MEIER, B. P., Nold, Keller & Partners Geophysics, Switzerland, M. M. SCHWANDER, A/S Norske Shell, Norway, and H. P. LAUBSCHER, University of Basel, Switzerland

ABSTRACT: The Tectonics of Tachira: Insight into North Andean Deformation

Tachira forms the connecting link between the Merida Andes of Venezuela, the Cordillera Oriental of Colombia, and the Santander massifs. On the basis of surface mapping and subsurface data, its structural style and kinematics are discussed. Tachira is a pronounced structural depression relative to the Andean branches it connects. It comprises a bewildering network of structures, containing all the elements of the surrounding Andean branches. The structural relief as well as the adjacent foredeep were mainly formed during Neogene deformation. The Tachira depression is composed of a system of southwestward plunging inverted Early Cretaceous troughs and a tectonic block mosaic, with mostly transpressive block boundaries; dextral for the northeast-trending, sinistral for the southeast-tre ding zones of motion. The complex combinations of folding and thrusting with steep faults in transpressive zones are interpreted as "flower structures." The structural pattern is supplemented by the more purely compressive north-south trends, revealing large-scale thrusting and disharmonic fault-bend folding, and by east-west trending transfer zones of dextral shear. Uplift of the Cordillera Oriental against the Tachira depression was established by movement in a sinistral transpressive zone (Bramon fault), coupled with thrusting towards the Llanos foredeep.

The structural patterns of the Tachira depression fit a kinematic scheme of east-west compression. The compressional

shortening, apparently not exceeding 10 km, and 60 km of distributed strike-slip movement, similar to the displacement along the Bocono fault at the northeast end of the Merida Andes, appears compatible with the Neogene block movement of northern South America.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.