fault interaction

Fault interaction creates areas of local concentration of stress and disturbances that affect the geometry and cognitive movement of faults. This stress concentration can create secondary structures in damage. In this article, the influence of the interaction or relationships between the Zagros and Arabian faults in the Zagros foreland on the formation and different geometry of the Rag Sefid and Tango anticlines is investigated and explained.

Due to the occurrence of the phenomenon of fault interaction between the faults that cause the Rag Sefid and Tengo anticlines, the three-dimensional interaction theoretical models between the fault segments are first introduced. By comparing the geometrical condition of fault parts in the study area with theoretical models, the types of fault interaction in this area are introduced. Finally, due to the dependence of the folds in the region on the fault, and with a detailed study of the geometry and dimensions of the underlying faults, the factors affecting changes in the pattern of folds are examined and the development of folds with different geometries, dimensions, extensions and mechanisms is justified.

Regarding the length more than 3 times of the Rag Sefid fault respect to the southern part of the Hendijan-Izeh strike-slip fault, the mean slope of 47 degrees of the Rag Sefid fault relative to the Hendijan-Izeh fault of 80 degrees and according to the general compression direction of N22E in the Southwest of Iran and the southern part of the Hendjan-Izeh fault trend (N20E), as well as the Rag Sefid fault trend, which is approximately perpendicular to the general compression direction, the deformation amount of the Rag Sefid fault is more than the Hendijan fault. In this condition, the stress field of the Rag Sefid thrust fault is dominated and due to the less resistance of the rising, folding with a larger amplitude occurs on the the Rag Sefid anticline; so that the folding amplitude in the Rag Sefid anticline is more than twice as large as the Tango anticline, and the tip of the Tango anticline is about 1,200 meters lower than the Rag Sefid anticline.

It can be concluded that in a set of faults where the interaction between the faults has occurred, larger, shallower faults with a plane slope of about 45 degrees, which have a diagonal or vertical orientation compared to the general orientation, they can create large and clearer folds similar to the folds with large dimensions in the Rag Sefid anticline compared to the Tango anticline.

The subduction systems are located in the continental collision phase. Due to the tectonic regime reversion from the tensile phase to the compressive phase and due to the reactivation of the normal and transverse basement faults, constitute folded belts that are in their tectonic evolution, have experienced multiple fault interactions. Therefore, based on the angle of post-collision shortening axis, relative to each of these old basement faults, their reactivation in the form of thrust or strike-slip components and their effects on the structural pattern of the folds, are justified. In the Zagros foreland, there are several fault lineaments with different trends and mechanisms. They have been reactivated during the collision phase of the Iranian and Arabian sheets. The present study tries to show the effect of the Zagros thrust faults and the Arabian strike slip faults interaction on the development and structural evolution of the Rag Sefid and Tango anticlines using analog modeling.

In order to model the interaction of basement faults in the South Dezful embayment, first, in accordance with the structural realities of the region, the topography of the basement is simulated using the cutting of wooden boards. The arrangement and cutting of the boards are according to the slope of the Rag Sefid thrust fault. First the wooden board is cut at an angle of 47 degrees. Also, considering that about one third of the Rag Sefid anticline has an axial curvature, and due to the slope of approximately 80 degrees at Hendijan fault, which is perpendicular to the Rag Sefid fault, the wooden board is cut into two unequal parts with an 80-degree slope. According to the interpretation of reflective seismic sections and drilled well data, the stratigraphic sequences detectable in the southern Dezful embayment are Aghajari, mobile Gachsaran formation and Middle Resistance Group between Asmari to Gadvan, respectively. Therefore, in order to model these stone units, clay with a thickness of 3 and 2 cm was used for upper and middle resistant units, respectively. Also, in order to simulate the Gachsaran moving formation, a combination of sifted rock powder and 3 cm thick engine oil is used at the beginning of modeling.

The northwest-southeast trend of Rag Sefid anticline located in Zagros foredeep, has been raised due to the Zagros orogenic phase in the Pliocene. After the collision of the plates from the Late Eocene onwards, in addition to folding on the northwest-southeast faults, the north-south basement faults during the Late Cenozoic have been reactivated by the entry of the Zagros deformation front into the study area. The reactivation of these faults has caused changes in sedimentary cover, such as facies change and sediment thickness as well as changes along the axis of surface anticlines. Oblique convergence after collision between Iran and the Arabian Plate has shortened the succession of the Zagros basement; so that the northwest-southeast longitudinal faults that were extensional at the time of the rift formation are now basement thrusts in this belt. The north-south faults, which have trends similar to those of the north-south basement fault in the eastern Arabian Block, are reactivated as a result of this compression.

Due to the general compression trend of N22E in southwestern Iran and the trend of the southern part of the Hendijan fault (N20E), the small-scale fold at the Tango anticline located on the Hendijan fault is due to the parallelism of this transverse fault and the overall compression direction. In contrast to the Rag Sefid anticline, where the trend of the main fault is approximately perpendicular to the direction of total compression (N100), a clear fold with large structural dimensions is created in the Rag Sefid anticline. Our results show that with the entry of the Zagros deformation front into the foreland of the orogenic region in the Pliocene, in addition to the Tango anticline rising at the top of the Hendijan fault, the Rag Sefid anticline has rotated axially by 30 degrees clockwise due to the movement of the right-hand slip in the Hendijan fault. Our modeling shows that the reason for the higher wavelength of the western part of the Rag Sefid anticline is due to the high thickness of the salt layer and also the high value of the ratio number. Due to the location of the western part of the Rag Sefid anticline in a right-sided fault zone and the resulting structural rotation, the widening and increasing wavelength of the anticline is facilitated.

Image logs of 14 wells in Rag sefid anticline are showing 6 main fracture sets treanding N45, EW, N35, N100, N150 and N162 respectively. Development of fractures in the eastern part of the Rag sefid anticline, especially in the frontal edge, are in effect of the propagation fault related folding so that the most open longitudinal and cross axial fractures show N100 and N10 trends. Due to fold axis rotation 30 degrees to the north In the western part of the anticline the orientation of the fractures have changed and major fractures are longitudinal type and have N160 trend. In the middle and curved part of anticline NE-SW trend fracture sets are more developed by reactivation of hendijan basement fault. Convergence of stress axises as a result of the fault triple interaction In effect of Rag sefid thrust dipping to the north east and dextral shears dut to reactivation of hendijan and southern part of Izef basement faults caused the Restraining bend and dextral shear zone In the western part of the Rag sefid anticline. Creation of this shear zone caused clockwise rotation of anticline axis, rising more in northen west culmination on asmari top formation than southern east culmination, change in fractures orientation and also development and increase the density of fractures in the curved part of Rag sefid anticline.