Déformation

DU RIFT DE L'OKAVANGO - COUPLAGE AVEC LE CLIMAT ET LES TRANSFERTS DE MATIERE.

Le projet de ce chercheur vise à analyser les couplages entre le fonctionnement sédimentologique récent du delta de l’Okavango, la tectonique extensive et le climat. Il s’agit d’un projet pluridisciplinaire dans le cadre de l’OSUR regroupant tectonicien, géophysicien, sédimentologue et géographe. Ce projet est financé par l’appel d‘offre “Défis émergents 2010” de l’université de Rennes

Ma contribution porte sur la quantification de la déformation actuelle par GPS autour et dans le delta. On attend à la fois une contribution tectonique et hydrologique. Une mission de mise en place du réseau est prévue en octobre. 

Rift deformation in the Okavango Delta 

The project aim is to analyse the coupling between the recent Okavango Delta sedimentology fonctionning, the tectonic expansion and the climat.  It is a pluridisciplinary project in the frame of l'OSUR, grouping tectonicians, geophysicists, sedimentologists et geographs.

 

Geometry and faults

tectonic activity of the Okavango Rift Zone, Botswana:

Evidence from magnetotelluric and electrical resistivity tomography imaging.

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We used Magnetotelluric (MT) and Electrical Resistivity Tomography (ERT) to investigate the geometry and nature of faults activity of the Okavango Rift Zone (ORZ) in Botswana, an incipient rift at the southern tip of the Southwestern Branch of the East African Rift System. The ORZ forms a subtle topographic depression filled with Quaternary lacustrine and fluvio-deltaic sediments and is bounded by NE-trending normal faults that are more prominent in the southeastern portion of the rift basin. An MT model from regional (140 km) NW–SE trending MT transect shows that much of the rift basin is underlain by a broad asymmetrical low resistivity anomaly that slopes gently (1) from NW to SE reaching a depth of 300 m. This anomaly suggests that faults in the southeastern part of the rift form a NW-dipping border fault zone and that the lacustrine and fluvio-deltaic sediments contain brackish to saline water filling the broad half-graben structure. Furthermore, MT and ERT models from detailed (4–13 km long) MT transects and resistivity profiles show that one border fault (Thamalakane) and two within-basin faults (Lecha and Tsau) in the southeastern part of the ORZ are characterized by a localized high conductivity anomaly while another border fault (Kunyere) lacks such an anomaly. These localized anomalies are attributed to channelized fresh surface water and saline groundwater percolating through these faults forming ‘‘fault zone conductors’’ and suggest actively displacing faults. The lack of a ‘‘fault zone conductor’’ in the Kunyere fault is interpreted as indicating diminishing displacement on this fault, and that strain was transferred to the Thamalakane fault further to the east. The fluids provide lubricant for the ORZ faults, hence preventing infrequent large magnitude earthquakes, but favoring frequent microseismicity.


Early structural

development of the Okavango rift zone, NW Botswana

 The East African Rift System: Dynamics, Evolution and Environment

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Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030–050° in the north and 060–070° in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.