The impact of splay fault initiation and evolution on the stress state in subduction-accretion complexes

Abstract:

We study the impact of upper-plate faults on the stress state of accreting sediments using an evolutionary numerical model that couples deformation with sediment compaction and simulates the initiation and evolution of faults. We use the program Elfen, and model sediments as poro-elastoplastic. First, tectonic loading brings the intact sediment to frictional failure. Subsequently, strain localizes and faulting initiates when plastic strain exceeds a threshold. We specify that the frictional strength of the faults decreases after their formation to a residual value, as is observed in laboratory experiments and along tectonic faults. We find that faults first form inside the wedge, where the intact sediment is at frictional failure, and propagate towards the trench. Because of their lower strength, faults limit the differential stress that neighboring sediments can sustain. This leads to spatial variation in the ratio of maximum to minimum principal effective stress, as well as the horizontal stress, from a higher value in the intact material to a much lower value near the faults. This decrease in stress ratio extends to a considerable area of the wedge and propagates as new families of faults form with progressive accretion of material and wedge growth. Stresses in the wedge are lower than predicted by continuum models using the intact sediment frictional strength, which is consistent with observations from seismic and borehole data. We find that the differential stress in outermost wedge near the trench and outboard (protothrust zone) is higher than in the faulted zone, because shear strains first localize inside the wedge. More importantly, our results reveal a lateral heterogeneity in differential stress driven by the formation and evolution of faults, which is not captured in models assuming a uniform strength of either the intact or the faulted material. This heterogeneity has potentially important mechanical and hydrological effects on sediment compaction and elastic properties, and on variations in stress that may lead to complex rupture processes or creep transients. This study is the first step in an effort to integrate weak discontinuities with forward modeling, realistic constitutive models, and a coupled hydromechanical approach to better understand the state of stress in faulted, accreted sediments.