Modeling the Role of Multiple Salt Horizons on Deformation in Fold-Thrust Belts

Hemin Koyi¹, Abbas Bahroudi¹, Maura Sans², and Fazil A. Lawa^3
1 Uppsala University, Uppsala, Sweden
² University of Barcelona, Barcelona, Spain
³ Slemani University, Slemani, Iraq

Deformed sedimentary successions in many FTBs (e.g. Zagros fold-thrust belt, Spanish Pyrenees) include layers of salt not only at their base but at different stratigraphic levels. Kinematics and dynamics of deformation in such FTBs is not only influenced by the mechanical behaviour of the basal décollement, but also by the weaker units embedded within the stratigraphic succession. Results of analogue models, containing multiple décollement horizons of viscous layers, are used here to argue for variation in deformation style, intensity and extent in different parts of the Zagros fold-thrust belt (ZFTB). Model results show that during shortening, deformation along a basal décollement steps up into a shallower décollement above which the deformation front propagates further and faster than at deeper levels. As a result, deformed shallow, and younger sediments are placed above deeper, older undeformed sediments. This deformation scenario suits reported field observations from the southwestern part of the Dezul Embayment, where upper Miocene and younger sediments, located above Gachsaran evaporites, show folds with Zagros trend (NW-SE) whereas deeper older units are unaffected by the Zagros orogeny and show the old Arabian trend (N-S). Decoupling between pre- and post- Gachsaran units by the Gachsaran evaporites result in disharmonic folding between the two units where geometry, size and age of the structures differ. Cross sections from the Kirkuk embayment and model results illustrate this scenario. Such decoupling between pre- and post-Gachsaran units in the Zagros may have significant impact on hydrocarbon exploration in the oil-rich Dezful and Kirkuk embayments.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005