The effect of upper detachment level on structural geometry of the deep and surface Sargelan Anticline, Lurestan, West Zagros

1-Introduction The Zagros fold and thrust belt with NW–SE trend is located in the Alpine–Himalayan orogeny, resulting from the Late Cretaceous to Cenozoic convergence between the Arabian and Eurasian plates (Berberian and King, 1981; Alavi, 1994). The northeastern boundary of this belt is the structural style, and sedimentary history divides the Main Zagros Reverse Fault into several structural zones. The Zagros fold and thrust belt can also be divided, along strike from east to west, into the Fars, Izeh, Dezful embayment and Lurestan zone. The Lurestan zone has a long history of hydrocarbon exploration, and production, and only a few wells have been successful. Seismic and well data in the area indicate that folding style has the primary role in the exploration and production of the hydrocarbon. Also, in this belt, thrust faults and detachment horizons have the main character in geometry and kinematics of the folded structures. The Sargelan anticline is one of the main folded structures in the Lurestan zone. So, in this research, deformational pattern and structural geometry of the Sargelan anticline have been studied using the interpretation of five 2D seismic profiles, drawing of structural cross-sections, drilling well data in the adjacent anticline and detailed analysis. 2-Methodology The Zagros fold and thrust belt with NW–SE trend is located in the Alpine–Himalayan orogeny, resulting from the Late Cretaceous to Cenozoic convergence between the Arabian and Eurasian plates (Berberian and King, 1981; Alavi, 1994). The northeastern boundary of this belt is the anticlines in the Lurestan zone host the hydrocarbon reserves in the west of Iran, hence are well-acknowledged regarding the style of folding and their response to appropriate detachment levels. This research indicates the results of a detailed study of the Sargelan anticline, based on field surveys, seismic profiles interpretation, well data, and drawing four structural cross-sections. We will successively focus on the upper (Amiran Formation) and middle (Garu Formation) detachment levels and deep-rooted thrust faults, causing the disharmonic folding beneath and above the upper detachment level restoration of each structural cross-section. At last, restoration of each structural cross-section obtained by a shortening percentage. 3-Results and discussion Recent studies indicate that mechanical stratigraphy has the main role in folding style in the Lurestan zone (Casciello et al., 2009; Farzipur-Saein et al., 2009). In the study area, full upper detachment level causes a reduction in wavelength of overlying anticlines and formation of disharmonic folding. So, in this research four structural cross-sections (from SE to NW: AA´, BB´, CC´, DD´) across the Sargelan anticline has been drawn transecting the anticline from the NE to SW to illustrate the vertical variations in folding style. Interpretation of seismic profile indicates that along the AA´ section deep thrust fault has not formed in the southwest limb. Also, regarding the geometrical parameters and the drawing the folding stereograph, the axial trend of the Sargelan anticline is 115/0, and its axial plane is 030/86 degrees. The BB´ structural cross-section indicates that in the southern part of the deep Sargelan anticline, another anticline is formed, which is introduced as the South Sargelan anticline. Along the South Sargelan anticline, a deep thrust fault rooted within the Garu Formation (middle detachment level) and cuts up-sections of the southern limb and flattens in the Amiran Formation (upper detachment level). In addition to the deep thrust fault, a low angle shallow thrust rooted within the upper detachment level (Amiran Formation) caused displacements and deformations in the post-Amiran formations. Along the CC´ cross-section, disharmonic folding caused the formation of the deep-seated Sargelan anticline beneath the surface syncline. The structure suggests that the deep-seated Sargelan anticline is formed along the s cross-section and continues to the north. The DD´ structural cross-section drew along the Sargelan, South Sargelan, Darreh-Baneh, and Chahar-Qaleh anticlines. Along with this structural cross-section, the Sargelan and South Sargelan anticlines have formed between the upper and middle detachment levels. Therefore, the study of these four structural cross-sections indicates that the deep-seated Sargelan anticline continues more than about 10 km just beneath the surface syncline. The reason for the formation of disharmonic folding is the high thickness of the upper detachment level (Amiran Formation) in the study area. 4- Conclusions The interpretation of structural profiles indicates two upper (Amiran Formation) and middle (Garu Formation) detachment level. Detachment levels and thrust faults have a significant influence on the geometry and kinematics of the structures, as the southwest limb thrust has formed and begin to ramp. Displacement of this thrust transported the anticline upward, and so the Sargelan anticline is a transported detachment folding. The high thickness of the upper detachment level cased formation of disharmonic folding that the deep Sargelan anticline continues to the northwest more than about 10 km and beneath the surface syncline. Measurement of the geometrical parameters indicates that the Sargelan anticline is an asymmetric, cylindrical fold, and concerning the aspect ratio and bluntness is fold-wide and semi-circle respectively. Interpretation of seismic profiles indicates that a deeper anticline is formed parallel to the Sargelan anticline, under the upper detachment level and it is introduced as the South Sargelan anticline in this study. References Alavi, M., 1994. Tectonics of the Zagros orogenic belt of Iran: new data and interpretations. Tectonophysics 229, 211–238. Berberian, M., King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Sciences 18, 210–265. Casciello, E., Verges, J., Saura, E., Casini, G., Fernandez, N., Blanc, E., Homke, S., Hunt, D.W., 2009. Fold patterns and multilayer rheology of the Lurestan Province, Zagros simply folded Belt (Iran). Journal of Geological Society 166, 947–959. Farzipour-Saein, A., Yassaghi, A., Sherkati, S., Koyi, H., 2009. Basin evolution of the Lurestan region in the Zagros fold-and-thrust belt, Iran. Journal of Petroleum Geology 32, 5–19.