The Role of the Basement and Salt Tectonics on the Cenozoic Contractional Deformation in the Southeastern Zagros Fold Belt

Abstract

OMV Upstream and the NIOC conducted a joint geoscientific study in the southern Fars area from 11/2016 until 10/2018. The study area is located within the Simply Folded Belt of the Zagros, famous for its large-scale whaleback folds and salt glaciers. However, despite decades of research there are still many uncertainties regarding aspects of the structural style. How much do basement structures and Hormuz salt control deformation? The amount of shortening in published sections in and around the study area varies from 6-20% (e.g. Molinaro et al., 2005; Alavi, 2007; Jahani et al., 2009). The main differences between the sections are variations of the primary salt and non-salt stratigraphic thicknesses, presence or absence of hidden thrust faults, and contribution of basement to the shortening or not. We use outcrop and surface geology observations, well data, reflection seismic (on- and offshore), and gravity data to assess the salt tectonic and structural history in the SE Fars and constrain two balanced and many local sections. We first assess uncertainties related to the stratigraphic thickness by sensitivity analysis of seismic reflection time-to-depth conversion and gravity inversion. Results show that the stratigraphic thickness has uncertainties of up to +- 4km. In a second step we use an area balancing method to calculate alternative sections and the associated shortening. Shortening values of 4-25% have a large spread and show that it is impossible to determine how much the basement is involved in the deformation based on section balancing alone. Consequently, other methods must be used to constrain basement effects on deformation. The folding pattern in areas of outcropping diapirs is complex and shows usually double plunging folds with two main orientations (approx. W-E, perpendicular to shortening) and ENE-WSW. We consider that these directions constrain some basement and halokinetic control on the young shortening deformation. The ENE-WSW direction is interpreted to be inherited from the Precambrian-Cambrian rifting. We consider that primary salt thickness is higher in graben areas with the ENE-WSW orientation. Halokinetic downbuilding since the Paleozoic increased the post-salt stratigraphic thicknesses especially in these graben areas. During N-S shortening these thicker packages are inverted and folded dominantly in a thin-skinned manner, obliquely interfering with the W-E folding direction. In addition to the thin-skinned deformation we postulate some additional thick skinned inversion below the Hormuz salt level. Minor inversion movements are considered to restore steps in the structured basement, thereby creating a smoother basal decollement plane. Therefore, we consider that the inversion movements partly control the propagation of the thin-skinned deformation. Furthermore, the deformation pattern and the halokinetic history of the diapirs is not uniform throughout the study area. Diapirs in the eastern part of the study area show pronounced phases of apparent higher activity. We interpret this to be caused by reactivation of basement structures due to far-field effects of tectonic movements at the plate margin (i.e. ophiolite obduction during Late Cretaceous). Even though some input parameters and section balancing have a large uncertainty range, detailed observations indicate a complex control of basement structures on the halokinetic evolution and on the young shortening deformation in the SE Fars.

AAPG Datapages/Search and Discovery Article #90362 ©2019 AAPG Middle East Region, Geosciences Technology Workshop, 2nd Edition Structural Styles of the Middle East, Muscat, Oman, December 9-11, 2019