EXCESS AREA AND DEPTH TO DETACHMENT

A plot of the excess area in cross section vs. the depth to a referenc e level for several stratigraphic horizons in a compressional fold pro duces a curve that gives the depth to detachment. The curve is a strai ght line for detachment folds. The slope of the line is the displaceme nt 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 o f the line is the displacement on the upper detachment. Added layer-pa rallel 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 basa l 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 d iagram is useful for testing a cross section for balance, determining the structural style (detachment anticline vs. fault-bend fold) and fo r examining alternatives in the construction of cross sections.