A numerical discrete element study on the influence of an embedded viscoelastic–plastic layer at different viscosity values

The major aim of this study was to examine the influence of an embedded viscoelastic-plastic layer at different viscosity values on accretionary wedges at subduction zones. To quantify the effects of the layer viscosity, we analysed the wedge geometry, accretion mode, thrust systems and mass transport pattern. Therefore, we developed a numerical 2D 'sandbox' model utilising the Discrete Element Method. Starting with a simple pure Mohr Coulomb sequence, we added an embedded viscoelastic-plastic layer within the brittle, undeformed 'sediment' package. This layer followed Burger's rheology, which simulates the creep behaviour of natural rocks, such as evaporites. This layer got thrusted and folded during the subduction process. The testing of different bulk viscosity values, from 1 × 10**13 to 1 × 10**14 (Pa s), revealed a certain range where an active detachment evolved within the viscoelastic-plastic layer that decoupled the over- and the underlying brittle strata. This mid-level detachment caused the evolution of a frontally accreted wedge above it and a long underthrusted and subsequently basally accreted sequence beneath it. Both sequences were characterised by specific mass transport patterns depending on the used viscosity value. With decreasing bulk viscosities, thrust systems above this weak mid-level detachment became increasingly symmetrical and the particle uplift was reduced, as would be expected for a salt controlled forearc in nature. Simultaneously, antiformal stacking was favoured over hinterland dipping in the lower brittle layer and overturning of the uplifted material increased. Hence, we validated that the viscosity of an embedded detachment strongly influences the whole wedge mechanics, both the respective lower slope and the upper slope duplex, shown by e.g. the mass transport pattern.

本研究的核心目标为探究不同黏度条件下，嵌入俯冲带（subduction zones）增生楔（accretionary wedges）内的黏弹塑性层所产生的影响。为量化该层黏度的调控效应，我们对楔体几何形态、增生方式、逆冲断层系（thrust systems）与物质运移模式（mass transport pattern）开展了分析。据此，我们采用离散元法（Discrete Element Method）构建了二维数值砂箱模型。模型初始采用简单的纯摩尔-库仑序列（Mohr Coulomb sequence），随后在脆性未变形的“沉积物”包体中嵌入了黏弹塑性层。该层遵循伯格流变学（Burger's rheology），可模拟蒸发岩（evaporites）等天然岩石的蠕变行为。在俯冲过程中，该层会发生逆冲变形与褶皱。我们对1×10¹³ 至1×10¹⁴ Pa·s区间内的不同体积黏度值开展测试后发现，存在特定黏度范围，此时黏弹塑性层内会形成活跃的拆离层（detachment），该拆离层可实现上覆与下伏脆性地层的解耦。该中层拆离层会促使其上方发育前锋增生楔，下方则形成长距离底冲并最终演化为基底增生序列。两类序列均呈现出与所用黏度值相关的专属物质运移模式。随着体积黏度降低，该弱中层拆离层上方的逆冲断层系会逐渐趋于对称，颗粒抬升量也随之减少，这与天然盐控弧前区域的典型特征相符。与此同时，下伏脆性地层中更易发育背形叠置（antiformal stacking）而非向内陆倾斜构造，抬升物质的倒转程度也会升高。综上，我们验证了赋存拆离层的黏度会显著影响整个楔体的力学行为，包括下斜坡与上斜坡双重构造（duplex），这一点可通过物质运移模式等特征得到印证。