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

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, for the first time, detailed 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 S(V) first-arrival traveltimes for the smoothest minimum-structure velocity models, imaging the shallowest few hundreds of metres along four in total similar to 60-km-long profiles. A comparison of a 2-D and 2.5-D (3-D forward and 2-D 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 S(V)-data due to greater data uncertainty, fewer number of picks and more limited source-receiver offsets compared with the P-data. The preferred final models compare favourably with the mapped surface geology, showing relatively high and uniform velocities (> 5.25 km s(-1)) for the flysch group in the southern part of the investigation area. Low velocities (similar to 4.5 km s(-1)) are found for the 'La Puebla de Guzman antiform' in the centre of the investigation area, where the phyllite-quartzite group is exposed. Velocities fluctuate the most along the northernmost similar to 20 km. Velocity variations reflect more the state of tectonic deformation than being directly correlated with the mapped lithologies. Based on a comparison with coincident seismic-reflection data along the southern half of the area, we suggest that two areas of low to intermediate V(P)/V(SV) ratios (similar to 1.85-1.9) correspond to occurrences of thick and less deformed flysch-group units, whereas high V(P)/V(SV) ratios (similar to 1.95) are interpreted to indicate increased porosity due to intense fracturing.