A plot of the excess area in cross section vs. the depth to a reference
level for several stratigraphic horizons in a compressional fold
produces a curve that gives the depth to detachment. The curve is a
straight line for detachment folds. The slope of the line is the
displacement on the lower detachment and the depth intercept (where
excess area = zero) is the depth to detachment. For a fault-bend fold,
the result is a line that abruptly changes slope at the position
corresponding to the upper detachment. The difference in slope between
the two parts of the line is the displacement on the upper detachment.
Added layer-parallel shear produces a curve on the graph. Tested on
detachment folds from the Jura Mountains, the method accurately predicts
the depth to detachment. The Tip Top field anticline in the Wyoming
thrust belt is found to be interpretable as either a detachment
anticline with a basal detachment close to the top of the basement or as
a fault-bend fold with a basal detachment at the base of the Triassic.
The excess-area diagram is useful for testing a cross section for
balance, determining the structural style (detachment anticline vs.
fault-bend fold) and for examining alternatives in the construction of
cross sections.