Three-dimensional structural evolution and kinematics of the Piedemonte Llanero, Central Llanos foothills, Eastern Cordillera, Colombia

Abstract

The Piedemonte Llanero is a wedge duplex zone in the Llanos foothills on the eastern flank of the Eastern Cordillera of Colombia. It is located northeast of the Cusiana and Cupiagua hydrocarbon fields. The area is characterized by a series of moderate to high-angle duplexes with east-southeast verging thin-skinned and thick-skinned tectonics. We present a structural model constrained by 2-D and 3-D seismic reflection data, surface geology, and well data. The structural analysis is based on backward modeling (kinematic restoration) and forward modeling using transfer-flexural slip and fault slip fold algorithms. The structures are significantly tighter in the northern segment compared to the southern segment of the overthrust trend. We estimate approximately 17 km of shortening in the northern duplex zone, and about 26 km total shortening for the southern duplex zone. We propose that thin-skinned in-sequence imbricate thrust stack deformation produced most of the shortening. The main Andean deformation (80% of the total shortening) commenced in the Piedemonte area about 6 Ma with rapid shortening and uplift in the area resulting in the development of an active-roof duplex structure as the cover was bulldozed forward by a horse block (Monterralo anticline) ramping up to a detachment at the base of C2, then ramping to the surface as the Yopal thrust. Later the horse blocks in the wedge rose, underthrusting the cover in a passive-roof duplex triangle zone. This was followed by an out-of-sequence Laramide-style thick-skinned basement-uplift of the range which produced much of the structural relief of the Eastern Cordillera. Cenozoic deformation in the Eastern Cordillera has been primarily range-normal, but has involved an increasing component of mountain-parallel right-lateral shear in the last 2 Ma.