Koen Van Noten, Manuel Sintubin

Tectonic structures that developed prior to folding, such as pre- and early-kinematic veins, hold valuable information on the stress state of the paleobasin in which these early structures formed. To derive the parental orientation of these prefolding brittle structures, folds need to “unfold.” A fold restoration methodology is presented in which fold limbs, and the structures they contain, are rotated back to their depositional horizontal position by removing the tilt of the fold hinge line and the dip of individual fold limbs. The method is applied on quartz veins emplaced in folded Lower Devonian sandstones from the High-Ardenne slate belt (Belgium, Germany) and allowed deducing NW-SE opening when the Ardenne-Eifel Basin was at maximum burial depth (early Carboniferous). This exercise can be used in structural geology classes to teach how to rotate data using stereonet techniques, thereby encouraging students in applying an unfolding strategy to derive information from prefolding structures.