Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-and-Thrust Belt
Abstract: The South Portuguese Zone (SPZ), which host world-class massive sulphide deposits, forms the southern fold-and-thrust belt of the Iberian Variscan orogeny. This thesis focuses on seismic-reflection and seismic-refraction processing efforts on a subset of the IBERSEIS deep seismic-reflection data set aiming at resolving the SPZ upper crust in high resolution.A comparison of different crooked-line seismic-reflection imaging schemes showed that a processing sequence involving dip-moveout corrections, a common-midpoint projection, and poststack time migration of common-offset gathers provided the most coherent images considering the crooked acquisition geometry. Correlation with surface-geological data allows four units of different reflection character to be identified: the ~0–2 km deep Upper Carboniferous Flysch group, the highly reflective ~2–4 km thick and up to ~5 km deep Volcano-Sedimentary Complex (VSC) group, and two deep Paleozoic metasedimentary units, with the shallower Phyllite-Quartzite group exposed in an antiform. Prominent diffracted energy was enhanced using a modified Kirchhoff imaging routine. High reflectivity and distinct diffractions mark extensive dike bands at 6–12 km depth, possibly related to the intense hydrothermal activity that led to the formation of the ore-bearing VSC group.Source-generated noise obscures potential signals from depths shallower than ~500m depth on the seismic-reflection sections. P- and SV-wave first-arrival traveltimes were inverted for velocity models imaging the shallowest crust. Overall, the velocity models correlate well with surface-geological data marking high (>5.25 km/s) and uniform P-velocities for the Flysch unit in the southern SPZ. A prominent P-wave low-velocity body (~4.5 km/s) is resolved where the Phyllite-Quartzite unit forms the core of an antiform. P-velocities fluctuate the most in the northern SPZ with Flysch group units exhibiting high velocities (>5.25 km/s) and VSC group bodies showing intermediate velocities (~5 km/s). Low VP/VS-ratios (~1.8) computed for the southern profile part are interpreted as less deformed Flysch-group units, whereas high VP/VS-ratios (~1.9) indicate fractured units.
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