Influence of Salt Thickness on the Style of Thrust Belts in Angola

Greg Schoenborn¹, Douglas Goff², and Gyorgy Marton^3
1 ChevronTexaco, San Ramon, CA
² ChevronTexaco Overseas Petroleum, Bellaire, TX
³ ChevronTexaco, Bellaire, TX

Deformation style in compressional belts is largely controlled by thickness and nature of the detachment horizon. Two thrust belts offshore Angola show this in an exemplary way.

About 150km off the coast of Cabinda (Angola) a classic thrust-belt developed during the Cenomanian (Late Cretaceous). Above a thin salt layer fault-propagation folds and detachment folds with breakthrough faults created a series of closely spaced, west-vergent duplexes with about 15km of shortening during a short time interval. Some of the duplexes were completely decapitated by erosion. Older growth strata were re-eroded and re-deposited again. The seafloor was highly accentuated during deformation with more than 800m of topographic relief and locally more than 40 deg slopes. The belt contracted at a rate of about 5 millimeters per year.

At the edge of the Aptian salt basin close to the abyssal plain an areally much more extensive toe-thrust belt developed from the Late Cretaceous to recent times. The inflated salt reaches thicknesses of several kilometers, anticlines are wide and have a low frequency. Cretaceous and Late Miocene to recent salt canopies account for most of the modest shortening in the northern part, whereas larger shortening further south resulted in a few widely spaced thrusts and related open folds. Deformation in this belt has occurred over a very long time (since Late Cretaceous) but at a rate (<0.5mm/year) about an order of magnitude slower than in the other belt.