Analog modeling of accretionary wedges with various décollement settings: Quantitative analysis of deformation process and strength evolution

Abstract

Deformation process and strength evolution in accretionary wedges are important factors that affect kinematics and dynamics at subduction zones. However, it is still challenging to understand the relationship between the short-term geodetic observations (coupling ratios or earthquakes) and the long-term geological structures (patterns of imbricated thrust sheets or fault networks) in natural systems. In this study, we performed 2-D, large-shortening (1 m) analog sandbox experiments to examine how the wedge deformation is affected by different décollement conditions including heterogeneities and numbers of weak layers in the incoming sediment. Four different settings (Types 1–4) of the incoming sediment layers were examined in this study. Serial side-view digital photographs were quantitatively analyzed with an open-source DIC software to characterize the accretion cycles, underthrusting/underplating, and reactivation of pre-existing thrusts (out-of-sequence thrusts).The reference models with single décollement (Type1) were dominated by periodic cycles of frontal accretion with landward propagation of strain, uplift, and reactivation of the pre-existing thrusts, which progressively increased in strength and then approached the critical state. Each cycle was composed of preparation (Phase 0), initiation (Phase 1), accretion (Phase 2), and reactivation (Phase 3). Through frontal accretion, the wedge accumulated the strain internally with landward migration of the basal coupled area along the plate interface, which caused uplift and reactivation of the landward preexisting thrusts in the wedge (hardening). When a new frontal thrust emerged at the deformation front (Phase 1), the basal coupling was suddenly lost (softening). Through this cycle, the entire accretionary wedge progressively increased in strength while experiencing hardening and softening and approached the critical state. The double décollement models (Type 2) showed a similar accretion cycle to Type 1 models, but it consisted of a combination of shallow-rooted and deep-rooted frontal thrusts, meaning that the décollement stepped up and down between the interbedded and basal weak layers. This promoted sediment underthrusting at the frontal part of the wedge during the early phase of the accretion cycle and favored the connection of pre-existing deep-rooted thrusts with shallow-rooted thrusts. A frictional interruption in the basal décollement (Type 3 or 4 models) produced a combination of a steep-taper inner wedge and a gentle-taper outer wedge, and disturbed the wavelengths of the accretion cycle. The single décollement models (Type 3) were dominated by high-angle out-of-sequence thrusts, while underplating was significantly promoted in the double décollement model (Type 4) where the interbedded décollement acted as a low-angle, smooth-surface megathrust.These results shed light on the impact of properties and homogeneity of the incoming sediment and the plate interface on the spatial and temporal evolution of internal structure and thrust activity in accretionary wedges through multiple accretion cycles. Comparisons of our results with natural subduction zones will contribute to understanding the mechanisms and dynamics of deformation process and strength evolution in natural subduction zones.