This study provides morphology and field survey data on Ghasr-e-Ghand and Kahorkan thrusts, in the central part of eastern Makran accretionary wedge, in southeast Iran. The N-dipping thrusts with E-W strike, which brought Upper Oligocene turbidites onto Lower-Middle Miocene sediments, are one of the most important structure in this area and separate Inner Makran to the north from Outer Makran in the south. A clear facies changes are recorded in hanging wall of Ghasr-e-Ghand thrust, i.e. from shale in the east to sandstone-dominated to the West.
We combined the field data with geomorphic indices and show how lithology changes along Ghasr-e-Ghand Thrust influenced topography and morphology and so reveal not reliable active tectonic areas in view of geomorphic processing.
2_Materials and Methodes
Morphotectonic indices, Stream Length-Gradient (SL), Mountain Front Sinuosity (Smf) and hypsometric integral (Hi), which are sensitive to uplift and topography changes, are used to determine the tectonic activities along Ghasr-e-Ghand and Kahorkan thrusts. 78 adjacent catchment draining are extracted by ArcGIS 10.1 and Arc hydro software in the study area.
Field survey data were used to determine kinematic and recent activities of the thrusts, based on measurements along the thrusts, lithologic observations and stratigraphic relations across the faults.
The results of morphotectonic and field data as well as geological map and Digital Elevation Model (DEM) are combined in ArcGIS software as different layer to determine relationship between the layers and their effect on morphology of the area.
3_Results and Discussion
The morphotectonic analysis consist of low value (<1.10) of mountain-front sinuosity index (Smf) for all part of the Ghasr-e-Ghand and Kahorkan thrust, which points to high activities along the faults. Hypsometric integral index (Hi) shows high tectonic activities in west and eastern part of the Ghasr-e-Ghand and suggest tectonic activities for all over the Kahorkan thrust, while high SL value in most western part of the Ghasr-e-Ghand indicate a high exhumation for this area.
Fault measurements and related slicken lines indicate same kinematic along the Ghasr-e-Ghand and Kahorkan faults, i.e. N-dipping thrust with hanging wall movements toward south. In other hand, the Ghasr-e-Ghand Thrust cut and bended Holocene conglomerate in the eastern segment clearly. This evidence as well as thick Pliocene-Pleistocene conglomerate, which has dips of about 30º in the foot wall of the eastern part of the thrust indicate high recent activities for this segment. The most western part of the thrust in not clear and reveal an inactive fault. The kahorkan thrust did not influenced the quaternary sediments in front, which points to low tectonic activities along the thrust in the study area.
As we shown, the field survey data in contrast with morphotectonic analysis. Detail investigation on field and geological map reveal different lithology along hanging walls of Ghasr-Ghand and Kahorkan thrust. A facies changes occur from weak shale with thin and fine sandstone in the eastern segment to rhythmic sandstone and shale and some time to thick to massive volcanoclastic coarse sandstone in the western segment of Ghasr-e-Ghand. This lithology changes caused an inhomogeneous strength and so different behavior of erosion along the thrusts. The field data show high recent tectonic activities in eastern segment, although this area show very low topography compare to the western part. In other hand, big difference in strength of lithology across the thrust in the western segment, thick sandstone in hanging wall and weak marl in foot wall, caused higher topography and effected the geomorphic indices. E-W mountains in south of the Ghasr-e-Ghand and Kahorkan with almost straight frontal mountain formed by differences of strength in lithology and erosion, i.e. in base weak marl and on top thick and hard sandstone. Filed observation consist of stratigraphy contact between these lithology and for sure absent of faults. For such stratigraphic contact, Smf and width-valley height ratio (Vf) indices were measured in previous work and show a high tectonic activities for this part.
4_Conclusions
Field survey data indicate a recent tectonic activity for the eastern segment of Ghasr-Ghand thrust, which is in contrast with morphotectonic analysis. We believe facies changes in hanging wall of the thrust, caused different lithology with different strength and so different behavior in erosion along the thrust, which caused lower topography in the eastern segment with outcrops of shale in hanging wall to higher topography in the western segment with outcrops of volcanoclastic sandstone. Some E-W mountain, south of Ghasr-e-Ghand and Kahorkan thusts, make a sharp front without present of faults in the northern flank. Different lithologic strength caused to have such a landscape in the southern part of Makran.