GRAVITY CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE SIBUNDOY VALLEY, EASTERN CORDILLERA OF COLOMBIA

Abstract

The Sibundoy Valley is a hinterland basin, located in the Eastern Cordillera of Colombia, near the Ecuadorian border. The valley trends to the northeast and has a rhomboidal shape with an approximate area of 100 km2. The hinterland basin is near the wedge top of a retro-arc basin between the Andean magmatic arc and the Guiana craton. Here, multiple fault types are active due to oblique convergence between the Nazca Plate and South America Plate. The North Andean Block is moving with a slip rate of 21-22 mm/year eastward and a 5-6 mm/year northward. Therefore, two potential models are proposed to explain the tectonic evolution of the Sibundoy Valley. In model 1, a step over between strike-slip faults produced a pull apart basin. In model 2, a system of reverse faults produced a contractional piggyback basin. To distinguish between these models, a combination of surface geology, gravity modeling and density measurements has been used. Complete Bouguer Anomaly values indicate that the Sibundoy Valley has a negative anomaly of 30 mGal. The density of the basin infill is approximately 1.8 g/cm3. This density estimate is constrained by measured values of surface rocks and comparison of variable density subsurface gravity models. Overall, the 3-D shape of the basin was determined by gridding together multiple 2-D gravity transect models. These show the basin has a half-graben shape where the deepest depression (-1300 m) is located at the southwestern side of the basin, and it thins to the northeast. It is likely that a normal fault created most of the accommodation space at the southwestern side of the basin, creating the half-graben shape. The northwest to southeast trending models suggests that a shallowly dipping fault outcrops at the northwestern side of the basin and extends as a master fault underneath the basin to the south. Gravity models also disallow a traditional strike-slip step over geometry as steep strike-slip basin bounding faults do not fit measured gravity data. Unfortunately, the gravity models do not provide the kinematics of the master fault, despite well constraining its geometry so the three different types of basins (piggyback, pull-apart, and hinterland basins) are still possible. Nevertheless, a rifting (or supradetachment) hinterland basin may be the most likely model to describe the tectonic evolution of the basin, considering the shape of the basin, geologic setting, and transtentional kinematics of the faults.