Abstract: Inversion Structures of the Central Zone of the Eastern Cordillera, Colombia

Freitas, Mario G. - Braspetro; João B.L. Françolin - Petrobras/Cenpes

The Eastern Cordillera of Colombia is a fold and thrust belt resulting from the Neogene inversion of a large Mesozoic-Cenozoic basin. The stratigraphic succesion comprises Paleozoic sediments resting on Proterozoic metamorphic basement, followed by marine Triassic and continental Jurassic deposits. A large marine basin persisted from Early Cretaceous to Maastrichtian, followed by dominantly continental Tertiary deposits.

The central zone of the Eastern Cordillera is dominated by a synclinorium cored by Paleogene sediments, with two adjacent anticlinoria composed of Lower Cretaceous or older rocks ( 1). Integration of a variety of geological and geophysical information revealed inversion structures comprising thick-skinned, thin-skinned and strike-slip faults and folds of various scales and styles. Primary controls on inversion were the Mesozoic rift basin configuration and the mechanical stratigraphy. Fault slip data indicate a main tectonic transport to azimuth 130 (NW-SE), nearly perpendicular to the rift orientation. This difficulted direct fault reactivation and favoured the generation of basement-involved shortcuts. Low angle thrusts, dominantly east-verging, are widespread. They present staircase flat-ramp profiles, with associated anticlines and synclines. Flats develop in Lower and Upper Cretaceous shales and ramps on sandy units of Mid and Upper Cretaceous. Ramps innitiate preferably on the edges of rift fault blocks. The majority of the large anticlines are nearly symmetrical fault bend folds. Fault propagation folds are also present, creating overturned structures. Subsidiarily, strike-slip faulting appears as sinistral NW-SE, N-S faults and E-W dextral ramps.

Structural style changes from north to south. The northern area is characterised by shallower basement and large, long-wavelength basement-involved structures (e.g., Boyacá, Soapaga thrusts), strongly SE-verging. Basement is deeper in the south, where thin-skin is dominant, facilitated by Lower Cretaceous evaporites. A transional domain consists of smaller, fragmented structures along a transpressional belt, E-W dextral in the east and N-S to NW-SE and sinistral in the west, reflecting a basement control, by southward narrowing of the rift through en echelon faults. Cross-section A-A´ ( 2) illustrates inversion structures in the central study area, including basement-involved shortcuts (eg Cormichoque) and fault reactivation (Soapaga) in a dominantly SE-verging imbricate fan, as well as preserved extensional faults.

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