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Deep crustal structure of the sheared South African continental margin

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Citation:
Parsiegla, N. , Gohl, K. and Uenzelmann-Neben, G. (2006): Deep crustal structure of the sheared South African continental margin , 12th International Symposium on Deep Seismic Profiling of Continents and their Margins, 24-29 Sept., Hayama, Japan. .
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Abstract:

The southern margin of South Africa offers an unrivalled possibility to enhance our understanding of the structure and processes involved in the formation of sheared continental margins. During the RV Sonne cruise SO-182 in 2005, the AWI acquired marine seismic reflection and refraction/wide-angle reflection data along two sub-parallel profiles (part of the Agulhas-Karoo Geoscience Transect) across the continental margin. A velocity depth-model was created using a forward modelling and an inversion approach. The observed crustal thickness along the western profile thins from 30 km on the inner continental shelf to 6 km in the Agulhas Passage (AP). A transition from continental to oceanic crust occurs at the Agulhas-Falkland fracture zone. A layer of unconsolidated sediments with P-wave velocities between 1.7 and 3.0 km/s and a thickness of about 2 km can be observed from the continental shelf to the Outeniqua Basin (OB). South of the OB, this layer becomes thinner and disappears in the AP. A layer with velocities between 3.5 and 5.0 km/s can be found along the entire profile. The seismic reflection data suggests that this layer may consist of alternating layers of volcanic flows and sediments. Metasediments may also play a role in the composition of this layer. Beneath the stratified sedimentary sequences of the OB, a 6 km thick zone with relatively low velocities of 5 6 km/s exists, suggesting pre-break-up metasediments which were altered by magmatic and tectonic processes. The top of the crystalline basement is marked by a sharp P-wave velocity increase from 5.7 to 6.6 km/s. Average velocities between 6.4 and 6.9 km/s were measured for the middle to lower crust. Uppermost mantle velocities of 7.8 to 8.0 km/s are observed from clear Pn phases.

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