Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

The architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation-based kinematics, to delineate the wedge growth history with four decollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded decollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high-friction patch in the basal decollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low-angle, long-lived forethrust and multiple cycles of frontal accretion on short-lived forethrusts at the deformation front. Our findings suggest that decollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of similar to 105 yr in natural accretionary systems. This result should be considered when investigating modern subduction zones. Accretionary wedges, which are one of the key components of subduction zones, comprise sediments scraped off from a downgoing plate. However, understanding the spatial and temporal changes of their growth and deformation patterns over geological timescales is challenging. In this study, we conducted laboratory experiments using different types of sands to quantify the deformation processes during wedge growth. We tested various layering conditions and, in particular, how single or double and continuous or discontinuous weak layers affect wedge growth. Our results show that wedge growth is achieved by repetition of a frontal accretion cycle, but the detailed nature of the cycle depends on the properties of the weak layers. In particular, an additional weak layer in the subducting sediment is critical in modifying the accretion cycle, strain distribution, and fault activity during wedge growth. Our findings suggest that weak layers are key in determining the stress-strain state in natural wedges and on the plate boundary as the wedge grows on a timescale of similar to 105 yr. An interbedded weak layer reduced the effective internal friction angle of the wedge, weakened the wedge, and formed a dense fault networkSubduction of a frictional barrier in the basal decollement resulted in out-of-sequence thrusting and facilitated sediment underplatingOur results contribute to an understanding of the growth processes, architectural evolution, and kinematics of natural accretionary wedges

Automated summary

This study investigates the architectural evolution of natural accretionary wedges using sandbox analog modeling. The results suggest that the wedge is formed by repeated episodic frontal accretion, and the configuration of the decollement layer plays a crucial role in controlling the accretion cycle and fault network.