MASS BALANCE AND THRUSTING IN DETACHMENT FOLDS

Detachment folds seem conceptually the simplest of all fault-related folds, yet they continue to offer substantial and rich challenges that are symptomatic of large-scale folding of sedimentary rocks in general. The standard picture of detachment folds goes back to August Buxtorf's widely reproduced 1916 regional cross sections of the Swiss Jura in which he interpreted the Jura box folds to be regionally detached from the basement along a weak layer of Triassic evaporites. Most theoretical, conceptual and mechanical models of detachment folding have followed Buxtorf's model with a two-fold mechanical stratigraphy composed of a competent flexural lid conserving bed length overlying a weak basal detachment layer that conserves only volume. However from a balancing perspective this standard model of detachment folding has classic fundamental problems that have been only partially recognized. Furthermore the classic Jura box folds drawn by Buxtorf were in fact not at all constrained by subsurface data, except along the Grenchenberg railroad tunnel where a surprising and much more complex structure was encountered, involving a strongly folded thrust fault that was intersected three times in the 8.5 km long tunnel. Laubscher argued that such complexly folded thrust faults are in fact typical of many Jura box folds. Analogous structures have been observed in Canada, Mexico and Spain. It is argued on the basis of mass-balance considerations that detachment folds with folded thrusts are one of several theoretically expected modes of detachment folding. More broadly the principle modes of detachment folding include [1] local flow of the weak basal layer, for example salt (Wiltschko and Chapple mechanism); [2] pure-shear detachment folding (Groshong and Epard mechanism); [3] far-field flow of the weak basal layer; [4] roof-detachments above the basal layer; and [5] roof-ramps above the basal layer such as the Grenchenberg anticline. All of these mechanisms are end members capable of existing in combination within actual folds or as nearly pure end members, as shown by recent observations of detachment folds from a variety of stratigraphic and tectonic environments.