The standard model of the thermal and chemical structure of cratons has been scrutinized in recent years as additional data have been collected. Recent seismological and petrological studies indicate that the notion of cratonic lithosphere as a thick thermal boundary layer with a very depleted and dehydrated composition may be too simplistic and does not fully explain all aspects of the seismological and petrological observations. We hypothesized that the cratonic lithosphere may be more complicated and designed an experiment to investigate its thermal, chemical, and mineralogical properties using a global database of fundamental mode Rayleigh surface waves. To test this hypothesis, the phase velocities of Rayleigh wave that travel paths primarily over cratons were selected. A 1-D global craton phase velocity profile was generated from these observations and compared to predicted phase-velocity curves using two different forward modeling techniques. With the first approach, profiles of shear velocity were generated based on educated guesses of upper mantle temperatures using geotherms. With the second approach, profiles of shear velocity were generated using random permutations about 1-D global model STW105. In total 5,625 geotherm and 80,000 random 1-D forward models were generated for comparison. Each shear velocity model was converted to phase velocity and compared to the observed range of cratonic phase velocities, defined as within one standard deviation of the mean. This method was able to constrain shear velocity in cratons relatively well though the 1-D profiles deviate at depths shallower than 100 km. Shear velocity is faster than PREM/STW105 to depths greater than 200 km with constantly increasing velocity with depth in the random model and a low velocity layer at 100-150 km.

Made available in DSpace on 2015-04-22T21:58:27Z (GMT). No. of bitstreams: 1 Elizabeth Tavares Pimentel.pdf: 1766165 bytes, checksum: 014e86903e1eaf04dbad226c2450a908 (MD5) Previous issue date: 2009-04-06 / From October 2007 to September 2008 a geothermal monitoring experiment was conducted at depths of 0.02 m, 0.5 m and 1.0 m to quantify the variations of temperature, thermal conductivity and the shallow geothermal heat flow at places with and without vegetation cover in the Humaitá city, Amazonas. The influence of the vegetation cover on the shallow geothermal system was observed in the sites studied. There were variations of monthly average values of temperature between the places with and without vegetation cover. During the "dry" period, this variation was up to 6.01ºC at the depth of 0.02 m, and 2.84ºC at the depth of 1.0 m. During the "rainy" period, however, the variation was up to 2.94ºC, at the depth of 0.02 m, and 2.51ºC at the depth of 1.0 m. The difference of the daily extreme values of temperature between sites with and without vegetation cover were 3.97ºC during the "rainy" period and 9.63ºC during the "dry" period, at the depth of 0.02 m. It was noticed that at 06:00 PM the magnitude of the temperature remained high compared to other times on the day studied. The values of the thermal conductivity were 0.54 W/mºC during the "dry" period, and 1.23 W/mºC during the "rainy" period. The values of the shallow geothermal flows at depths of 0.5 m and 1.0 m, were 2.51 W/m² and 0.64 W/m², respectively. These values are 10³ larger than the terrestrial heat flow in the region. The thermal variations at0.5 m to 1.0 m depth are influenced by external sources that reach the surface and cannot be neglected. The thermal variations recorded in this work are important and fundamental to better understanding the shallow geothermal structure in the southern, part of Amazonas state, and they also contribute as input to models that allow the mitigation or elimination of the effects caused by anthropogenic actions / No período de outubro de 2007 a setembro de 2008 foi realizado monitoramento geotermal, às profundidades de 0,02 m, 0,5 m e 1,0 m, em locais com e sem cobertura vegetal, na cidade de Humaitá (AM), a fim de quantificar as variações de temperatura, condutividade térmica e fluxo geotermal raso local. Constatou-se a influência da cobertura vegetal sobre o regime geotermal raso na região estudada. Houve variação dos valores médios mensais da temperatura nos locais cc e sc. No período "seco", esta variação foi de até 6,01ºC à profundidade de 0,02 m, e de 2,84ºC à profundidade de 1,0 m; já no período "chuvoso", a variação foi de até 2,94ºC a 0,02 m de profundidade e de 2,51ºC à profundidade de 1,0 m. Na profundidade de 0,02 m, a diferença entre os valores diários máximos, nos locais cc e sc, foi de 3,97ºC no período "chuvoso" e de 9,63ºC no período "seco". Às 18 h, as magnitudes da temperatura permaneceram elevadas em relação aos outros horários estudados. Os valores de condutividade térmica foram de 0,54 W/mºC no período "seco" e de 1,23 W/mºC no período "chuvoso". Os valores do fluxo geotermal raso, às profundidades de 0,5 m e 1,0 m, variaram até 2,51 W/m² e 0,64 W/m², respectivamente. Tais valores são da ordem de 10³ acima do valor do fluxo térmico terrestre profundo na região. As variações termais a 0,5 m e a 1,0 m de profundidade são influenciadas por fontes externas que atingem a superfície e não podem ser negligenciadas. As variações térmicas registradas neste trabalho são importantes e fundamentais para o melhor conhecimento da estrutura geotermal rasa na cidade de Humaitá (AM), como também, contribuem para a elaboração de modelos que possibilitem mitigar ou eliminar os efeitos causados por ações antrópicas