A hybrid structure of fault-related folds: an outcrop-scale case study in the Zagros Fold-Thrust Belt, SW Iran
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GEOSITES
A hybrid structure of fault‑related folds: an outcrop‑scale case study in the Zagros Fold‑Thrust Belt, SW Iran
The ~ NW–SE trending Zagros Fold-Thrust Belt (ZFTB), in SW Iran, resulted from closing of the the Neo-theys and subsequent collision of the Arabian and Central Iran plates (Allen et al. 2013) from the Late Cretaceous until Late Miocene (Saura et al. 2011). The ZFTB comprises of various compressional structures. Thrust fault-related folds are common occurrences in the ZFTB, and can be classified into three main end members: detachment folds, fault propagation folds and fault-bend folds (Jamison 1987; Brandes and Tanner 2014). An outcrop-scale fault propagation fold (Fig. 1) was formed in the Pabdeh Formation in the core of the Kaseh Mast Anticline in the southernmost part of the Lurestan zone of the ZFTB. The Pabdeh Formation consists of an alternation of the thin-bedded shale (as incompetent uint) and marly limestone (competent unit). The NE verging fold has developed on the hanging wall of the thrust (1 in Fig. 1b), and is characterized by a rounded hinge. The steeply dipping, overturned forelimb of the fold is shorter than the backlimb (Fig. 1). This is one of the main characteristics of the faultpropagation folds (e.g., Mitra 1990; McClay et al. 2004). The shale and marly limestone layers on the footwall have not been folded. The ramp dips ~ 25 on an average; the dip decreases towards the NE. The backlimb parallels the lower portion of the ramp and dips ~ 35° towards SW. The fault is flattened upward into a thin shale unit, as upper detachment horizon (2 in Fig. 1b), so that the uppermost layer of the forelimb (marked by yellow dash line) was horizontally displaced along this portion of the fault (Fig. 1b). The flat
part of the fault has about 0.3 m long (3 in Fig. 1b). The fault is locked in the end of the flat part and formed the fault tip (4 in Fig. 1b). The correlation of the geological units in both sides of the fault unravels that the fault slip decreases upward (Fig. 1b). A tight syncline has developed in front of the anticline and above the fault tip (Fig. 1b). The secondary structures are formed around this syncline. One of these structures is a southwesterly dipping out of syncline fault (5 in Fig. 1b) commonly encountered alongside fault-propagation folds (McClay et al. 2004). This fault along with the northeasterly dipping fault in the upper units exhibits a fishtail geometry (Letouzey et al. 1995) and form a triangle zone (Harrison and Bally 1988) (6 in Fig. 1b). A small rabbit ear fold (Dahlstrom 1990; Letouzey et al. 1995) has also developed in the forelimb of the anticline (7 in Fig. 1b). A few other secondary structures can also be observed: (i) a low-angle thrust fault and two back-thrusts (Bose and Mukherjee 2019) near the flat portion of the syncline that form a pop-up structure (8 in Fig. 1b). (ii) Steeply dipping reverse fault to the SW of the anticline—this has brought the gently dipping lithounits over the backlimb (9 in Fig. 1b). These faults were also predicted in
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