Estimating the detachment depth in cross sections involving detachment folds

Abstract

This paper evaluates four different methods that have been proposed for the estimation of the detachment depth beneath detachment folds. Guidelines are presented in order to use the most suitable method in a particular region deformed by detachment folds. These guidelines are constructed considering the assumptions of each method, the influence of different parameters on the estimation of the detachment depth (folding mechanisms, cross-sectional area variations, position and orientation of the regional datum, number of stratigraphic horizons, bed length and thickness, dip and position of the bounding lines, initial thickness of the ductile unit, single anticlines or fold trains) and the available data (ductile unit thickness, regional datum, detachment depth). Moreover, a new, simple method based on two previous methods is presented. The advantages of this new method are that the calculations are extremely simple and it uses information from more than one stratigraphic horizon; therefore, it does not depend strongly on the accuracy of the data at one level. The precision of the proposed methods is tested by their application to a number of natural and experimental single anticlines and fold trains. It appears that better predictions are obtained when analysing a complete detachment fold train than when dealing with a single detachment anticline. The reason might be that ductile material flow beneath the folds along the cross-sectional plane is taken into account when dealing with a long-enough cross section. We also suggest some hypotheses concerning what the shape and dimensions of the detachment folds indicate about the detachment depth in those cases where insufficient data are available to apply the methods proposed.

Folding and inversion tectonics in the Anti‐Atlas of Morocco

“…[47] The top of the lower Cambrian has been reconstructed using down-plunge projections and depth to detachment calculations [Epard and Groshong, 1993;Bulnes and Poblet, 1999]. In the western Anti-Atlas the Adoudounian (Lower Cambrian) consists mainly of massive platform carbonate series [Destombes et al, 1985;Benssaou and Hamoumi, 2003].…”

A geometric and kinematic model for double‐edge propagating thrusts involving hangingwall and footwall folding. An example from the Jaca–Pamplona Basin (Southern Pyrenees)

“…The Epard and Groshong (1993) and Bulnes and Poblet (1999) methods to estimate the detachment depth are based on construction of a graph and finding the best-fit function for the data plotted, so that, the depth to detachment is given by the point of intersection of the best-fit function with one of the graph axis. In the Epard and Groshong (1993) method the excess areas above the regional data underneath various stratigraphic horizons are plotted versus the heights of these horizons measured with respect to an arbitrary reference level; the depth to detachment is the height of the horizon whose excess area is zero.…”

“…In the Epard and Groshong (1993) method the excess areas above the regional data underneath various stratigraphic horizons are plotted versus the heights of these horizons measured with respect to an arbitrary reference level; the depth to detachment is the height of the horizon whose excess area is zero. In the Bulnes and Poblet (1999) method the depths to detachment calculated for various stratigraphic horizons using the classical excess area method (Chamberlin, 1910) are plotted versus the stratigraphic heights of these horizons; the depth to detachment is the height of the horizon whose depth to detachment value is zero, i.e. the horizon whose excess area is zero.…”

“…This means that its hangingwall excess area minus its footwall depressed area equals zero. Since the Epard and Groshong (1993) and Bulnes and Poblet (1999) methods are able to determine the stratigraphic horizon whose excess area is zero, these methods are useful tools to unravel the stratigraphic position of the horizon where the fault nucleation point was located. In addition, the slope of the best-fit function in the Epard and Groshong (1993) method is the shortening, and if this function is not linear but curved it indicates that simple shear took place.…”

“…If the beds above the upper fault tip are considered on the graph, then the nucleation point depth will be overestimated, and the shear, calculated using a polynomial fit of the data will be underestimated. presents two graphs constructed for the last stage of the fold shown in Figure 8 using the Epard and Groshong (1993) and Bulnes and Poblet (1999) methods. According to both graphs, the nucleation point was situated two units above the lowest stratigraphic horizon.…”