romberg gravimeter (G-type) at about a 50 meter interval, and also carried out leveling surveys on the same points using a Wild NA3000 digital level. Moreover, we paid much attention for terrain corrections using the 50 meter DEM (Digital Elevation Model) provided by Geographical Survey Institute and partially using a 10 meter DEM compiled by ourselves. Consequently, we achieved 0.1 mgal level precisions for almost all the survey points. Using the gravity anomaly data, density structures in both survey areas were estimated and compared with the structures obtained from seismic reflection survey. The main results are as follows ; (1) comparatively simple structures are obtained to explain the gravity anomaly in the Katagihara fault, (2) the density contrast between the basement and the sedimentary layer is 0.58 g/cm3 in the Katagihara fault, (3) no gravity anomaly due to the displacement on the fault plain is observed in the Fumotomora fault, and (4) possibility of a high density layer is observed beneath the Tokai Group. Although the gravity survey has some limitations and drawbacks, it certainly gives us useful information about the density contrasts. Moreover, one-dimensional gravity survey is quite easy to conduct with very low cost. We therefore recommend that this kind of gravity surveys should be carried out whenever seismic reflection survey is conducted.
Este artículo constituye una serie de reflexiones sobre la inmigración en la Regió Metropolitana de Barcelona, siendo el objetivo central el analizar el peso de los inmigrantes como factor diferencial de la fecundidad, sin aislarlo de otros factores explicativos de caracter tanto demográfico como socioeconómico. A tal efecto, el artiiacute;culo se estructura en las siguientes partes : Primero, tras un repaso a los estudios más destacados que se han realizado en España en relación con el tema aquí planteado, se hace una exposición breve de la evolución de la inmigración y su distribución geográfica en la zona objeto de estudio. De esta manera, se enmarca el presente trabajo en el contexto de la consolidación de la Regió, en un período en que pasadas las décadas de fuerte desarrollo industrial e inmigración masiva los inmigrantes ya ocupan una posición sólida en la sociedad metropolitana. A continuación, se procede a la estimación de la fecundidad transversal según origen geográfico en Barcelona capital, para to cual se ha calculado el índice de fecundidad general utilizando el método que desarrolló en los años 1960 un grupo de la Universidad de Princeton. Los valores calculados muestran con nitidez que a la altura de 1990 siguen existiendo diferencias claras en fecundidad entre los distintos grupos, y que éstas constituyen un reflejo fiel de los comportamientos reproductivos en sus respectivas regiones de origen. Con todo, las diferencias eran mucho más acusadas en 1970, momento en que la afluencia inmigratoria aún estaba en su fase culminante. Tras realizar un análisis sobre el origen de los cónyuges al contraer el matrimonio, se llega a la conclusión de que el cambio apuntado se debe, al menos en parte, a la disminución de matrimonios endogámicos, con la consiguiente reduccion del número de hijos que tienen padres de un mismo origen. Después, la incidencia de la inmigración en la fecundidad se contrasta con otros factores explicativos. Para ello, se ha llevado a cabo un análisis de regresión múltiple sobre la fecundidad longitudinal, calculada para los municipios de la Regió. Este análisis ha permitido detectar, además de la inmigración de la zona de alta fecundidad, tres variables determinantes : peso relativo de las solteras, nivel de instrucción de las mujeres y tasa de actividad femenina. La fuerza explicativa de cada una, sin embargo, cambia de una cohorte a otra : el origen geográfico pierde importancia segúm desciende la generación, mientras que el estado civil aumenta su peso. Por otro lado, el nivel de instrucción muestra una fuerza considerable en la cohorte nacida en la década de los 1950, a to que ha contribuido probablemente la modernización de la educación formal, así como una mayor escolarización femenina a partir de los años 60. Y ya en la cohorte de los añbs 60, la participación femenina en el mercado laboral constituye un factor adicional que propicia la bajada de la fecundidad, aunque, dada su edad en el momento de la investigacion, min se desconoce la descendencia final que pueda alcanzar esta generación. Finalmente, la distribución territorial de la fecundidad longitudinal refleja no sólo el legado de la inmigración, sino el alejamiento progresivo del lugar de residencia que eligen las generaciones jovenes para la formación de su familia. Se constata asimismo la existencia de algunos subgrupos de población cuyo nivel de fecundidad escapa a las pautas generales recogidas por el análisis de regresión.
This study classified Japanese mountains based on mountain ordering using 1 : 500000 topographic maps, and examined the relationships of relief, relative relief and perimeter fractal dimension for the classified mountains. Mountain order was defined in terms of closed contour lines on the topographic map. A set of closed, concentric contour lines defines a first-order mountain. Higher-order mountains can be defined as a set of closed contour lines that contain lower-order mountains and that have only one closed contour line for each elevation. Relief, relative relief and fractal dimension were measured for ordered mountains using personal computer, and were defined as follows : relief E = H/A1/2, where H and A are the height and area of each ordered mountain, respectively; relative relief R= ∑ hi/H, where hi is the height of the enclosed, lower-order mountains, and represents the degree of vertical roughness of the ordered mountain; fractal dimension was measured for perimeter contour line by the pixel dilation method, and represents the degree of horizontal roughness of the ordered mountain. Japanese mountains were classified into 74 third order mountains and 11 fourth order mountains. The area of a third order mountain varies from 50 to 4712 km2, and that of a fourth order mountain is 2498 to 15563 km2. A significant relationship was found among relief E, relative relief R, and fractal dimension D for the ordered mountains (r=0.91, n=85), and can be defined by the expression : LogE=-aD-bLogR-c This relationship shows that Japanese mountains have the following morphological characteristics : a high relief mountain has low vertical and horizontal roughness, and a low relief mountain has high vertical and horizontal roughness. These characteristics suggest that slope angle of Japanese mountains converges within a certain range.