GEOPHYSICAL STUDY OF THE GARZóN BASEMENT THRUST FAULT, UPPER MAGDALENA VALLEY, COLOMBIA

The mechanics and kinematics of basement tectonic uplifts, such as the Laramide Rocky Mountain orogeny, remain poorly understood and controversial. The debate continues in part because of the paucity of well-documented Present Day analogs. The Garzón Massif rising between the Upper Magdalena Valley and the Llanos Basin of Colombia is an active basement uplift with well, seismic, gravity, and magnetic data available. In the past 10 Ma, PreCambrian age granitic rocks of the Garzón Massif have been uplifted and displaced against Cretaceous and Tertiary sediments of the Upper Magdalena Valley along the Garzón fault.

Aerogravimetric data calibrated by well data and 2D seismic data was used to model the geometry of the Garzón fault and the top of basement (Saldaña Fm) in 2 dimensions. The density models provide an independent estimate of fault orientation. A high density airborne gravity and magnetic survey was flown over the Garzón fault in 2000, including 2,663 line kms along 1 x 5 and 1 x 4 km flight lines at elevations of 2564 and 4589 m above mean sea level.

An initial depth model was derived from the well logs, seismic reflection profile, and down-hole velocity surveys. Airborne gravity data was used to produce a Bouguer Anomaly Gravity Map. Average rock densities were estimated from density logs, seismic velocities, and formation rock types. The regional gravity field was estimated and 2-dimensional forward models were constructed with average densities from the wells, seismic velocities, and rock types, and the initial depth model. Since the model fit is dependent on the density assumed for the Garzón Massif rocks, multiple densities and dip angles were tested, and the errors between calculated and observed were compared for 11 densities and 8 dip angles (88 models).

Our gravity analysis indicates that the Garzón fault is a basement thrust fault dipping at a shallow angle under the Massif. Best-fit models show a true dip of 12 to 17 degrees to the southeast. This study provides a well-documented example of an active basement uplift by low angle thrust faulting. A regional profile will test if the uplift is symmetric or asymmetric, and estimate the extent of basement involvement, the depth of the Garzón fault, and orogenic uplift and shortening.