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Abstract: Kinematics of a Raft / Listric Fault System From 3D Restoration

Rouby, Delphine and François Guillocheau - Géosciences Rennes CNRS; Stéphane Raillard - Elf; Renaud Bouroullec - Imperial College

In order to (1) understand the sedimentary and tectonic evolution through time of the albo-Cenomanian platform offshore Congo and (2) quantify the displacements associated with the raft structures, we built a 3D geometrical model of a raft/listric faults system. The 3D structural model covers : (1) the upslope and downslope lower-Albian rafts, respectively footwall and hanging wall of the main listric fault, (2) the upper-Albian and Cenomanian rollover associated with this fault and (3) the Cenomanian graben located between the roll-over and the downslope raft.

To study the interactions between tectonics and sedimentation processes, we performed two sets of measurements.

(1) From the sequential stratigraphic analysis of 11 wells located throughout the area, we have defined 13 sets of genetic units by correlation. For each regressif or trangressif half cycles, we measure the amount of deposited sediments and the associated variation of bathymetry. This gives us the variations through time of vertical accommodation rates and by correction from eustasy: the vertical displacement rate. These measures indicate a significant increase of the subsidence at the end of the Albian.

(2) From the interpretation of 3D seismic data we built a depth converted geometrical model showing 6 albo-Cenomanian horizons. We restored these horizons in 3D. The difference between the restored state and current state gives us the 3D displacement fields for 5 increments of time (defined by the 6 horizons). We quantified the displacements between the upslope and downslope rafts associated with faults and turtle structures. This indicates: (i) horizontal displacements of 6km between the upslope and slope raft at the end of the Albian and 16km at the end of the Cenomanian; this displacement is accommodated by both the main listric fault and the graben; (ii) a significant counterclockwise rotation about a vertical axis of the downslope raft with respect to the upslope raft (about 15°); (iii) a seaward divergent displacement field associated with the main fault resulting in a significant radial extension; (iv) the graben is actually a fault relay between an east-verging fault to the north and a west-verging fault to the south ; the graben is active only at the end of the Cenomanian; (v) the main fault is active during the whole albo-Cenomanian.

We compare the displacements of the well markers measured by the restoration (horizontal component) and by accommodation measurements (vertical component). We show that the amount of horizontal displacements is 10 to 20 times larger than vertical displacements and should therefore not be neglected. We built new paleogeographic maps taking into account these horizontal displacements.

We discuss the distribution of 3D displacements and sediments in space and time.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil