Normal faulting and landsliding in morpho-structural domes related to buried salt stocks, Zagros Mountains, Iran. Insights into salt breakout

Abstract

Normal faulting in domal structures above diapirs has important implications in hydrocarbon geoscience, but studies on surface faulting on exposed growing domes are scarce. Moreover, there is limited work addressing the erosional processes involved in the unroofing of active diapirs preceding salt emergence. This investigation analyses two morpho-structural domes (Gavbast Dome: GD; Kooh Bedoo Dome: KBD) associated with growing anticlines related to the rejuvenation of precursor buried salt stocks in the Zagros Mountains. The structural relief of the domes (1330 m in GD and 400 m in KBD) and the available chronological data on fold development indicate long-term uplift rates of around 0.1–0.4 mm/yr. GD displays three domains with different fault patterns: (1) fold-normal grabens in one half; (2) polygonal faulting with multiple enclosed graben depressions in the crest; and (3) fold-parallel faulting on the other half. KBD shows a major fold-normal graben restricted to the crest and one flank of the dome. The lack of the typical radial pattern related to multidirectional hoop extension can be attributed the suppression of extension in the fold-normal direction by regional shortening. Other factors such as complexities in the folding structure, outer-arc extension in the anticline crests, and plan geometry seem to play a local control on fault patterns. The studied domes display an extraordinarily high density of large slope movements, indicating that they play an instrumental role in the unroofing process. The slope failures, with sliding planes mainly developed in argillaceous units overlain by thick limestone packages, include incipient planar rock slides more than 5 km2 in area expressed as buckle folds in the lower part of the slopes. The main preparatory factors involved in the development of these slope collapses include rock mass weakening by faulting and slope over-steepening, both related to doming. Our detailed mapping and the examination of a number of salt diapirs in the Fars Arc suggest that episodic landsliding and more gradual fluvial dissection, often controlled by grabens, are the main erosional processes involved in the unroofing of active diapirs leading to salt emergence.