Determinación de la longitud de la falla y velocidad de ruptura para terremotos de magnitud media

En el proceso de inversión de la función de directividad para la obtención de la longitud de falla y velocidad de mptura, en el caso de terremotos de magnitud media, es muy dificil obtener ambos parámetros independientemente y deben hacerse hipótesis sobre uno de los dos parámetros. Proponemos una solución a este problema mediante un proceso iterativo, que iniciamos con un valor hipotético de la velocidad de ruptura. El resultado final resulta ser independiente del valor de partida. Mediante este proceso hemos obtenido la longitud de falla y velocidad de ruptura de dos terremotos, el del 18-nov-1970, ocurrido en la Dorsal Oriental del Pacifico, y el del 4-jul-1966, ocurrido en la Dorsal Centro-Atlántica. Para el sismo del 18-nov-1970 los valores obtenidos en la b = (18 4- 1) km v = (2.3 f 0.1) kmls y para el sismo del 4-jul-1966 b = (19 f 2) km, v = (2.1 i 0.1) kmls. Las desviaciones tipicas de estos parámetros, del orden del 5-10 %, son inferiores a las obtenidas en trabajos previos.

Archaeology on the Road, July 2010

Not only are we excited that Team Archaeology is back for our third ride, we are energized to be part of a “Human and Natural History” partnership that allows us expanded opportunities to share the story of Iowa’s amazing past. Once again there will be archaeologists along for the ride, as well as at Expo and this year at roadside locations Day One, Five and Six. Don’t hesitate to ask about the history of the first people to travel this landscape as well as the stories of each generation that has contributed to what we know of ourselves today. We will also feature information about the landscape and natural resources of Iowa you will encounter along the route through our partnering colleagues specializing in geology, hydrology, and other earth sciences. Enjoy using this booklet as your guide to the week’s activities and please help yourself to free materials from our outreach booth about our shared past and the natural world we depend on. Ride smart, be safe, and when you get home, be sure to tell your friends and neighbors about Iowa archaeology!

Metallogenic model of the Trepca Pb-Zn-Ag skarn deposit, Kosovo: Evidence from fluid inclusions, Rare Earth Elements, and stable isotope data

The Trepca Pb-Zn-Ag skarn deposit (29 Mt of ore at 3.45% Pb, 2.30% Zn, and 80 g/t Ag) is located in the Kopaonik block of the western Vardar zone, Kosovo. The mineralization, hosted by recrystallized limestone of Upper Triassic age, was structurally and lithologically controlled. Ore deposition is spatially and temporally related with the postcollisional magmatism of Oligocene age (23-26 Ma). The deposit was formed during two distinct mineralization stages: an early prograde closed-system and a later retrograde open-system stage. The prograde mineralization consisting mainly of pyroxenes (Hd(54-100)Jo(0-45)Di(0-45)) resulted from the interaction of magmatic fluids associated with Oligocene (23-26 Ma) postcollisional magmatism. Whereas there is no direct contact between magmatic rocks and the mineralization, the deposit is classified as a distal Pb-Zn-Ag skarn. Abundant pyroxene reflects low oxygen fugacity (<10(-31) bar) and anhydrous environment. Fluid inclusion data and mineral assemblage limit the prograde stage within a temperature range between 390 degrees and 475 degrees C. Formation pressure is estimated below 900 bars. Isotopic composition of aqueous fluid, inclusions hosted by hedenbergite (delta D = -108 to -130 parts per thousand; delta O-18 = 7.5-8.0 parts per thousand), Mn-enriched mineralogy and high REE content of the host carbonates at the contact with the skarn mineralization suggest that a magmatic fluid was modified during its infiltration through the country rocks. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, quartz, and various carbonates. Increases in oxygen and sulfur fugacities, as well as a hydrous character of mineralization, require an open-system model. The opening of the system is related to phreatomagmatic explosion and formation of the breccia. Arsenopyrite geothermometer limits the retrograde stage within the temperature range between 350 degrees and 380 degrees C and sulfur fugacity between 10(-8.8) and 10(-7.2) bars. The principal ore minerals, galena, sphalerite, pyrite, and minor chalcopyrite, were deposited from a moderately saline Ca-Na chloride fluid at around 350 degrees C. According to the isotopic composition of fluid inclusions hosted by sphalerite (delta D = -55 to -74 parts per thousand; delta O-18 = -9.6 to -13.6 parts per thousand), the fluid responsible for ore deposition was dominantly meteoric in origin. The delta S-31 values of the sulfides spanning between -5.5 and +10 parts per thousand point to a magmatic origin of sulfur. Ore deposition appears to have been largely contemporaneous with the retrograde stage of the skarn development. Postore stage accompanied the precipitation of significant amount of carbonates including the travertine deposits at the deposit surface. Mineralogical composition of travertine varies from calcite to siderite and all carbonates contain significant amounts of Mn. Decreased formation temperature and depletion in the REE content point to an influence of pH-neutralized cold ground water and dying magmatic system.

Mineralogical, K-Ar, stable and Sr isotope systematics of K-white micas during very low-grade metamorphism of limestones (Helvetic nappes, Western Switzerland)

Mineralogical, K-Ar, Rb-Sr and stable isotope analyses have been carried out on K-white micas from Helvetic Malm limestones in order to examine their evolution during very low- to low-grade Alpine metamorphism, associated with intense ductile deformation. Metamorphic temperatures were estimated al approximately 300-degrees-C from stable isotopes (quartz-calcite thermometry), occurrence of chloritoid, and `'epizonal'' illite crystallinity index. K-white micas consist of variable mixtures of 2M, phengite and muscovite, as revealed by detailed X-ray diffraction analyses using peak decomposition of the (060, 331) spectra. K-Ar apparent ages display a strong grain-size dependence in which mainly fine-grained size fractions (< 2 mum) record Alpine ages (37-15 Ma). However, these ages provide a relative rather than an absolute chronology of the diachronous Alpine metamorphic evolution of the Helvetic nappes. The resetting of the K-Ar isotopic system of K-white micas to Alpine metamorphic conditions reflects an apparent combination of crystallization/recrystallization and radiogenic Ar-40 diffusion loss. The oxygen isotope compositions of micas (+ 15 to + 22 parts per thousand) are intermediate between detrital and O-18-enriched values expected for micas neoformed within an abundant marine carbonate matrix. No isotopic equilibrium has been reached between calcite and micas. The variable depletion of hydrogen isotope compositions (- 126 to - 82 parts per thousand) is influenced by the interaction with organic matter under closed-system conditions. Organic matter, if not removed, may also represent a serious source of error in K-Ar age determination, by introducing radiogenic Ar-40 contamination. Sr-87/Sr-86 isotope ratios of micas range from 0.70879 to 0.70902 with one outlier at 0.71794. The low values reflect Sr exchange with calcite occurring during crystallization/recrystallization of micas under closed-system conditions.

Late Jurassic continental flood basalt doleritic dykes in northwestern Cuba: remnants of the Gulf of Mexico opening

Accreted terranes, comprising a wide variety of Late Jurassic and Early Cretaceous igneous and sedimentary rocks are an important feature of Cuban geology. Their characterization is helpful for understanding Caribbean paleogeography. The Guaniguanico terrane (western Cuba) is formed by upper Jurassic platform sediments intruded by microgranular dolerite dykes. The geochemical characteristics of the dolerite whole rock samples and their minerals (augitic clinopyroxene, labradorite and andesine) are consistent with a tholeiitic affinity. Major and trace element concentrations as well as Nd, Sr and Pb isotopes show that these rocks also have a continental affinity. Sample chemistry indicates that these lavas are similar to a low Ti-P2O5 (LTi) variety of continental flood basalts (CFB) similar to the dolerites of Ferrar (Tasmania). They derived from mixing of a lithospheric mantle Source and an asthenopheric component similar to E-MORB with minor markers of crustal contamination and sediment assimilation. However, the small quantity of Cuban magmatic rocks, similarly to Tasmania, Antarctica and Siberia differs from other volumetrically important CFB occurrences Such as Parana and Deccan. These dolerites are dated as 165-150 Ma and were emplaced during the separation of the Yucatan block from South America. They could in fact be part of the Yucatan-South America margin through which the intrusive system was emplaced and which was later accreted to the Cretaceous arc of central Cuba and to the Palaeogene arc of eastern Cuba. These samples could therefore reflect the pre-rift stage between North and South America and the opening of the gulf of Mexico.

Estructura del macizo ofiolítico de Moa (NE de Cuba) según la interpretación del levantamiento aeromagnético 1:50.000

La reducción al polo, los gradientes horizontales, los mapas de relieves sombreados y la continuación analítica ascendente (CAA) constituyen transformaciones del campo magnético ampliamente utilizadas durante el estudio de la estructura geológica de una región. En la región de Moa (NE de Cuba) estas transformaciones se emplean para resaltar alineaciones en los datos magnéticos que constituyen estructuras tectónicas disyuntivas o zonas de contactos abruptos, así como la estructura geológica en profundidad. Con el análisis de estas transformaciones del campo se comprobó que los principales sistemas de fallas de la región de estudio se manifiestan en el comportamiento del campo magnético, particularmente en los mapas de relieve sombreados a partir de zonas alineadas. En estos mismos mapas se repiten alineaciones con dirección noroeste y noreste que no coinciden con las estructuras descritas en los mapas morfotectónico y geológico, destacando zonas que pueden constituir contactos tectónicos o litológicos no citados en trabajos anteriores. Para la mayoría de las estructuras, los mapas analizados sugieren posiciones, longitudes e incluso formas algo distintas a las señaladas en los mapas morfotectónico y geológico. A partir del modelaje interactivo en los perfiles de interpretación trazados a través de las principales anomalías, se deduce que loscuerpos anómalos poseen formas de cuñas y de capas verticales o ligeramente inclinadas en algunos de sus extremos. Además, yacen a poca profundidad (0-400 m), con la excepción de algunos cuerpos que pueden alcanzar más de 1000 m de profundidad en su límite inferior (perfil III-III' y XII-XII'). Estos resultados corroboran el carácter alóctono de las unidades ofiolíticas en esta región.

Estudi microtermomètric dels filons de quars de les Esquerdes de Rojà, Massís del Canigó, Pirineu Oriental

Els materials pre-hercinians i el granit de Costabona estan travessats per un conjunt de filons de quars de fins a 15 km de longitud, coneguts com a 'Esquerdes de Roja'. Aquests filons estan localment afectats per un sistema de bandes milonítiques i per un altre posterior de fractures. A partir de l'anilisi microtermometrica de les inclusions fluides primàries en quars, ha estat obtinguda una temperatura de formació de l'ordre de + 280 C per la major part del quars, i una temperatura de + 250 C per als monocristalls que omplen esquerdes i cavitats. Els filons s'haurien format a partir de solucions mineralitzants que contenien ~ a + C, a++, K+ i, en menor proporció, ~ g + + .fins una salinitat total de l'ordre del 20/@ en pes de la Na CI.

El análisis canónico y su aplicación en Geologia

Se exponen los fundamentos del análisis canóiiico, combinación del análisis de la varianza y del factorial, así como sus posibles aplicaciones a los problemas geológicos.

Los depósitos aluviales sintectónicos de la Pobla de Segur: alogrupos y su significado tectonoestratigráfico

El análisis estratigráfico y estructural de los conglomerados finieocenos y oligocenos de La Pobla de Segur (Formación de Collegats) localizados en los Pirineos centrales, ha permitido correlacionar diferentes episodios de sedimentación y de estructuración tectónica. Los materiales que constituyen el relleno de la cuenca intramontañosa de La Pobla de Segur (facies de abanico aluvial y deltaico, con intercalaciones lacustres subordinadas) reposan discordantemente sobre un substrato Mesozoico previamente deformado y estan organizados en una serie de cuñas clásticas que alcanzan un espesor acumulativo mínimo de 3500 m. El reconocimiento de superficies de discontinuidad, cartografiables a lo largo de toda la cuenca, ha permitido la definición de 5 alogrupos (Pessonada, Ermita, Pallaresa, Senterada y Antist) que a su vez pueden subdividirse en aloformaciones o secuencias. Las relaciones estructurales entre el suhstrato y los depósitos aluviales han permitido establecer la cronologia de la deformación compresiva tardía que afectó a este sector de la cadena sur-pirenaica y precisar el orígen claramente tectónico de las discontinuidades que constituyen los límites de los alogrupos.

Primer Seminario Iberoamericano de Cuencas de Antepaís: Andes, Pirineos y Varíscides. Proyecto TRANSCUYO

El presente volumen, que corresponde a los dos primeros números del volumen 32 de ACTA GEOLOGICA HISPANICA, está dedicado a algunos de los aspectos geológicos más importantes de la zona de los Andes Centrales Argentino-Chilenos, y constituye una recopilación de trabajos de diferentes autores americanos y españoles.