DISSECTION OF THE TABACO ANTICLINE: DETAILED RECORD USING DGPS

With approximately 246 km of lineal mapping in more than 16 horizontal slices, the Tabaco anticline is an outstanding laboratory to map the geometry and kinematics of deformation, and to test techniques to map, interpret, and quickly distribute geological data. The Tabaco anticline is located in northern Colombia where its core is currently being open-cast mined for coal in an area covering approximately 3,6 km² and containing over 20 different coal seams at nearly even intervals (~20 m) so that the mapped seam traces portray a comprehensive picture through a stratigraphic thickness of about 400 m. This anticline folds an overall southeast-dipping monocline that is wedged between an east-west dextral strike-slip plate boundary fault system (Oca fault) and a northwest-verging, northeast-trending thrust fault (the Cerrejon fault). The unconventional attitude of its axis (N20E), oblique to both structures, suggests the influence of escape in a structurally tight corner. Geologists have mapped over 3,600 intersections between the mining levels and the roof of the seams (200 km of traces) and over 1,000 fault traces (21 km of faults) using differentially-corrected GPS handhelds. The emerging picture of this seemingly simple anticline shows a three-dimensional network of northeast-trending, northwest-verging thrust faults with an overall fault density of up to 1.2 cm/m² arranged in right-stepping, en echelon segments with average throws of 69m and average lengths of 115m. Detailed observations on the hanging wall of a thrust fault with 16m of throw show a fault-damage zone 100m-wide with a poorly connected, but dense array of joints perpendicular to the main fault. Kinematic markers collected in old mining pits constrain a tectonic transport direction to the northwest (between 310 and 330 azimuth) consistent with shortening perpendicular to the strike of the Cerrejon fault. Geologic maps of the anticline are updated daily with extrapolation of geological structures and seam traces based on the new information. These maps are distributed to web users in the mine.