Geophysical investigation of the thermo-rheological state of Yellowstone Caldera

The thermal rheological state along with the depth to the Curie isothermal surface have been investigated in this thesis in order to provide a solid and a consistent subsurface image of the crust under Yellowstone caldera. This latter well-known caldera represents a unique geological and geophysical laboratory, where complex geodynamic processes in a continental hotspot are manifested, and it has been the center of interest for decades monitored since the fifties. In the first part of this manuscript, I started by mapping the distribution of the depth to the Curie isothermal surface. To this end, I assume that the bottom of the magnetic crust corresponds to the depth of the iso-Curie surface where the magnetic minerals lose their magnetization due to the increase of temperature. Comparing with previous findings, our outcomes are computed with new aeromagnetic dataset with higher resolution, which play a key role in the improvement of the final results. I used two techniques based on spectral analysis of the magnetic anomalies: the first approach is the modified centroid method assuming a statistical ensemble of blocks of varying depth, width, thickness, and magnetization, each one uniformly distributed. It is simple to argue that this kind of distribution is not uncorrelated but it is instead a correlated distribution of magnetic sources, with a sloping exponent of about 3 that is within the fractal range. The misunderstanding may be explained in this way: even though the magnetization spectral factor is constant (a white power spectrum), the magnetization distribution must instead be defined by the product of spectral factors related to the prism size, magnetization and thickness, which is a red power spectrum, as discussed above. The second approach is the fractal distribution of magnetic sources, for which a spectral factor exists with a sloping exponent within the fractal range (from 2 to 4). The application of the two methods to our dataset suggest that in both cases, a shallow Curie isothermal surface ranging from 1 km to 5 km is retrieved. The second part of the thesis is focused on building a 3D thermal model. It was constructed through solving a 3D finite element problem of heat transfer in a conductive system using Comsol Multiphysics software involving a trial and error optimization of the density, velocity models jointly with the geothermal heat source parameters. The previous obtained depth to the Curie isothermal map was a key constraint parameter to check the validity of the thermal model. Subsequently, a 3D rheological model was built using the 3D temperature model as an input along with the geological and the geophysical information from literature. The most part of earthquakes epicentres were found to be concentrated in the brittle zone of the volume while the ductile zone is not totally homogeneous confirming the sandwich theory of the crust suggested by different authors which states that the ductile zone contains some brittle stratification. This brittle-ductile volume is the first 3D mapping of the rheological features of the crust under Yellowstone caldera. The results obtained from this work can be considered as a new valuable information that provide a new insights into Yellowstone caldera, improve the monitoring of the volcanic evolution and assess the risk of its hazard.