Richard H. Groshong
3-D Structure Research and University of Alabama, Northport, AL
This paper shows how area-balance, in the form of the area-depth relationship derived from multiple horizons, can be used to interpret compressional folding above a ductile detachment zone. The example to be discussed is from Shaw et al. (2005) AAPG Studies in Geology #53. They interpret the anticline to be a detachment fold above Aptian salt. They demonstrate that for this structure the classic depth-to-detachment calculation indicates a detachment far deeper than that actually observed. Their result casts doubt on the applicability of area balancing to this and other similar structures.
The major issues in area balanced folding above a ductile detachment are 1) the transfer of material from beneath the synclines into the anticlines and 2) the effect of growth stratigraphy. Quantitative models demonstrate that both of these effects can be recognized on an area-depth graph. If a single horizon within the fold maintains constant bed length, then the subsidence of the synclines required by area balance can be determined. The area-depth relationship of the pre-growth interval demonstrates whether the fold was formed by compression, extension, or vertical displacement. The area-depth graph of the Angolan fold indicates that it is compressional and consists of two no-growth units separated by a thin growth interval and is overlain by a thick growth sequence. The fold and the salt detachment zone are area balanced above the observed detachment if enough ductile-detachment material migrated from beneath the synclines into the anticline to cause a 0.3 km subsidence of the synclinal troughs.