TOPOGRAPHIC AND GRAVITY EXPRESSIONS OF LITHOSPHERE REMOVAL

Observations of topography and gravity anomalies, provide information about the subsurface. The Sierra Nevada de Santa Marta (SNSM) is a 5.8 km high mountain in NW South America, which is isolated from the Andes Cordillera and is not associated with the convergence between the South American and Nazca plates. The Bouguer anomaly over the SNSM has a maximum value of ±170mGal, suggesting that the mountain is not isostatically compensated and is underlain by thin crust. However, the cause of the isostatic non-equilibrium and the origin of its height is debated. As mantle dynamics can play an important role in geological evolution even in the absence of tectonic processes, it is possible that the SNSM 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 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 ±1 km in 75 Ma. Lower crustal removal enhances crustal thinning, resulting in a gravity anomaly of ±30 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 drip produces surface subsidence of ±1.3 km in 120 Ma.

None of the three models produces a simultaneous positive gravity anomaly and high topography that matches the observations. Future work will test the effects of crustal elasticity and variations in crustal strength, as well as extending the models to 3D.