Analog Models and Trishear Kinematics: A Sandbox Experiment

Abstract

Numerous studies have focused on comparing the kinematics of fault-propagation folds using numerical models, yet there is a scarcity of research directly comparing these features with velocity fields obtained from analog models. This study aims to bridge that gap by conducting laboratory experiments specifically designed to simulate fault-propagation folds.Sandbox experiments are a commonly used experimental approach in geology to investigate geological processes. These models generally emphasize geometry and kinematics while attempting to simulate natural prototypes. In this research, we focused on the kinematics involved in the formation of a simple fault-propagation fold composed of a single reverse fault.To simulate the development of this thrust, we used a rigid plastic sheet inside an acrylic box with a moving wall filled with sand. The kinematic field was obtained by applying particle image velocimetry (PIVlab), and the resulting vectors were compared with those proposed by the theoretical trishear model. The data collected from the analog models aligned well with the theoretical framework.The observed translations and rotations across the phases of the experiment corroborate the proposed kinematics. There are two main rotations in the kinematic field. The first consists of horizontal vectors rotating counterclockwise until they become parallel to the reverse fault, occurring from the backlimb and in the hanging block of the main fault. The second rotation, of smaller magnitude, occurs exclusively in the frontal sector close to the tip. The vectors that were parallel to the fault begin a new clockwise rotation, tending to zero around the footwall block.To compare the stages of the analog model with the trishear method, the apical angle was tested at intervals of 5°, ranging from 10° to 100°. The difference between the velocity field of the analog model and the theoretical trishear model was calculated to determine the trishear apical angle that best approximates the analog kinematic field in the experiment. Subsequently, the angular differences between the trishear method and the analog experiment were computed after scaling the vectors. The best approximation was obtained with an angle of 30°.