International Geologiical Congress - Oslo 2008


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EIE-07 High-resolution geophysical imaging of geological structures and processes in environmental studies


P- and SV-velocity structure of the South Portuguese Zone fold-and-thrust belt, SW Iberia, from traveltime tomography


Cedric Schmelzbach, Earth Science, Uppsala University (Sweden)
Christopher Juhlin, Uppsala University (Sweden)
Colin Zelt, Rice University (United States)


Imaging the architecture of the shallow crust of the South Portuguese Zone fold-and-thrust belt is essential to extend surface mapped geological information to depth and to help in developing models of the ore-bearing Iberian Pyrite Belt part of the Variscan orogeny. The recently acquired IBERSEIS seismic-reflection data set provides high-resolution images of the entire crust, but source-generated noise masks the earliest reflections and limits the shallowest observed signals to depths >500 m. We inverted P- and SV-first arrival traveltimes for the smoothest minimum-structure velocity models, imaging the shallowest few hundreds of meters along four in total 60 km-long profiles. A comparison of a 2D and 2.5D (3D forward and 2D inverse problem) crooked-line inversion scheme revealed that the crooked-line geometry has a negligible effect on the final images. Resolution of the final preferred models was assessed on the basis of an extensive series of checkerboard tests, showing a slightly lower resolution capability of the SV-data due to greater data uncertainty, a fewer number of picks, and more limited source-receiver offsets compared to the P -data. The preferred final models compare favorably with the mapped surface geology, showing relatively high and uniform velocities (>5.25 km/s) for the Flysch group in the southern part of the investigation area. Low velocities (∼4.5 km/s) are found for the La Puebla de Guzman antiform in the center of the investigation area, where the Phyllite-Quartzite group is exposed. Velocities fluctuate the most along the northernmost ∼20 km, with Flysch group units characterized by high velocities (>5.25 km/s) and large bodies of the Volcano-Sedimentary Complex group showing intermediate velocities (∼5 km/s). Based on a comparison with coincident seismic-reflection data along the southern half of the area, we suggest that two areas of low VP/VSV-ratios (∼1.8) correspond to occurrences of thick and less deformed Flysch-group units, whereas high VP/VSV-ratios (∼1.9) are interpreted to indicate increased porosity due to intense fracturing.


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