Imaging the Chilean continental margin using seismic wide-angle data
Chalbaud, Daniel

HaupttitelImaging the Chilean continental margin using seismic wide-angle data
TitelvarianteAbbildung der chilenischen Kontinentalrand mit seismischen Weitwinkel-Daten
AutorChalbaud, Daniel
Geburtsort: Caracas, Venezuela
GutachterProf. Dr. Serge Shapiro
weitere GutachterProf. Dr. Rainer Kind
Freie Schlagwörterseismic imaging; wide-angle data; subduction zone; Chile
DDC550 Geowissenschaften
ZusammenfassungWide-angle experiments represent one of the most common methods to study deep structures of the lithosphere in different tectonic settings. The principal benefit of these methods is the recording of strong amplitude signals reflected at larger incidence angles, originating not only from first order discontinuities, but also from higher discontinuities located in zones with small velocity contrast. Since 1993, in the frame of the Collaborative Research Center (SFB 267) “Deformation Processes in the Andes”, several active and passive seismic projects have been conducted along the Chilean continental margin with the aim to understand the subduction orogeny of the Andes and related surface processes. Between 19°S and 26°S the CINCA-95 project combined off-onshore seismic measurements including wide-angle and near-verticalreflection (NVR) data to investigate the evolution and structure of the erosive convergent margin. In southern Chile, between 36°S and 40°S, active seismic experiments during the SPOC-2001 project were carried out to compile data of the seismogenic plate interface and to study the geometry of the accretive convergent margin. In both projects the wide-angle experiments were performed using marine, amphibious as well as onshore layouts, covering a total area that extends from west of the trench to the recent volcanic arcs. The Kirchhoff prestack depth migration (KPSDM) is a robust algorithm that has provided successful results in the imaging of the Chilean convergent margin. Until now, only KPSDM has been applied to near-vertical-reflection (NVR) data on the region. We present the application of KPSDM to the CINCA and SPOC wide-angle datasets, with the aim to obtain new images of the seismogenic coupling zone in northern and southern Chile. In order to reduce the generation of artifacts during the migration process, the seismic data were pre-processed using a flow designed to attenuate the background noise originated by heterogeneities in the subsurface and the reverberations generated by the air-gun source. The velocity models used for migration were derived from interpretation and inversion of the most prominent arrivals interpreted on the record sections. The KPSDM of the CINCA wide-angle data imaged major features below the forearc region of the erosive continental margin in northern Chile. The migration results show the oceanic Moho as a strong dipping reflector at 25-32 km depth. The crystalline upper crust is imaged by a series of horizontal reflectors visualized in the amphibious and onshore data between 10-20 km depth, which reveals layering in a region composed mainly by remnants of the Paleozoic continental crust and the volcanic and granitoid rocks of the ancient Jurassic magmatic arc. Additionally, an east dipping reflector occurring above of the low velocity zone has been interpreted as the top of the subduction channel. A possible location of the continental Moho or downward continuation of the subduction channel below the Coastal Cordillera is indicated by a sharp east dipping reflector observed at 47-55 km depth in the migrated section of the onshore data. Fresnel Volume Migration (FVM), a novel extension of KPSDM, was applied to the data in northern Chile providing successful results with an improved resolution and signal to noise ratio. Reflection Image Spectroscopy (RIS) was applied by imaging the data in three narrow frequency bands in order to characterize the medium in terms of scattering concentration. Additional structural details were revealed and the occurrence of those imaged in the full frequency section was confirmed. Also, the prestack depth migration of narrower offset ranges provided results that support the structures imaged in the full offset section. In southern Chile the oceanic Moho has been imaged from the trench area at 13-14 km depth down to 42-49 km depth under the Coastal Cordillera. The migration of the amphibious data images structures in the upper continental crust beneath the offshore forearc. The reflectors observed on the migrated section may be associated to changes in the acoustic impedance caused by the intercalation of the basally accreted rocks of the Western Series, characterizing the upper continental plate as strongly segmented. The KPSDM results are compared with results of other geophysical methods applied in the region including gravimetry, magnetotellurics, hypocentre locations, heat-flow density and temperature distribution, seismic tomography, Vp/Vs ratios in order to describe the observed structures from a tectonic and geological point of view. Further, two geological models are proposed for the off-onshore forearc area of the erosive and accretive convergent margins, based on the results of previous investigations and those obtained in this study. In summary, the prestack depth migrated sections derived in the present study provide a first insight into the structural architecture of the Chilean seismogenic coupling zone which has not been imaged before.
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SeitenzahlXIV, 158 S.
Fachbereich/EinrichtungFB Geowissenschaften
Rechte Nutzungsbedingungen
Tag der Disputation19.05.2010
Erstellt am01.07.2010 - 13:44:02
Letzte Änderung13.07.2010 - 13:11:39
Statische URLhttp://www.diss.fu-berlin.de/diss/receive/FUDISS_thesis_000000017918