Who is working on this?
Chris A. Guzofski,
John H. Shaw,
Freddy Corredor

In collaboration with:
Frank Bilotti of Chevron

put online:
January 2006

Abstract
Using a new implementation of critical taper wedge mechanics, we investigate the relationship between structural styles and calculated mechanical strength properties in two unique fold and thrust belts. Through the analysis of regional seismic reflection profiles and detailed bathymetric data, we define the wedge taper geometry of the fold and thrust belts and delineate various three-dimensionally varying internal strength parameters, including frictional coefficients and density. We subsequently calculate other strength parameters, including fluid pressures, and compare these with structural features and styles imaged in the seismic reflection data.

Regions

We utilize this approach to investigate and compare the compressive toe of the Niger Delta and the Southern Caribbean accretionary prism of Colombia. Both fold and thrust belts have a regional detachment that dips down toward the continent, while imbricate thrust faulting builds a bathymetric slope that dips away from the shelf, defining an internally deforming wedge. This fold and thrust belt geometry is consistent with a Coulomb critical taper wedge model, where an influx of sediments causes the wedge to internally deform, thus maintaining its critical taper angle. In the case of the Southern Caribbean accretionary prism deformation results from the collision of the Caribbean and South American plates, while in the Niger Delta, deformation is driven by gravitational collapse of the shelf sediments. While the driving mechanisms in these wedges are therefore quite different, the fold and thrust belts share many similarities including a low angle taper, which is likely controlled by the fact that they both have a weak basal detachment.

Results

We use these observations to create a 2.5-D model of the fold and thrust belts using critical taper wedge mechanics (CTWM). Regional 2-D seismic reflection profiles, detailed bathymetric data, and three dimensional maps of the main detachment (where available) are used to define the regional geometry of the wedge. We supplement this dataset with deep-water well pressure data and three dimensional fault maps to investigate variations in the strength of the wedge and frictional parameters throughout the fold and thrust belt.
In our first calculations, shown in Figs. 3 and 4, we use the determined strength parameters to try to model the bathymetry of the wedge using an average detachment dip for each fold and thrust belt. We find that the strength parameters and the average dip allow us to accurately model the expected bathymetry in both the deepwater Niger delta and the Colombian margin.
We also find evidence that in both fold and thrust belts, regions where the basal detachment is calculated to be stronger (lower calculated fluid pressures) we see increased imbrication and shortening above the basal detachment. This is in contrast to regions where the basal detachment is calculated to be weaker (higher calculated fluid pressures) where we see broader structures and more distributed or ductile deformation above the basal detachment.

Fig. 4: a. Observed bathymetry from the Colombia margin                 b. 3D CTWM model predicted bathymetry

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