F. Simancas

La Orogenia Cadomiense y el rifting del Paleozoico Inferior en el Sudoeste de Iberia

La intensidad y la duracion de la Orogenia Cadomiense en el sudoeste de Iberia han sido temas controvertidos. El magmatismo calcoalcalino del Vendiense superior, que se encuentra ampliamente representado en la Zona de Ossa Morena, es el efecto orogenico cadomiense mas significativo. Ademas, en las rocas del Proterozoico superior que afloran en Peraleda del Zaucejo (sur de la Zona Centroiberica) se ha establecido firmemente la existencia de foliacion y metamorfismos tardiprecambricos. En la Zona de Ossa Morena, sin embargo, la deformacion penetrativa de las rocas precambricas parece corresponder casi exclusivamente a pliegues y cabalgamientos de edad varisca. En conjunto, la intensidad de la orogenia cadomiense en el sudoeste de Iberia es, como en el resto del Macizo Iberico, moderada. Los datos geocronologicos existentes aparentan la existencia de un magmatismo initerrumpido desde el Proterozoico superior hasta el Ordovicico; sin embargo, se sugiere que tal continuidad no es real. Del analisis de los datos estratigraficos se deduce que la orogenia cadomiense se habria desarrollado exclusivamente en el Proterozoico terminal, siendo el Cambrico inferior un periodo de transicion entre el ciclo orogenico cadomiense y el ciclo varisco. Desde el Cambrico Inferior alto, se desarrollo en la Zona de Ossa Morena el rifting pre-varisco, cuyo reflejo estratigrafico es la sustitucion de carbonatos de plataforma (Cambrico Inferior) por sedimentos terrigenos y rocas volcanicas. Localmente, el rifting pre-varisco origino una fabrica milonitica y un metamorfismo de grado medio-alto, como el que se observa en Valuengo y en Monesterio. Como consecuencia del rifting cambro-ordovicico, la Zona de Ossa-Moreno debio de ser en el Silurico un fragmento aislado de corteza continental.

Diapiric emplacement in the upper crust of a granitic body: the La Bazana granite (SW Spain)

Abstract The ascent and emplacement of granites in the upper crust is a major geological phenomenon accomplished by a number of different processes. The active processes determine the final geometry of the bodies and, in some favourable cases, the inverse problem of deducing mechanisms can be undertaken by relying on the geometry of plutons. This is the case of the La Bazana granitic pluton, a small Variscan igneous body that intruded Cambrian rocks of the Ossa-Morena Zone (SW Iberian Massif) in the core of a large late upright antiform. The granite shows no appreciable solid-state deformation, but has a late magmatic foliation whose orientation, derived from field observations, defines a gentle dome. The regional attitude of the main foliation in the country rock (parallel to the axial plane of recumbent folds) is NW–SE, but just around the granite, it accommodates to the dome shape of the pluton. Flattening in the host rock on top of the granite is indicated by boudinaged and folded veins, and appears to be caused by an upward pushing of the magma during its emplacement. The dome-shaped foliation of the granite, geometrically and kinematically congruent with the flattening in the host rock, can be related in the same way to the upward pushing of the magma. The level of final emplacement was deduced from the mineral associations in the thermal aureole to be of 7–10 km in depth. Models of the gravity anomaly related to the granite body show that the granite has a teardrop–pipe shape enlarged at its top. Diapiric ascent of the magma through the lower middle crust is inferred until reaching a high viscous level, where final emplacement accompanied by lateral expansion and vertical flattening took place. This natural example suggests that diapirism may be a viable mechanism for migration and emplacement of magmas, at least up to 7–10 km in depth, and it provides natural evidence for theoretical discussion on the ability of magmatic diapirs to pierce the crust.

Pedological nodules with cone in cone structure in the Permian of Sierra Morena (Spain) and Central Morocco

The occurrence of (1) calcite aureoled quartz, and (2) vertical cylindrical features (“rhizocretions” in literature), and subspherical or unevenly shaped nodules resulting from the coalescence of smaller elements (“nodular calcrete”) is reported in various Permian continental series from Western Europe (Sierra Morena, Spain) and North Africa (Central Meseta of Morocco). These features are developed in flood plain pelites (mudstones) interbedded with gravelly channel material and debris flow deposits. These nodules are interpreted as pedological concretions formed in hydromorphic soils under tropical climatic conditions with contrasting seasons and not under desertic environments.These nodules show the close association of a classical micritic/microsparitic internal sediment with areas where cone in cone structures prevail. The distribution of facies suggests an early pedogenic origin. At this early stage, vegetal roots were alive (probably Cordaitales) and were encrusted by the redistribution of calcitic components in the soil profile resulting from oscillations of the water table. The cone-in-cone structure probably resulted from bacterial activity.

Dependence with air density of the response of the PTW SourceCheck ionization chamber for low energy brachytherapy sources

PURPOSE Air-communicating well ionization chambers are commonly used to assess air kerma strength of sources used in brachytherapy. The signal produced is supposed to be proportional to the air density within the chamber and, therefore, a density-independent air kerma strength is obtained when the measurement is corrected to standard atmospheric conditions using the usual temperature and pressure correction factor. Nevertheless, when assessing low energy sources, the ionization chambers may not fulfill that condition and a residual density dependence still remains after correction. In this work, the authors examined the behavior of the PTW 34051 SourceCheck ionization chamber when measuring the air kerma strength of (125)I seeds. METHODS Four different SourceCheck chambers were analyzed. With each one of them, two series of measurements of the air kerma strength for (125)I selectSeed(TM) brachytherapy sources were performed inside a pressure chamber and varying the pressure in a range from 747 to 1040 hPa (560 to 780 mm Hg). The temperature and relative humidity were kept basically constant. An analogous experiment was performed by taking measurements at different altitudes above sea level. RESULTS Contrary to other well-known ionization chambers, like the HDR1000 PLUS, in which the temperature-pressure correction factor overcorrects the measurements, in the SourceCheck ionization chamber they are undercorrected. At a typical atmospheric situation of 933 hPa (700 mm Hg) and 20 °C, this undercorrection turns out to be 1.5%. Corrected measurements show a residual linear dependence on the density and, as a consequence, an additional density dependent correction must be applied. The slope of this residual linear density dependence is different for each SourceCheck chamber investigated. The results obtained by taking measurements at different altitudes are compatible with those obtained with the pressure chamber. CONCLUSIONS Variations of the altitude and changes in the weather conditions may produce significant density corrections, and that effect should be taken into account. This effect is chamber-dependent, indicating that a specific calibration is necessary for each particular chamber. To our knowledge, this correction has not been considered so far for SourceCheck ionization chambers, but its magnitude cannot be neglected in clinical practice. The atmospheric pressure and temperature at which the chamber was calibrated need to be taken into account, and they should be reported in the calibration certificate. In addition, each institution should analyze the particular response of its SourceCheck ionization chamber and compute the adequate correction factors. In the absence of a suitable pressure chamber, a possibility for this assessment is to take measurements at different altitudes, spanning a wide enough air density range.