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 be ‘unfold’.

In this chapter a fold restoration methodology is presented in which fold limbs, and 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 hereby encouraging students in applying an unfolding strategy to derive information from prefolding structures.

The exercise is published as Chapter 9 in the new Book Developments in Structural Geology and Tectonics, Volume 5: https://www.sciencedirect.com/bookseries/developments-in-structural-geology-and-tectonics/vol/5/suppl/C

A KMZ file with additional information supporting this chapter can be downloaded here: https://cloud-as.oma.be/index.php/s/aYYGgajACmwcMLD.

Enjoy and don't hesitate to contact me to translate this chapter into a classroom example.

Reference: Van Noten, K., Sintubin, M., 2019. Chapter 9 - Unfolding Veined Fold Limbs to Deduce a Basin’s Prefolding Stress State, in: Billi, A., Fagereng, Å.B.T.-D. (Eds.), Problems and Solutions in Structural Geology and Tectonics. Elsevier, pp. 105–116. https://doi.org/https://doi.org/10.1016/B978-0-12-814048-2.00009-0