Dynamic topography and the uplift of the Sierra Nevada de Santa Marta, Colombia

Abstract:

Surface observations of topography and gravity anomaly provide important information about the subsurface. Gravity observations over the Sierra Nevada de Santa Marta (SNSM), a 5.8 km high mountain in northern Colombia, South America, show that there is a positive Bouguer anomaly with a maximum value of ~170mGal. This suggests that the mountain is not isostatically compensated and is underlain by thin crust. However, the cause of the isostatic non-equilibrium is debated. One hypothesis is that this region experienced recent gravitational removal of the deep lithosphere, causing a local increase in the gravity anomaly, and dynamic uplift. We test this hypothesis with three 2D numerical models that use the ASPECT code. Model 1 starts with an area of topographic relief and thickened lithosphere, assuming this region underwent an earlier phase of shortening. Owing to its cool temperature, the mantle lithosphere is unstable, creating a downwelling that removes the lowermost mantle lithosphere. This is accompanied by a mantle upwelling that together with isostatic adjustment of the crust, induces dynamic uplift and generates an elevation of ~4.9 km in 90 Ma. However, the crust does not significantly thin, leaving a negative Bouguer gravity anomaly of -500 mGal. Model 2 follows the geometry of model 1, but eclogitization of the lower crust is included. Both the mantle lithosphere and the dense lower crust are unstable and founder into the deeper mantle. Following removal, the surface uplifts to 0.2 km in 40 Ma. Lower crustal removal enhances crustal thinning, resulting in a gravity anomaly of 20 mGal. In model 3, there is no lithosphere thickening, but a dense block is placed in the lower crust to simulate the formation of magmatic eclogite. The lower dense block founders leaving a thin crust that produces a positive gravity anomaly of ~140mGal. However, the crustal drip produces surface subsidence of ~1.3 km in 120 Ma. None of the three models produce a simultaneous positive gravity anomaly and high topography. Future work will test the effects of crustal elasticity and variations in crustal strength, as well as extending the models to 3D.