Paper No. 4
Presentation Time: 8:50 AM
THE FORMATION AND EVOLUTION OF TRIANGLE ZONES IN FOLD-AND-THRUST-BELTS: INSIGHTS FROM DEM SIMULATIONS
Studying the frontal structures of fold and thrust belts can provide valuable insights into the formation of mountain belts. Triangle zones, characterized by frontal backthrusts, occur in some settings, but the mechanical processes responsible for them is not well understood. One hypothesis is that the presence of a mechanically weak cover unit promotes the formation of a frontal triangle zone, whereas a strong and homogenous package will not form a triangle zone. Simulations carried out using the discrete element method (DEM) tested the effects of a mechanically weak layer overlying a stronger unit under horizontal compression. The models consisted of multiple layers with different interparticle bond strengths to simulate mechanical stratigraphy. Each model was 60 km long and 4 km thick, and composed of 25,000 – 50,000 particles with radii of 60 and 80 m. Horizontal compression was imposed by moving the left wall inward, above a weak décollement over an elastic basement. The deformation fields were quantified by plotting the 1st and 2nd strain invariants every 160 m of displacement, showing the cumulative strain over time for each simulation. Triangle zones formed only rarely during simulations, although backthrusts commonly formed as part of popup structures in front of the deforming wedge. The backthrusts were generally short lived, and subsequent deformation was focused on the conjugate forethrust. Triangle zone geometries did form in a few of the simulations, but were very transient structures, quickly overridden or passively uplifted and incorporated into the deforming wedge. Therefore, while the presence of an upper detachment surface and weak cover sequence did promote the formation of backthrusts in the simulations, convincing triangle zones were rare, and not found in all the simulations, indicating that while a weak cover unit may aid in the formation of a frontal triangle zone, it is not an essential parameter.