Opposing Structural Vergence on Eastern Margin of Central Cordillera, Upper Magdalena Valley, Colombia

D. Q. Coffield, C. A. Barrio

Structural vergence along margins of compressional orogenic belts generally verge toward the foreland. Structures along the Ibague-Chaparral segment of the eastern Central Cordillera of Colombia contradict this and verge toward the orogen. Field observations of large-scale structures and small-scale kinematic indicators in Cretaceous and Paleogene rocks of the Upper Magdalena Valley along the cordilleran margin record two periods of deformation, each with distinct style and vergence, and explain the apparent discrepancy.

The bulk of the structures consists of Eocene to Oligocene west-verging tight and asymmetric anticlines floored by basement-rooted, moderate-angle, east-dipping thrust-fault ramps. These structures abruptly end against crystalline rocks of the Central Cordillera at the Calarma fault, a post-Miocene, very high-angle, west-dipping reverse fault. This fault consists of a rhombic system of margin-parallel and oblique segments with a component of right-lateral displacement. Basinward deformation is negligible.

The older west-verging structural grain continues south to the western margin of the Natagaima high, a major basement uplift within the Upper Magdalena Valley riding on the east-verging Chusma thrust system. The west-verging structures of the Ibague-Chaparral region are interpreted to be back-thrusts of the Chusma fault system and not "foreland structures" of the adjacent Central Cordillera. The lack of foreland deformation in front of the younger Calarma fault is due to its very high angle and component of right-lateral displacement which limited foreland-directed shortening.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.