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Seismic-reflection imaging over the South Portuguese Zone fold-and-thrust belt
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2008 (English)In: Journal of Geophysical Research, ISSN 0148-0227, Vol. 113, no B8, B08301- p.Article in journal (Refereed) Published
Abstract [en]

We reprocessed an ∼35-km-long part of the IBESREIS seismic reflection profile which runs over the Iberian Pyrite Belt section of the South Portuguese Zone, SW Iberia, with the goal to image the upper crust (<15-km depth). The applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5-s travel time relative to high-amplitude source-generated noise. A complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked-line prestack migration scheme for coherent imaging. To interpret sources of diffracted energy, we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections.The final seismic images show south-vergent imbricate fold-and-thrust tectonics, documenting the contractive deformation that the South Portuguese Zone experienced during the Variscan Orogeny. Based on surface geological information, we correlate a low reflective unit with the shallow Upper Carboniferous Flysch Group, a highly reflective unit ranging in depth from 2 to 4 km with the Middle Carboniferous Volcano–Sedimentary Complex Group, which hosts massive sulfide deposits, and a moderately reflective unit with the Upper Devonian Phyllite–Quartzite Group. Below these units, another low-reflective facies is present, which may represent older Paleozoic metasediments. In addition, the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions. These proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in Early Carbonifereous times.

Place, publisher, year, edition, pages
2008. Vol. 113, no B8, B08301- p.
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-96410DOI: 10.1029/2007JB005341ISI: 000258156100002OAI: oai:DiVA.org:uu-96410DiVA: diva2:170976
Available from: 2007-11-08 Created: 2007-11-08 Last updated: 2013-11-01Bibliographically approved
In thesis
1. Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-and-Thrust Belt
Open this publication in new window or tab >>Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-and-Thrust Belt
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 87 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 362
Geophysics, Dip-moveout, Prestack time migration, Crooked-line geometry, Traveltime tomography, Inversion, Seismic velocity, IBERSEIS profile, Variscan orogeny, Iberian Pyrite Belt, Geofysik
National Category
Earth and Related Environmental Sciences
urn:nbn:se:uu:diva-8302 (URN)978-91-554-7016-6 (ISBN)
Public defence
2007-11-30, Axel Hambersalen, Geocentrum, Villavägen 16, 752 36 Uppsala, 10:00 (English)
Available from: 2007-11-08 Created: 2007-11-08 Last updated: 2009-03-24Bibliographically approved

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