Role of Mechanical Properties of Sediments in the Formation of Landward Versus Seaward Verging Thrust Faults at Accretionary Prisms
Abstract
In most accretionary wedges at subduction zones, thrust faults are oriented, or verge, seaward, away from the overriding continental plate. However, in a few regions like Cascadia and Sumatra — the site of the destructive 2004 tsunami — thrust faults verge landward. To understand why this occurs, numerical tests were run using a 2D finite differencing code, which solves for conservation of mass and momentum in a visco-elastic-plastic medium, simulating thrusting in accretionary prisms. Various parameters were tested, including the thickness of a basal layer below the faulting sediments that deforms diffusely (e.g., due to cataclastic flow), the shear strength of the basal layer, and the elastic (shear) modulus of the sediments. Results show that landward-verging faults occur when the diffusely deforming basal layer has a thickness 15% of the entire sediment package, and is weak in shear (< about 10 MPa). This requirement for a low basal shear stress is in agreement with previous studies. Landward-vergence is also promoted by weaker elastic deformation of the sediment as a whole, due to a lower elastic modulus. We propose the following explanation. First, the weaker basal layer promotes the formation of conjugate faults at the front of the accretionary wedge. As the basal layer deforms diffusely, the overlying sediments are pushed up, and a lower elastic modulus allows the overlying layer to flex upwards in response. This concave-up curvature increases horizontal compression near the surface, which promotes faulting closer to the wedge and encourages slip along landward-verging thrusts. This finding is evident by concentrated zones where the sediment is at or near brittle failure close to the wedge, promoting faulting so that slip occurs along the closer, landward-verging thrust of the conjugate pair. For seaward-verging cases, there is a much broader band where the sediment is at or near failure, which promotes the more distal seaward-verging thrust to slip. Analytical solutions for the bending plate will be compared with these numerical solutions. To understand the implications of these results, they will further be compared with rock properties observed in the field for different accretionary wedges.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T51F0346H
- Keywords:
-
- 8045 Role of fluids;
- STRUCTURAL GEOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS