CONSTRAINING PRE-MIDDLE MIOCENE ANDEAN DEFORMATION BASED ON FORELAND BASIN GEOMETRY AND GEODYNAMIC MODELING: A CASE FROM THE NORTHERN ANDES

Major structures bounding the Eastern Cordillera of Colombia (EC) resulted from post-Middle Miocene tectonic activity, obscuring evidences of previous deformation events, however, the analysis of foreland sediments as tectonic sequences allow to reconstruct most of the tectonic pulses. A foreland tectonic sequence is related to one pulse of deformation bounded at the base by amalgamated sandstones in the foredeep and correlative unconformity in the distal foreland. Uppermost strata of a foreland sequence consists of muddy intervals with upsection increase of detrital content in coincidence with decrease of accommodation space. The proximal foredeep presents accumulation of dark-colored mudstones while deposition of amalgamated sandstones occur in the distal foreland, as migration of the flexural uplift advance.

Four pulses of deformation in the EC were identified by the analysis of palinspastically restored foreland tectonic sequences. The Maasthrichtian-lower Paleocene sequence consists of marginal to marine quartzarenites covered by paralic to fluvial coal-bearing strata. The upper Paleocene foreland sequence records northward and eastward migration of amalgamated fluvial quartzarenites, litharenites and subarkoses, which are overlain by dark-colored mudstones and channel-fill litharenites. Paleocene orogenic loads (<3 km high) covered the axial EC and migrated eastward 60 km. The Lower to Middle Eocene foreland sequence is composed mostly of amalgamated fluvial quartzarenites to sublitharenites. Orogenic loads (<1.5 km high) were located westward of the axial EC. The fourth sequence is recorded by the onset of >1-km-thick mud-rich strata and deltaic quartzarenites, sublitharenites and arkoses, and amalgamated fluvial quartzarenites in the distal foreland basin. Orogenic loads of 3-6 km high correspond to exhumed massifs that latter reached area near the Llanos foothills.

Foreland geometry and basin fill analysis help constructing kinematic models of the EC. Even though thermochronology and paleoecological techniques contribute to understand the process of orogenic exhumation and surface uplift, the study of post-Middle Miocene volcanics and associated hydrothermal activity in the EC, and analysis of Paleogene humid climate conditions, emerge as new techniques constraining the study of deformation in the EC.