Three-dimensional Structure and Kinematics of the Piedras-Girardot Fold Belt<subtitle>Surface Expression of Transpressional Deformation in the Northern Andes</subtitle>

Abstract

This paper presents a new kinematic model for the structure and tectonic evolution of part of the northern Andes of Colombia based on detailed geologic mapping, strain analysis, seismic interpretation, and 3-D forward modeling. This model indicates that the oblique convergence vector imposed by the Caribbean Plate is not fully partitioned in space, but instead must be distributed in a diffuse zone of transpressional deformation spanning the Cordilleras Oriental and Central. The Piedras-Girardot fold belt, located between the two cordilleras, contains the termination of two regional scale structures of the northern Andes: the Ibagu fault and the Guaduas syncline. The Ibagu fault, with a minimum dextral displacement of 30 km, is the southern boundary of a rigid indenter that was inserted to the east-northeast, causing contrasting deformation styles in the Mesozoic sedimentary sequence to the east and to the south of it. South of the rigid indenter, a complex array of diverging north- to northeast-trending faults and folds delineate a sigmoidal stepover to the left with faults verging outwardly in opposite directions that define the Piedras-Girardot fold belt. The kinematics of deformation indicate that this fold belt is a dextral transpressional system where oblique contraction is accommodated along north- and northwest-trending segments of faults, dextral strike-slip along their northeast-trending segments, and extension along the east-trending faults. This doubly vergent system has northwest-verging thrust faults with oblique displacements approximately parallel to the Ibagu fault (17-km Cambao fault; 7-km Camaito fault), as well as a southeast-verging roof thrust with approximately 8 km of oblique displacement to the east-northeast (Cotomal fault). A geometric fit constrained with a stratigraphic piercing point also requires counterclockwise rotation of the easternmost two thrust sheets by 7 and 13. A three-dimensionally admissible and valid palinspastic restoration of this fold belt requires approximately 52% east-northeast contraction (about 32 km), which is consistent with the minimum displacement of the Ibagu fault and other estimates of shortening in the eastern margin of the Cordillera Oriental. This component of deformation had not been previously identified and quantified in the northern Andes.