GRAVITY CONSTRAINTS ON THE SIBUNDOY VALLEY, EASTERN CORDILLERA OF COLOMBIA: STRIKE-SLIP OR THRUST DOMINATED TECTONICS?

The Sibundoy Valley is a hinterland basin, located in the Eastern Cordillera of Colombia, near the Ecuadorian border. The valley has a rhomboidal shaped with an approximate area of 100 km², trending to the northeast. This hinterland basin is at the wedge top of a retro-arc basin between the Andean magmatic arc and the Guiana craton. Here, thrust faults are active due to a constant oblique convergence between the Nazca Plate and the South American Plate. In addition, strike-slip faults are also active due to the northwestern movement of the North Andean Sliver (NAS) with a slip rate of 7.7 ± 0.4 to 11.9 ± 0.7 mm/year. 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.

Gravity measurements indicate the Sibundoy basin is characterized by a 25-30 mGal negative Bouguer anomaly. Sedimentary rocks from the basin have a density of 2.2 g/cm³. South of the basin there are higher density Mesozoic sedimentary and plutonic rocks including the Jurassic Mocoa batholith, and associated metamorphic basement with densities that range from 2.5-2.7 g/cm³. North and west of the valley exist Neogene volcanic rocks that range from andesitic lavas to pyroclastic deposits. Such younger volcanic rocks have densities of 2.4-2.6 g/cm³. Our model reveals that the basin is asymmetric with the largest steps on the southeastern side and more gradual changes on the northwestern side. Thus, this suggests that the southern Massif thrusted upward and blocked the flow out of the basin causing it to fill with sediment. This uplift event may have created the valley.

Overall, initial gravity modeling indicates a sedimentary thickness for the Sibundoy valley of 1 km to 2.5 km, and that the higher density Mesozoic rocks to the south may be partially thrust over the basin itself. This observation, along with the apparent lack of step-over related normal faults, suggest the thrust fault dominated model (model 2) may better fit the existing data.