Date of Award

8-9-2014

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

James N. Kellogg

Abstract

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 limited number 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 were 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 were flown over the Garzón fault in 2000, including 2,663 line km 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 two-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.

The 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. A regional density and magnetic susceptibility model of the entire Massif is consistent with dense basement rocks throughout the Garzón Massif and asymmetric loading (sedimentary basin is much deeper on NW flank – Upper Magdalena Valley). Crust thickens to the NW toward the Central Cordillera. Euler deconvolution of the magnetic field shows pronounced NE-SW trending features under the Massif which are interpreted as faults bounding a possible pre-Cambrian sedimentary rift graben. Retrodeformed 2D regional models indicate 13 km of shortening on the Garzón basement thrust in the last 12 Ma. Approximately 9 km of shortening occurred on the SE marginal basement thrust fault, probably also in the last 12 Ma. This was preceded by approximately 43 km of shortening by thin-skinned imbricate thrusting to the southeast (12 - 25 Ma). This study provides a well-documented example of an active basement uplift on low angle thrust faults.

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