A 4D Analysis of a Minor Graben Structure at Ekolsund, South Central Sweden

Abstract: The objective of this project is about a kinematic analysis of a shear zone, specifically a graben structure. The surface of the study area has also been investigated, this is to understand if a faulting reactivation has occurred due to glacial impact. The graben structure is exposed along a road cut, next to the E18 highway at Ekolsund, 14km east of Enköping. The results of the report are based on field studies of the geometrical relationship pattern of fracture planes, relative displacement along fracture and a kinematic analysis of movements along fractures. A high resolution terrain model was acquired in the analyzing of the graben structure formation and to create a geometrical 3D brittle deformation model. This model was applied to reconstruct the deformation process. The area contains a more than 1,1km long dextral NE-trending strike-slip fault feature in a polydeformed Precambrian bedrock. The fault is connected NE to one of the regional structure that delimits the bedrock blocks. The fault is visible both as a topographical feature from the ground and from aerial photos. While the project followed through it was clear that the area had been subjected to reactivation. The evidence for this claim stems from an observed semivertical fracture plane with conjugating subhorizontal striations markers that contain an alternating plunge and trend. Other evidence for a reactivation scenario is the fault gouges that contain horizontal striation as well as steep plunging striation, indicating a dip-slip faulting movement. The main zone of faulting displays lower altitude than the surrounding rock. This indicates an asymmetric graben structure that has been subjected to polydeformation. When analyzing the results, it is concluded that a pull-apart basin event has occurred. It’s highly possibly that it has been undergoing modifications during its geological history, resulting in a polydeformed reactivation of the minor faults outlining the graben structure.