Numerical Models of Lithosphere Removal in the Sierra Nevada de Santa Marta, Colombia
Creators: Quiroga, David
Publisher: name: University of Alberta Library
Year: 2022

<p>The Sierra Nevada de Santa Marta (SNSM) in northern Colombia, is one of the highest coastal mountains<br>
in the world with a maximum height of more than 5.7 km. Geophysical measurements report that there<br>
is a high Bouguer gravity anomaly (&gt;+130 mGal) in the region, indicating that the mountain does not<br>
have a crustal root. Previous studies attributed the support of the mountain to the underthrusting<br>
buoyant Caribbean plateau or to elastic stresses within the continental lithosphere. However, thermo-<br>
chronological studies report a recent uplift episode (in the last 2 Ma) that does not have a clear driving<br>
force. This thesis proposes that this region experienced a localized lithosphere removal event, whereby<br>
the dense lower lithosphere sank into the mantle as a drip, resulting a thin crust and lithosphere and<br>
causing surface uplift.<br>
To test this hypothesis, 2D and 3D numerical models are developed using the software ASPECT. The<br>
models examine the dynamics of a lithosphere drip and the associated observations, including surface<br>
topography, gravity anomaly and surface heat flow. The general model setup has a four-layer structure<br>
consisting of an upper crust, lower crust, mantle lithosphere and sublithosphere mantle. There is an<br>
initially thickened crust in the middle of the domain, representing the SNSM. This induces a perturbation<br>
in the lower lithosphere which is gravitationally unstable and undergoes foundering. The first 2D models<br>
are used to define a reference setup, by testing the effect of initial crustal root thickness, lateral<br>
compression rate, perturbation width, and plastic rheology parameters. Subsequent 2D models consist<br>
of modifications of the reference and explore the effects of crustal root eclogitization, lithosphere<br>
rheology, upper crustal density, initial crustal thickness, and lateral compression. In the preferred 2D<br>
model, the surface first undergoes subsidence, but post-removal non-isostatic forces induce uplift rates<br>
above 0.2 mm/yr for about 1 Ma, holding a region of high topography (3.3 km) and a positive Bouguer<br>
gravity anomaly of 103 mGal, consistent with SNSM observations. Additional calculations show that<br>
lithosphere removal generates a low seismic wave velocity anomaly under the Moho (-5% and -8% for<br>
Vs and Vp, respectively) and potentially inducing lower crustal and upper mantle melting. The final set<br>
of models uses a simplified 3D geometry to show that a 3D drip is wider and is associated with up to<br>
66% of less subsidence and positive gravity anomalies that are up to 112% higher than an equivalent 2D<br>
model.<br>
The models show that lithosphere removal is a viable explanation for the origin of the SNSM. The<br>
removal event could have occurred either very recently (&lt; 2 Ma ago), potentially explaining a recent<br>
uplift episode, or earlier (50-40 Ma ago), possibly affecting the Paleocene-Eocene magmatism in this<br>
region. In either case, lithosphere removal would have affected the continental lithosphere structure.<br>
This work shows that local lithosphere geodynamics may have played an important role in the geological<br>
history of the SNSM, northern Colombia, and the Caribbean. The insights and quantitative predictions<br>
from the numerical models provide important constraints for future studies of this complex region.</p>